By: @BTroncone
Also check out my lesson @ngrx/store in 10 minutes on egghead.io!
Update: Non-middleware examples have been updated to ngrx/store v2. More coming soon!
- Overview
- Building Blocks of @ngrx/store
- Walkthrough
- The Sample Application
- Setting Up The First Reducer
- Configuring Store Actions
- Utilizing Container Components
- Utilizing the AsyncPipe
- Taking Advantage of ChangeDetection.OnPush
- Expanding State
- Slicing State for Views
- Projecting State for View with combineLatest and withLatestFrom
- Extracting Selectors for Reuse
- Introducing Store Middleware
- Middleware with Dependencies
- Rehydrating Application State on Bootstrap
- Implementing A Meta-Reducer
- More to come...
- Additional Resources
With the advent of Angular 2 new patterns, best practices, and libraries empowered by new framework features and functionality are emerging. One such library, championed by Angular developer advocate Rob Wormald (huge props to @MikeRyan52 as well), is @ngrx/store. Store builds on the concepts made popular by Redux, a popular state management container in the React community, and supercharges it with the backing of RxJS. The result is a tool and philosophy that will revolutionize your applications and development experience.
Before we dive into the nuts and bolts of what makes store work, let's first take a high-level look at the core concepts. Each application built around store will contain three main pieces, reducers, actions, and a single application store. Let’s take a moment to explore each of these concepts.
Like a traditional database represents the point of record for an application, your store can be thought of as a client side ‘single source of truth’, or database. By adhering to the 1 store contract when designing your application, a snapshot of store at any point will supply a complete representation of relevant application state. This becomes extremely powerful when it comes to reasoning about user interaction, debugging, and in the context of Angular 2, performance.
1 Single, immutable state tree updated only through explicitly defined and dispatched actions
The second integral part of a store application is reducers. A 2 reducer is a 3 pure function, accepting two arguments, the previous state and an action with a type and optional data (payload) associated with the event. Using the previous analogy, if store is to be thought of as your client side database, reducers can be considered the tables in said database. Reducers represent sections, or slices of state within your application and should be structured and composed accordingly.
export interface Reducer<T> {
(state: T, action: Action): T;
}3 A function whose return value is determined only by its input values, with no observable side-effects.
export const counter: Reducer<number> = (state: number = 0, action: Action) => {
switch(action.type){
case 'INCREMENT':
return state + 1;
case 'DECREMENT':
return state - 1;
default:
return state;
}
};Store encompasses our application state and reducers output sections of that state, but how do we communicate to our reducers when state needs to be updated? That is the role of 4 actions. Within a store application, all user interaction that would cause a state update must be expressed in the form of actions. All relevant user events are dispatched as actions, flowing through the 5 action pipeline defined by store, before a new representation of state is output. This process occurs each time an action is dispatched, leaving a complete, serializable representation of application state changes over time.
export interface Action {
type: string;
payload?: any;
}
//simple action without a payload
dispatch({type: 'DECREMENT'});
//action with an associated payload
dispatch({type: ADD_TODO, payload: {id: 1, message: 'Learn ngrx/store', completed: true}})Finally, we need to extract, combine, and project data from store for display in our views. Because store itself is an observable, we have access to the typical JS collection operations you are accustom to (map, filter, reduce, etc.) along with a wide-array of extremely powerful RxJS based observable operators. This makes slicing up store data into any projection you wish quite easy.
//most basic example, get people from state
store.select('people')
//combine multiple state slices
Observable.combineLatest(
store.select('people'),
store.select('events'),
(people, events) => {
//projection here
})In the previous section I mentioned abiding by the store contract when developing your application. What exactly does this mean in the context of the typical Angular setup and workflow which you have grown accustomed? Let's take a look.
If you are coming from an Angular 1 background you are familiar with 6 two-way data binding. The controller model binds to the view and vice versa. The problem with this approach presents itself as your view becomes more complex, requiring controllers and directives to manage and represent significant state changes over time. This can quickly turn into a nightmare both to reason about and debug as one change effects another, which effects another, and so on.
Store promotes the idea of 7 one-way data flow and explicitly dispatched actions. All state updates are handled above your components in store, delegated to reducers. The only way to initiate a state update in your application is through dispatched actions, corresponding to a particular reducer case. This not only makes reasoning about state changes in your application easier, as updates are centralized, it leaves a clear audit trail in case of error.
(demo)
@Component({
selector: 'counter',
template: `
<div class="content">
<button (click)="increment()">+</button>
<button (click)="decrement()">-</button>
<h3>{{counter}}</h3>
</div>
`
})
export class Counter{
counter = 0;
increment(){
this.counter += 1;
}
decrement(){
this.counter -= 1;
}
}(demo)
@Component({
selector: 'counter',
template: `
<div class="content">
<button (click)="increment()">+</button>
<button (click)="decrement()">-</button>
<h3>{{counter$ | async}}</h3>
</div>
`,
changeDetection: ChangeDetectionStrategy.OnPush
})
export class Counter{
counter$: Observable<number>;
constructor(
private store : Store<number>
){
this.counter$ = this.store.select('counter')
}
increment(){
this.store.dispatch({type: 'INCREMENT'});
}
decrement(){
this.store.dispatch({type: 'DECREMENT'});
}
}Throughout the overview we touched briefly on the advantages of utilizing Store over a typical, Angular 1 style approach but let's take a moment to recap. Why take the time to invest in this particular library, architecture pattern, and learning curve? The primary advantage to a Store-based application are centralized state, performance, testability, and tooling.
All relevant application state exists in one location. This makes it easier to track down problems, as a snapshot of state at the time of an error can provide important insight and make it easy to recreate issues. This also makes notoriously hard problems such as undo/redo trivial in the context of a Store application and enables powerful tooling.
Since state is centralized at the top of your application, data updates can flow down through your components relying on slices of store. Angular 2 is built to optimize on such a data-flow arrangement, and can disable change detection in cases where components rely on Observables which have not emitted new values. In an optimal store solution this will be the vast majority of your components.
All state updates are handled in reducers, which are pure functions. Pure functions are extremely simple to test, as it is simply input in, assert against output. This enables the testing of the most crucial aspects of your application without mocks, spies, or other tricks that can make testing both complex and error prone.
A centralized, immutable state also enables powerful tooling. One such example is ngrx developer tools, which provides a history of actions and state changes, allowing for 8 time travel during development. The patterns provided by Store also allow for a rich ecosystem of easy to implement middleware. Because store provides an entry point both before and after dispatched actions hit application reducers, problems such as syncing slices of state to local storage, advanced logging, and implementing sagas are easily solved with a quick package include and a few lines of code. This ecosystem will only grow over the coming months.
8 Manipulating the history of dispatched actions and state changes to emulate a point in time of application interaction.
Before we build a Store application, let's first take a look at the RxJS concepts on which @ngrx/store is built. By understanding these concepts first, we can more effectively utilize the library in the future. For a more detailed explantion of each of the topics below, please check out these additional resources.
Disclaimer: The actual @ngrx/store code by Mike Ryan and Rob Wormald is significantly more robust. These examples are meant to demonstrate the RxJS concepts involved and remove the 'magic' from the library.
The messenger of Rx, you tell me, I'll tell them...
(demo)
The two pillars of @ngrx/store, the store and dispatcher, both extend RxJS Subjects. Subjects are both Observables and Observers, meaning you can subscribe to a Subject, but can also subscribe a Subject to a source. At a high-level Subjects can be thought of as messengers, or proxies.
Because Subjects are Observers, you can 'next', or pass values into the stream directly. Subscribers of that Subject will then be notified of emitted values. In the context of Store, these subscribers could be a Angular 2 service, component, or anything requiring access to application state.
//create a subject
const mySubject = new Rx.Subject();
//add subscribers
const subscriberOne = mySubject.subscribe(val => {
console.log('***SUBSCRIBER ONE***', val);
});
const subscriberTwo = mySubject.subscribe(val => {
console.log('***SUBSCRIBER TWO***', val);
});
//publish values to observers of subject
mySubject.next('FIRST VALUE!'); '***SUBSCRIBER ONE*** FIRST VALUE! ***SUBSCRIBER TWO*** FIRST VALUE!'
mySubject.next('SECOND VALUE!'); '***SUBSCRIBER ONE*** SECOND VALUE! ***SUBSCRIBER TWO*** SECOND VALUE!'In Store (and Redux), it is convention to dispatch actions to the application store. To maintain this API, the Dispatcher extends Subject, adding a dispatch method as a passthrough to the classic next method. This is used to pass values into the Subject before emitting these values to subcribers.
/*
redux/ngrx-store has a concept of a dispatcher, or method to send actions to application store
lets extend Rx.Subject with our Dispatcher class to maintain familiar terms.
*/
//inherit from subject
class Dispatcher extends Rx.Subject{
dispatch(value : any) : void {
this.next(value);
}
}
//create a dispatcher (just a Subject with wrapped next method)
const dispatcher = new Dispatcher();
//add subscribers
const subscriberOne = dispatcher.subscribe(val => {
console.log('***SUBSCRIBER ONE***', val);
});
const subscriberTwo = dispatcher.subscribe(val => {
console.log('***SUBSCRIBER TWO***', val);
});
//publish values to observers of dispatcher
dispatcher.dispatch('FIRST DISPATCHED VALUE!');
dispatcher.dispatch('SECOND DISPATCHED VALUE!');Similar to Subject but, what's the last thing you said?...
(demo)
While vanilla Subjects work perfectly as a 'dispatcher', they have one issue that prevents them from being a good fit for store. When subscribing to a Subject, only values emitted after the subscription are received. This is unacceptable in an environment where components will be consistently added and removed, requiring the latest, on-demand sections of state from the application store at the time of subscription.
/*
Now that we have a dispatcher, let's create our store to receieve dispatched actions.
*/
class FirstStore extends Rx.Subject{}
const myFirstStore = new FirstStore();
//add a few subscribers
const subscriberOne = myFirstStore.subscribe(val => {
console.log('***SUBSCRIBER ONE***', val);
});
const subscriberTwo = myFirstStore.subscribe(val => {
console.log('***SUBSCRIBER TWO***', val);
});
//For now, lets surpass dispatcher and manually publish values to store
myFirstStore.next('FIRST VALUE!');
/*
Let's add another subscriber.
Since our first implementation of store is a subject, subscribers will only have visibility into values emitted *AFTER* they subscribe. In this case, subscriber three will have no knowledge of 'FIRST VALUE!'
*/
const subscriberThree = myFirstStore.subscribe(val => {
console.log('***SUBSCRIBER THREE***', val);
});Luckily, RxJS offers a convienant extension of Subject to handle this problem, the BehaviorSubject. BehaviorSubject's encapsulate all of the functionality of Subject, but also return the last emitted value to subscribers upon subscription. This means components and services will always have access to the latest (or initial) application state and all future updates.
/*
Because our components will need to query current state, BehaviorSubject is a more natural fit for Store. BehaviorSubjects have all the functionality of Subject, but also allow for an initial value to be set, as well as outputting the last value received to all observers upon subscription.
*/
class Store extends Rx.BehaviorSubject{
constructor(initialState : any){
super(initialState);
}
}
const store = new Store('INITIAL VALUE!');
//add a few subscribers
const storeSubscriberOne = store.subscribe(val => {
console.log('***STORE SUBSCRIBER ONE***', val);
});
const storeSubscriberTwo = store.subscribe(val => {
console.log('***STORE SUBSCRIBER TWO***', val);
});
//For demonstration, manually publish values to store
store.next('FIRST STORE VALUE!');
//Add another subscriber after 'FIRST VALUE!' published
//output: ***STORE SUBSCRIBER THREE*** FIRST STORE VALUE!
const subscriberThree = store.subscribe(val => {
console.log('***STORE SUBSCRIBER THREE***', val);
});Single state tree and one way data flow in Angular 2...
(demo)
In order to function correctly in the context of store, the dispatcher still needs some work. In a Store application, all dispatched actions must be passed through a specific pipeline before a new representation of state is passed into store, to be emitted to all observers. You can think of this as a factory assembly line, in this case the stations on the line are pre-middleware -> reducers -> post-middleware -> store.
The creation of this pipeline is handled by passing the dispatcher into store when it is created. The store next method is then overridden in order to delegate all actions first to the dispatcher pipeline before passing the new representation of state into store. This also allows the store to be subscribed directly to action streams, funneling received actions first through the dispatcher.
For now, the implementation of middleware and reducers will be stubbed out with comments.
/*
All actions should pass through pipeline before newly calculated state is passed to store.
1.) Dispatched Action
2.) Pre-Middleware
3.) Reducers (return new state)
4.) Post-Middleware
5.) store.next(newState)
*/
class Dispatcher extends Rx.Subject{
dispatch(value : any) : void {
this.next(value);
}
}
class Store extends Rx.BehaviorSubject{
constructor(
private dispatcher,
initialState
){
super(initialState);
/*
all dispatched actions pass through action pipeline before new state is passed
to store
*/
this.dispatcher
//pre-middleware
//reducers
//post-middleware
.subscribe(state => super.next(state));
}
//delegate store.dispatch first through dispatched action pipeline
dispatch(value){
this.dispatcher.dispatch(value);
}
//override store next to allow direct subscription to action streams by store
next(value){
this.dispatcher.dispatch(value);
}
}
const dispatcher = new Dispatcher();
const store = new Store(dispatcher, 'INITIAL STATE');
const subscriber = store.subscribe(val => console.log(`VALUE FROM STORE: ${val}`));
/*
All dispatched actions first flow through action pipeline, calculating new state which is
then passed to store. To recap, our ideal behavior:
dispatched action -> pre-middleware -> reducers -> post-middleware -> store.next(newState)
*/
//both methods are same behind the scenes
dispatcher.dispatch('DISPATCHED VALUE!');
store.dispatch('ANOTHER DISPATCHED VALUE!');
const actionStream$ = new Rx.Subject();
/*
Overriding store next method allows us to subscribe store directly to action streams, providing same behavior as manually calling store.dispatch or dispatcher.dispatch
*/
actionStream$.subscribe(store);
actionStream$.next('NEW ACTION!');Like a snowball rolling down hill, reducers accumulate value through iteration...
(demo)
Reducers are the foundation of any Store or Redux-based application, describing sections of state and their potential transformations based on dispatched action types. It is the combination of your reducers that makes up a representation of application state at any given time.
Before we discuss how reducers are created and implemented, let's first look at the reduce function. Reduce takes an array of values, running a function against an accumulated value and current value, reducing your array into a single value upon completion. You can think of the accumulator as a snowball rolling downhill, gaining mass with each revolution. In the same way, the reduce accumulator is the result of applying your defined function to the current value through iteration.
/*
You can think of reduce as a snowball rolling downhill. Each rotation (or iteration) mass is accumulated until we reach the bottom. Similarly with reduce, the returned value is passed to the next invocation of the supplied function until all values in the source array are exhausted. Let's see some examples to solidify this concept.
*/
const numberArray = [1,2,3];
/*
1.) accumulator: 1, current: 2
2.) accumulator: 3, current: 3
Final: 6
*/
const total = numberArray.reduce((accumulator, current) => accumulator + current);
console.log(`***TOTAL***: ${total}`);
//Reduce with objects
const personInfo = [{name: 'Joe'}, {age: 31}, {birthday: '1/1/1985'}];
/*
1.) accumulator: {name: 'Joe'}, current: {age: 31}
2.) accumulator: {name: 'Joe', age: 31}, current: {birthday: '1/1/1985'}
Final: {name: 'Joe', age: 31, {birthday: '1'/1/1985'}}
*/
const fullPerson = personInfo.reduce((accumulator, current) => {
return Object.assign({}, accumulator, current)
});
console.log('***FULL PERSON***:', fullPerson);
//We can also provide an initial value for reduce as a second parameter
const personInfoStart = [{name: 'Joe'}, {age: 31}, {birthday: '1/1/1985'}];
/*
1.) accumulator: {favoriteLanguage: 'JavaScript'}, current: {name: 'Joe'}
2.) accumulator: {favoriteLanguage: 'JavaScript', name: 'Joe'}, current: {age: 31}
3.) accumulator: {favoriteLanguage: 'JavaScript', name: 'Joe', age: 31}, current: {birthday: '1/1/1985'}
Final: {favoriteLanguage: 'JavaScript', name: 'Joe', age: 31, birthday: '1/1/1985'}
*/
const fullPersonStart = personInfo.reduce((accumulator, current) => {
return Object.assign({}, accumulator, current)
}, {favoriteLanguage: 'JavaScript'});
console.log('***FULL PERSON START:', fullPersonStart);Inspired by Redux, @ngrx/store has the concept of Reducer functions used to manipulate specific slices of state. Reducers accept a state and action parameter, exposing a switch statement (generally, although this could be handled multiple ways) defining action types in which the reducer is concerned. 9 Each time an action is dispatched, each reducer registered to store will be called (through the root reducer, created during provideStore at application bootstrap), passed the current state for that state slice (accumulator) and the dispatched action. If the reducer is registered to handle that action type, the appropriate state calculation will be performed and a new representation of state output. If not, the current state for that section will be returned. This is the core of Store and Redux state management.
//Redux-Style Reducer
const person = (state = {}, action) => {
switch(action.type){
case 'ADD_INFO':
return Object.assign({}, state, action.payload)
default:
return state;
}
}
const infoAction = {type: 'ADD_INFO', payload: {name: 'Brian', framework: 'Angular'}}
const anotherPersonInfo = person(undefined, infoAction);
console.log('***REDUX STYLE PERSON***: ', anotherPersonInfo);
//Add another reducer
const hoursWorked = (state = 0, action) => {
switch(action.type){
case 'ADD_HOUR':
return state + 1;
case 'SUBTRACT_HOUR':
return state - 1;
default:
return state;
}
}
//Combine Reducers Refresher
const myReducers = {person, hoursWorked};
const combineReducers = reducers => (state = {}, action) => {
return Object.keys(reducers).reduce((nextState, key) => {
nextState[key] = reducers[key](state[key], action);
return nextState;
}, {});
};
/*
This gets us most of the way there, but really want we want is for the value of firstState and secondState to accumulate
as actions are dispatched over time. Luckily, RxJS offers the perfect operator for this scenario., to be discussed in next lesson.
*/
const rootReducer = combineReducers(myReducers);
const firstState = rootReducer(undefined, {type: 'ADD_INFO', payload: {name: 'Brian'}});
const secondState = rootReducer({hoursWorked: 10, person: {name: 'Joe'}}, {type: 'ADD_HOUR'});
console.log('***FIRST STATE***:', firstState);
console.log('***SECOND STATE***:', secondState);
Similar to reduce, but value is accumulated over time...
Scan behaves in a similar fashion to reduce, except the accumulator value is maintained over time, or until the observable with scan applied has completed. For instance, as actions are dispatched and new state output, the accumulator in the scan function will always be the last output representation of state. This alleviates the need to maintain a copy of state in store to be passed to our reducers, the scan operator handles this functionality.
const testSubject = new Rx.Subject();
//basic scan example, sum over time starting with zero
const basicScan = testSubject.scan((acc, curr) => acc + curr, 0);
//log accumulated values
const subscribe = basicScan.subscribe(val => console.log('Accumulated total:', val));
//pass values into our test subject, adding to the current sum
testSubject.next(1); //1
testSubject.next(2); //3
testSubject.next(3); //6
const testSubjectTwo = new Rx.Subject();
//scan example building an object over time
const objectScan = testSubjectTwo.scan((acc, curr) => Object.assign({}, acc, curr), {});
//log accumulated values
const subscribe = objectScan.subscribe(val => console.log('Accumulated object:', val));
//pass values into our test subject, adding properties to object
testSubjectTwo.next({name: 'Joe'}); // {name: 'Joe'}
testSubjectTwo.next({age: 30}); // {name: 'Joe', age: 30}
testSubjectTwo.next({favoriteFramework: 'Angular 2'}); // {name: 'Joe', age: 30, favoriteFramework: 'Angular 2'}To utilize scan in the application store, it simply needs to be applied as an operator to the dispatcher. All dispatched actions will pass through scan, invoking the combined reducer with current state and action, outputting a new representation of state. The new application state is then next'ed, or pushed to the store, and emitted to all subscribers.
class Store extends Rx.BehaviorSubject{
constructor(
private dispatcher,
private reducer,
initialState = {}
){
super(initialState);
this.dispatcher
//pre-middleware?
/*
Scan is a reduce over time. In the previous lesson we compared reduce to a snowball rolling downhill, accumulating mass
(or calculated value). Scan can be thought of similarly, except the hill has no certain end. The accumulator (in this
case, state) will continue to collect until destroyed. This makes it the ideal operator for managing application state.
*/
.scan((state, action) => this.reducer(state, action), initialState)
//post-middleware?
.subscribe(state => super.next(state));
}
//...store implementation
}let me have the entire observable...
(let demo | store demo)
Middleware has been removed in ngrx/store v2. It is still worth reading through this section to understand let as it can be used with selectors.
While most operators are passed emitted values from the observable, let is handed the entire observable. This allows the opportunity to tack on extra operators and functionality, before returning the source observable. While this may seem like a small nuance, it fits perfectly into situations like middleware or selectors (to be discussed later), where the consumer would like to define a composable, reusable block of code to be inserted at a particular slot in an observable chain.
const myArray = [1,2,3,4,5];
const myObservableArray = Rx.Observable.fromArray(myArray);
const test = myObservableArray
.map(val => val + 1)
//this fails, let behaves differently than most operators
//.let(val => val + 2)
.subscribe(val => console.log('VALUE FROM ARRAY: ', val));
const letTest = myObservableArray
.map(val => val + 1)
//'let' me have the entire observable
.let(obs => obs.map(val => val + 2))
.subscribe(val => console.log('VALUE FROM ARRAY WITH let: ', val));
//let provides flexibility to add multiple operators to source observable then return
const letTestThree = myObservableArray
.map(val => val + 1)
//'let' me have the entire observable
.let(obs => obs
.map(val => val + 2)
//also, just return evens
.filter(val => val % 2 === 0)
)
.subscribe(val => console.log('let WITH MULTIPLE OPERATORS: ', val));
//pass in your own function to add operators to observable
const obsArrayPlusYourOperators = (yourAppliedOperators) => {
return myObservableArray
.map(val => val + 1)
.let(yourAppliedOperators)
};
const addTenThenTwenty = obs => obs.map(val => val + 10).map(val => val + 20);
const letTestFour = obsArrayPlusYourOperators(addTenThenTwenty)
.subscribe(val => console.log('let FROM FUNCTION:', val));The let operator is a perfect fit for @ngrx/store middleware as an entry point is required for the user to add custom functionality before or after reducers output state. This is the basis for how pre and post middleware is applied in @ngrx/store.
class Store extends Rx.BehaviorSubject{
constructor(
private dispatcher,
private reducer,
preMiddleware,
postMiddleware,
initialState = {}
){
super(initialState);
this.dispatcher
//The let operate accepts the source observable, returning a new observable
//@ngrx/store composes middleware so you can supply more than 1 function,
//for our simple example we will accept one pre and post middleware
//Middleware signature: (obs) => obs
.let(preMiddleware)
.scan((state, action) => this.reducer(state, action), initialState)
.let(postMiddleware)
.subscribe(state => super.next(state));
}
//...store implementation
}
//create basic middleware that logs actions before reducer, and newly outputted state
const preMiddleware = obs => { return obs.do(val => console.log('ACTION: ', val))};
const postMiddleware = obs => { return obs.do(val => console.log('STATE: ', val))};
...create store supplying middlewareTo recap up to this point:
- Dispatched actions are
next'ed into the dispatcher (Subject) - The Dispatcher has 3 operators applied:
let- Passed an observable of actionsscan- Calls each reducer with current state and action, outputting new representation of statelet- Passed an observable of state
- The new representation of state is then
nexted intostore(BehaviorSubject), to be emitted to subscribers
This is the gist of the inner workings of store.
I'll take the corner piece...
(demo)
The cornerstone function for projecting data from a collection is map. Map applies a specified function to each item, returning a new representation of that item. Because application state is a key/value object map of sections of state, it's simple to provide a helper function to return the requested slice of state based on a string, or any other relevant selector.
class Dispatcher extends Rx.Subject{
dispatch(value : any) : void {
this.next(value);
}
}
class Store extends Rx.BehaviorSubject{
constructor(
private dispatcher,
private reducer,
preMiddleware,
postMiddleware,
initialState = {}
){
super(initialState);
this.dispatcher
.let(preMiddleware)
.scan((state, action) => this.reducer(state, action), initialState)
.let(postMiddleware)
.subscribe(state => super.next(state));
}
//Map makes it easy to select slices of state that will be needed for your components
//This is a simple helper function to make grabbing sections of state more concise
select(key : string) {
return this.map(state => state[key]);
}
//...store implementation
}
//...create store
//utilizing the store select helper
const subscriber = store
.select('person')
.subscribe(val => console.log('VALUE OF PERSON:', val));Don't call me until you've changed...
(distinctUntilChanged demo | store demo)
Each view in our application generally is concerned with their own particular slices of state. For perfomance reasons, we would prefer not to emit new values from the selected state slices unless an update has been made. Luckily for us, RxJS has an operator for exactly this scenario (notice the trend). The distinctUntilChanged operator will only emit when the next value is unique, based on the previously emitted value. In cases of numbers and strings, this means equal numbers and strings, in the case of objects, if the object reference is the same a new object will not be emitted.
//only output distinct values, based on the last emitted value
const myArrayWithDuplicatesInARow = new Rx.Observable
.fromArray([1,1,2,2,3,1,2,3]);
const distinctSub = myArrayWithDuplicatesInARow
.distinctUntilChanged()
//output: 1,2,3,1,2,3
.subscribe(val => console.log('DISTINCT SUB:', val));
const nonDistinctSub = myArrayWithDuplicatesInARow
//output: 1,1,2,2,3,1,2,3
.subscribe(val => console.log('NON DISTINCT SUB:', val));
const sampleObject = {name: 'Test'};
const myArrayWithDuplicateObjects = new Rx.Observable.fromArray([sampleObject, sampleObject, sampleObject]);
//only out distinct objects, based on last emitted value
const nonDistinctObjects = myArrayWithDuplicateObjects
.distinctUntilChanged()
//output: 'DISTINCT OBJECTS: {name: 'Test'}'
.subscribe(val => console.log('DISTINCT OBJECTS:', val));Recall that store reducers always have a default case, returning the previous state if dispatched actions are not relevant. This means, when selecting slices of state within your applications, you will not receive updates unless your particular slice has been updated. This aids in making your Store application more efficient.
class Dispatcher extends Rx.Subject{
dispatch(value : any) : void {
this.next(value);
}
}
class Store extends Rx.BehaviorSubject{
constructor(
private dispatcher,
private reducer,
preMiddleware,
postMiddleware,
initialState = {}
){
super(initialState);
this.dispatcher
.let(preMiddleware)
.scan((state, action) => this.reducer(state, action), initialState)
.let(postMiddleware)
.subscribe(state => super.next(state));
}
/*
distinctUntilChanged only emits new values when output is distinct, per last emitted value.
In the example below, the observable with the distinctUntilChanged operator will emit one less value then the other with only the map operator applied
*/
select(key : string) {
return this
.map(state => state[key])
.distinctUntilChanged();
}
//...store implementation
}
// add reducers...
// configure store...
const subscriber = store
//with distinctUntilChanged
.select('person')
.subscribe(val => console.log('PERSON WITH DISTINCTUNTILCHANGED:', val));
const subscriberTwo = store
//without distinctUntilChanged, will print out extra time
.map(state => state.person)
.subscribe(val => console.log('PERSON WITHOUT DISTINCTUNTILCHANGED:', val));
//dispatch a few actions
dispatcher.dispatch({
type: 'ADD_INFO',
payload: {
name: 'Brian',
message: 'Exploring Reduce!'
}
});
//person will not be changed
dispatcher.dispatch({
type: 'ADD_HOUR'
});The 10 sample application we will be building is a simple party planner. The user should be able to enter a list of attendees and their guests, keep track of who has confirmed attendance, filter attendees by particular criteria, and quickly view important statistics regarding the event. Throughout the creation of the application we will explore the core concepts of @ngrx/store and discuss popular patterns and best practices.
There are two links provided above each section, Work Along and Completed Lesson. The Work Along link picks up at the beginning of each lesson if you wish to code along as the concepts are presented. Otherwise, the Completed Lesson link allows you to start at the end point of the current lesson.
Without further ado, let's get started!
(Work Along | Completed Lesson)
Reducers are the foundation to your store application. As the application store maintains state, reducers are the workhorse behind the manipulation and output of new state representations as actions are dispatched. Each reducer should be focused on a specific section, or slice of state, similar to a table in a database.
Creating reducer's is quite simple once you get used to one common idiom, never mutate previous state and always return a new representation of state when a relevant action is dispatched. If you are new to store or the Redux pattern this takes some practice to feel natural. Instead of using mutative methods like push, or reassigning values to previously existing objects, you will instead lean on none mutating methods like concat and Object.assign to return new values. Still confused? Let's see what this looks like in practice with our people reducer.
The people reducer needs to handle five actions, adding a person, removing a person, adding guests to a person, removing guests from a person, and toggling whether they are attending the event. To do this, we will create a reducer function, accepting previous state and the currently dispatched action. We then need to implement a case statement that performs the correct state recalculation when a relevant action is dispatched.
const details = (state, action) => {
switch(action.type){
case ADD_GUEST:
if(state.id === action.payload){
return Object.assign({}, state, {guests: state.guests + 1});
}
return state;
case REMOVE_GUEST:
if(state.id === action.payload){
return Object.assign({}, state, {guests: state.guests - 1});
}
return state;
case TOGGLE_ATTENDING:
if(state.id === action.payload){
return Object.assign({}, state, {attending: !state.attending});
}
return state;
default:
return state;
}
}
//remember to avoid mutation within reducers
export const people = (state = [], action) => {
switch(action.type){
case ADD_PERSON:
return [
...state,
Object.assign({}, {id: action.payload.id, name: action.payload.name, guests:0, attending: false})
];
case REMOVE_PERSON:
return state
.filter(person => person.id !== action.payload);
//to shorten our case statements, delegate detail updates to second private reducer
case ADD_GUEST:
return state.map(person => details(person, action));
case REMOVE_GUEST:
return state.map(person => details(person, action));
case TOGGLE_ATTENDING:
return state.map(person => details(person, action));
//always have default return of previous state when action is not relevant
default:
return state;
}
}(Work Along | Completed Lesson)
The only way to modify state within a store application is by dispatching actions. Because of this, a log of actions should present a clear, readable, history of user interaction. Actions are generally defined as string constants or as static string values on services encapsulating a particular action type. In the latter case, functions are provided to return an appropriate action given the correct input. These methods, which help standardize your actions while providing additional type safety, are known as action creators.
For the case of our starter application we will export a string constant for each application action. These will then be used as the keys to our reducer case statements and the type for every dispatched action.
//Person Action Constants
export const ADD_PERSON = 'ADD_PERSON';
export const REMOVE_PERSON = 'REMOVE_PERSON';
export const ADD_GUEST = 'ADD_GUEST';
export const REMOVE_GUEST = 'REMOVE_GUEST';
export const TOGGLE_ATTENDING = 'TOGGLE_ATTENDING';(Work Along | Completed Lesson)
Components in your Store application will fall into two categories, 11 smart and dumb. So what does it mean to be a smart component vs a dumb component?
Smart, or Container components should be your root level, routable components. These components generally have direct access to store or a derivative. Smart components handle view events and the dispatching of actions, whether through a service or directly. Smart components also handle the logic behind events emitted up from child components within the same view.
Dumb, or Child components are generally for presentation only, relying exclusively on @Input parameters, acting on the receieved data in an appropriate manner. When relevant events occur in dumb components, they are emitted up to be handled by a parent smart component. Dumb components will make up the majority of your application, as they should be small, focused, and reusable.
The party planner application will need a single container component. This component will be responsible for passing appropriate state down to each child component and dispatching actions based on events emitted by our dumb components, person-input, person-list, and in the future party-stats. For now, we will manually subscribe to store in the constructor, setting updates to a component-level property. In future lessons we will explore how to utilize the AsyncPipe to reduce some of this boilerplate.
@Component({
selector: 'app',
template: `
<h3>@ngrx/store Party Planner</h3>
<person-input
(addPerson)="addPerson($event)"
>
</person-input>
<person-list
[people]="people"
(addGuest)="addGuest($event)"
(removeGuest)="removeGuest($event)"
(removePerson)="removePerson($event)"
(toggleAttending)="toggleAttending($event)"
>
</person-list>
`,
directives: [PersonList, PersonInput]
})
export class App {
public people;
private subscription;
constructor(
private _store: Store
){
/*
demonstrating use without the async pipe,
we will explore the async pipe in the next lesson
*/
this.subscription = this._store
.select('people')
.subscribe(people => {
this.people = people;
});
}
//all state-changing actions get dispatched to and handled by reducers
addPerson(name){
this._store.dispatch({type: ADD_PERSON, payload: {id: id(), name})
}
addGuest(id){
this._store.dispatch({type: ADD_GUEST, payload: id});
}
removeGuest(id){
this._store.dispatch({type: REMOVE_GUEST, payload: id});
}
removePerson(id){
this._store.dispatch({type: REMOVE_PERSON, payload: id});
}
toggleAttending(id){
this._store.dispatch({type: TOGGLE_ATTENDING, payload: id})
}
/*
if you do not use async pipe and create manual subscriptions
always remember to unsubscribe in ngOnDestroy
*/
ngOnDestroy(){
this.subscription.unsubscribe();
}
}@Component({
selector: 'person-list',
template: `
<ul>
<li
*ngFor="let person of people"
[class.attending]="person.attending"
>
{{person.name}} - Guests: {{person.guests}}
<button (click)="addGuest.emit(person.id)">+</button>
<button *ngIf="person.guests" (click)="removeGuest.emit(person.id)">-</button>
Attending?
<input type="checkbox" [(ngModel)]="person.attending" (change)="toggleAttending.emit(person.id)" />
<button (click)="removePerson.emit(person.id)">Delete</button>
</li>
</ul>
`
})
export class PersonList {
/*
"dumb" components do nothing but display data based on input and
emit relevant events back up for parent, or "container" components to handle
*/
@Input() people;
@Output() addGuest = new EventEmitter();
@Output() removeGuest = new EventEmitter();
@Output() removePerson = new EventEmitter();
@Output() toggleAttending = new EventEmitter();
}@Component({
selector: 'person-input',
template: `
<input #personName type="text" />
<button (click)="add(personName)">Add Person</button>
`
})
export class PersonInput {
@Output() addPerson = new EventEmitter();
add(personInput){
this.addPerson.emit(personInput.value);
personInput.value = '';
}
}
(Work Along | Completed Lesson)
The AsyncPipe is a unique, stateful pipe in Angular 2 meant for handling both Observables and Promises. When using the AsyncPipe in a template expression with Observables, the supplied Observable is subscribed to, with emitted values being displayed within your view. This pipe also handles unsubscribing to the supplied observable, saving you the mental overhead of manually cleaning up subscriptions in ngOnDestroy. In a Store application, you will find yourself leaning on the AsyncPipe heavily in nearly all of your component views. For a more detailed explanation of exactly how the AsyncPipe works, check out my article Understand and Utilize the AsyncPipe in Angular 2 or free egghead.io video Using the Async Pipe in Angular 2.
Utilizing the AsyncPipe in our templates is easy. You can pipe any Observable (or promise) through async and a subscription will be created, updating the template value on source emission. Because we are using the AsyncPipe, we can also remove the manual subscription from the component constructor and unsubscribe from the ngOnDestroy lifecycle hook. This is now handled for us behind the scenes.
@Component({
selector: 'app',
template: `
<h3>@ngrx/store Party Planner</h3>
<person-input
(addPerson)="addPerson($event)"
>
</person-input>
<person-list
[people]="people | async"
(addGuest)="addGuest($event)"
(removeGuest)="removeGuest($event)"
(removePerson)="removePerson($event)"
(toggleAttending)="toggleAttending($event)"
>
</person-list>
`,
directives: [PersonList, PersonInput]
})
export class App {
public people;
private subscription;
constructor(
private _store: Store
){
/*
Observable of people, utilzing the async pipe
in our templates this will be subscribed to, with
new values being dispayed in our template.
Unsubscribe wil be called automatically when component
is disposed.
*/
this.people = _store.select('people');
}
//all state-changing actions get dispatched to and handled by reducers
addPerson(name){
this._store.dispatch({type: ADD_PERSON, payload: name})
}
addGuest(id){
this._store.dispatch({type: ADD_GUEST, payload: id});
}
removeGuest(id){
this._store.dispatch({type: REMOVE_GUEST, payload: id});
}
removePerson(id){
this._store.dispatch({type: REMOVE_PERSON, payload: id});
}
toggleAttending(id){
this._store.dispatch({type: TOGGLE_ATTENDING, payload: id})
}
//ngOnDestroy to unsubscribe is no longer necessary
}(Work Along | Completed Lesson)
Utilizing a centralized state tree in Angular 2 can not only bring benefits in predictability and maintability, but also performance. To enable this performance benefit we can utilize the changeDetectionStrategy of OnPush.
The concept behind OnPush is straightforward, when components rely solely on inputs, and those input references do not change, Angular can skip running change detection on that section of the component tree. As discussed previously, all delegating of state should be handled in smart, or top level components. This leaves the majority of components in our application relying solely on input, safely allowing us to set the ChangeDetectionStrategy to OnPush in the component definition. These components can now forgo change detection until necessary, giving us a free performance boost.
To utilize OnPush change detection in our components, we need to set the changeDetection propery in the @Component decorator to ChangeDetection.OnPush. That's it! Angular will now ignore change detection on our dumb components and children of these components until there is a change in their input references.
@Component({
selector: 'person-list',
template: `
<ul>
<li
*ngFor="let person of people"
[class.attending]="person.attending"
>
{{person.name}} - Guests: {{person.guests}}
<button (click)="addGuest.emit(person.id)">+</button>
<button *ngIf="person.guests" (click)="removeGuest.emit(person.id)">-</button>
Attending?
<input type="checkbox" [(ngModel)]="person.attending" (change)="toggleAttending.emit(person.id)" />
<button (click)="removePerson.emit(person.id)">Delete</button>
</li>
</ul>
`,
changeDetection: ChangeDetectionStrategy.OnPush
})
/*
with 'onpush' change detection, components which rely solely on
input can skip change detection until those input references change,
this can supply a significant performance boost
*/
export class PersonList {
/*
"dumb" components do nothing but display data based on input and
emit relevant events back up for parent, or "container" components to handle
*/
@Input() people;
@Output() addGuest = new EventEmitter();
@Output() removeGuest = new EventEmitter();
@Output() removePerson = new EventEmitter();
@Output() toggleAttending = new EventEmitter();
}(Work Along | Completed Lesson)
The majority of store applications will be made up of multiple reducers, each managing their own slice of state. For this example we will have two, one to manage party attendees and the other to represent the currently active filter being applied to this list. Let's first write our action constants, specifiying the allowed filters to be applied by the user.
It's now time to create the partyFilter reducer. For this functionality we have a couple of options. The first would be to simply return a string representation of which filter should be applied. We could then write a method, either in a service or component, that filters the list based on the current active party filter. While this works, it is more extensible to return the function to be applied to the party list based on the current party filter state. In the future, adding more filters is as simple as creating a new case statement to return the appropriate projection function.
import {
SHOW_ATTENDING,
SHOW_ALL,
SHOW_WITH_GUESTS
} from './actions';
//return appropriate function depending on selected filter
export const partyFilter = (state = person => person, action) => {
switch(action.type){
case SHOW_ATTENDING:
return person => person.attending;
case SHOW_ALL:
return person => person;
case SHOW_WITH_GUESTS:
return person => person.guests;
default:
return state;
}
};//Party Filter Constants
export const SHOW_ATTENDING = 'SHOW_ATTENDING';
export const SHOW_ALL = 'SHOW_ALL';
export const SHOW_WITH_GUESTS = 'SHOW_GUESTS';import {Component, Output, EventEmitter} from "angular2/core";
import {
SHOW_ATTENDING,
SHOW_ALL,
SHOW_WITH_GUESTS
} from './actions';
@Component({
selector: 'filter-select',
template: `
<div class="margin-bottom-10">
<select #selectList (change)="updateFilter.emit(selectList.value)">
<option *ngFor="let filter of filters" value="{{filter.action}}">
{{filter.friendly}}
</option>
</select>
</div>
`
})
export class FilterSelect {
public filters = [
{friendly: "All", action: SHOW_ALL},
{friendly: "Attending", action: SHOW_ATTENDING},
{friendly: "Attending w/ Guests", action: SHOW_WITH_GUESTS}
];
@Output() updateFilter : EventEmitter<string> = new EventEmitter<string>();
}(Work Along | Completed Lesson)
Store can be thought of as your client-side database. Because store is the aggregate of state in our application we need to be able to query against it, returning relevant state slices and projections. This is an area where the RxJS foundation of store really shines.
To select appropriate slices of state for consumption you can start by using the Rx implementations of classic JavaScript collection operations in which you have grown accustom. Store also provides a helper function, select, which accepts a string or function, applying map and distinctUntilChanged behind the scenes to return an Observable of the appropriate section of state. As your needs progress, RxJS offers a multitude of powerful operators to suit any use-case.
In this example, we are going to slice our state into multiple Observables, representing the party attendees, current filter, attendance and guest count. In the next lesson we will see how to streamline this utilizing popular RxJS operators.
@Component({
selector: 'app',
template: `
<h3>@ngrx/store Party Planner</h3>
<party-stats
[invited]="(people | async)?.length"
[attending]="(attending | async)?.length"
[guests]="(guests | async)"
>
</party-stats>
<filter-select
(updateFilter)="updateFilter($event)"
>
</filter-select>
<person-input
(addPerson)="addPerson($event)"
>
</person-input>
<person-list
[people]="people | async"
[filter]="filter | async"
(addGuest)="addGuest($event)"
(removeGuest)="removeGuest($event)"
(removePerson)="removePerson($event)"
(toggleAttending)="toggleAttending($event)"
>
</person-list>
`,
directives: [PersonList, PersonInput, FilterSelect, PartyStats]
})
export class App {
public people;
private subscription;
constructor(
private _store: Store
){
this.people = _store.select('people');
/*
this is a naive way to handle projecting state, we will discover a better
Rx based solution in next lesson
*/
this.filter = _store.select('partyFilter');
this.attending = this.people.map(p => p.filter(person => person.attending));
this.guests = this.people
.map(p => p.map(person => person.guests)
.reduce((acc, curr) => acc + curr, 0));
}
//...rest of component
}Now that we have all the necessary data, we can pass it down to our "dumb" components to be presented accordingly. Our container component will handle any actions emitted back up the chain, dispatching the appropriate events.
@Component({
selector: 'person-list',
template: `
<ul>
<li
*ngFor="let person of people.filter(filter)"
[class.attending]="person.attending"
>
{{person.name}} - Guests: {{person.guests}}
<button (click)="addGuest.emit(person.id)">+</button>
<button *ngIf="person.guests" (click)="removeGuest.emit(person.id)">-</button>
Attending?
<input type="checkbox" [checked]="person.attending" (change)="toggleAttending.emit(person.id)" />
<button (click)="removePerson.emit(person.id)">Delete</button>
</li>
</ul>
`,
changeDetection: ChangeDetectionStrategy.OnPush
})
export class PersonList {
@Input() people;
//for now, we will pass filter down and apply
@Input() filter;
@Output() addGuest = new EventEmitter();
@Output() removeGuest = new EventEmitter();
@Output() removePerson = new EventEmitter();
@Output() toggleAttending = new EventEmitter();
}(Work Along | Completed Lesson)
While building out your @ngrx/store application, many of your components will require slices of state output from multiple reducers. One example of this would be a list that is adjusted by a filter, both managed through store. When the filter is changed, the list needs to be updated to reflect this change. So how do we facilitate this interaction? RxJS provides two operators that you will consistently utilize, combineLatest and withLatestFrom.
The combineLatest operator accepts an unspecified number of observables, emitting the last emitted value from each when any of the provided observables emit. These values are passed to a projection function for you to form the appropriate projection.
(demo)
//timerOne emits first value at 1s, then once every 4s
const timerOne = Rx.Observable.timer(1000, 4000);
//timerTwo emits first value at 2s, then once every 4s
const timerTwo = Rx.Observable.timer(2000, 4000)
//timerThree emits first value at 3s, then once every 4s
const timerThree = Rx.Observable.timer(3000, 4000)
//when one timer emits, emit the latest values from each timer as an array
const combined = Rx.Observable
.combineLatest(
timerOne,
timerTwo,
timerThree
);
const subscribe = combined.subscribe(latestValues => {
//grab latest emitted values for timers one, two, and three
const [timerValOne, timerValTwo, timerValThree] = latestValues;
/*
Example:
timerOne first tick: 'Timer One Latest: 1, Timer Two Latest:0, Timer Three Latest: 0
timerTwo first tick: 'Timer One Latest: 1, Timer Two Latest:1, Timer Three Latest: 0
timerThree first tick: 'Timer One Latest: 1, Timer Two Latest:1, Timer Three Latest: 1
*/
console.log(
`Timer One Latest: ${timerValOne},
Timer Two Latest: ${timerValTwo},
Timer Three Latest: ${timerValThree}`
);
});
//combineLatest also takes an optional projection function
const combinedProject = Rx.Observable
.combineLatest(
timerOne,
timerTwo,
timerThree,
(one, two, three) => {
return `Timer One (Proj) Latest: ${one},
Timer Two (Proj) Latest: ${two},
Timer Three (Proj) Latest: ${three}`
}
);
//log values
const subscribe = combinedProject.subscribe(latestValuesProject => console.log(latestValuesProject));The withLatestFrom operator is similar, except it only combines the last emitted values from the provided observables when the source observable emits. This is useful when your projection depends first on a single source, aided by multiple other sources.
(demo)
//Create an observable that emits a value every second
const myInterval = Rx.Observable.interval(1000);
//Create an observable that emits immediately, then every 5 seconds
const myTimer = Rx.Observable.timer(0, 5000);
//Every time interval emits, also get latest from timer and add the two values
const latest = myInterval
.withLatestFrom(myTimer)
.map(([interval, timer]) => {
console.log(`Latest Interval: ${interval}`);
console.log(`Latest Timer: ${timer}`);
return interval + timer;
});
//log total
const subscribe = latest.subscribe(val => console.log(`Total: ${val}`));Now that we have an understanding of these combination operators, we can clean up our previous example by using Observable.combineLatest. Instead of creating a new observable for each statistic, the state from people and partyFilter can be combined any time either updates, executing our projection function to calculate party statistics and properly filter the people list.
@Component({
selector: 'app',
template: `
<h3>@ngrx/store Party Planner</h3>
<party-stats
[invited]="(model | async)?.total"
[attending]="(model | async)?.attending"
[guests]="(model | async)?.guests"
>
{{guests | async | json}}
</party-stats>
<filter-select
(updateFilter)="updateFilter($event)"
>
</filter-select>
<person-input
(addPerson)="addPerson($event)"
>
</person-input>
<person-list
[people]="(model | async)?.people"
(addGuest)="addGuest($event)"
(removeGuest)="removeGuest($event)"
(removePerson)="removePerson($event)"
(toggleAttending)="toggleAttending($event)"
>
</person-list>
`,
directives: [PersonList, PersonInput, FilterSelect, PartyStats]
})
export class App {
public model;
constructor(
private _store: Store
){
/*
Every time people or partyFilter emits, pass the latest
value from each into supplied function. We can then calculate
and output statistics.
*/
this.model = Observable.combineLatest(
_store.select('people')
_store.select('partyFilter'),
(people, filter) => {
return {
total: people.length,
people: people.filter(filter),
attending: people.filter(person => person.attending).length,
guests: people.reduce((acc, curr) => acc + curr.guests, 0)
}
});
}
//...rest of component
}(Work Along | Completed Lesson)
Through the course of building your application you will often utilize similar queries, or projections of state in your views. A common way to eliminate the duplication of this logic is to place popular selections into services, injecting these services where needed in your components or other services. While this certainly works, there is another more flexible, composable way to tackle this issue.
Because nothing about these projections is Angular specific, we can export each small query, or 12 selector independantly, without the need for Angular service wrapping. Leveraging the let operator, these selectors can then be mixed and matched for the desired result, whether in components, services, or middleware. This toolbox of targeted, composable queries is called the selector pattern.
To accomplish this we will create a new file to house our application selectors. We can then extract the projection function being applied in combineLatest to filter people and produce statistics into a selector.
export const partyModel = () => {
return state => state
.map(([people, filter]) => {
return {
total: people.length,
people: people.filter(filter),
attending: people.filter(person => person.attending).length,
guests: people.reduce((acc, curr) => acc + curr.guests, 0)
}
});
};For futher demonstration, let's create two more selectors, one to return an obervable of party attendees and another, building on the previous selector, to calculate the percent of people attending based on those invited. This shows how easy it is to compose these small, focused selectors into powerful queries for use in views and middleware.
export const attendees = () => {
return state => state
.map(s => s.people)
.distinctUntilChanged();
};
export const percentAttending = () => {
return state => state
//build on previous selectors
.let(attendees())
.map(p => {
const totalAttending = p.filter(person => person.attending).length;
const total = p.length;
return total > 0 ? (totalAttending / total) * 100 : 0;
});
};Applying selectors is easy, simply apply the let operator to the appropriate Observable, supplying the selector(s) of your choosing.
export class App {
public model;
constructor(
private _store: Store
){
/*
Every time people or partyFilter emits, pass the latest
value from each into supplied function. We can then calculate
and output statistics.
*/
this.model = Observable.combineLatest(
_store.select('people'),
_store.select('partyFilter')
)
//extracting party model to selector
.let(partyModel());
//for demonstration on combining selectors
this.percentAttendance = _store.let(percentAttending());
}
//...rest of component
}interface Selector<T,V> {
(state: Observable<T>): Observable<V>
}(Work Along | Completed Lesson)
Note: Middleware has been remove in ngrx/store v2. Please see the meta-reducers section for how to create similar functionality.
Middleware provides an easy-to-utilize entry point for inserting custom functionality into the action pipeline. Store middleware comes in two flavors, preMiddleware and postMiddleware, implemented using the RxJS let operator. Pre-middleware is applied before dispatched actions hit your reducers, passed an Observable<Action>. Post-middleware is invoked before the new representation of state is nexted into store, passed an Observable<State>
The uses for middleware are many, from handling side-effect with sagas, to advanced logging of actions, to automatically syncing slices of state to local storage. In the Redux community, an entire ecosystem of custom middleware has been established. For our use, we'll be implementing simple logger middleware to keep track of dispatched actions and subsequent updates to state.
Because middleware is passed the entire observable, either of Action or State, we simply need to provide functions that receieve the source observable, returning an observable. This allows us to utilize the wide-array of operators RxJS exposes to obtain the desired functionality. One such operator, do, provides a transparent way to perform aribtrary actions as values flow through the dispatcher pipeline.
Let's implement basic logging middleware, making use of the do operator in both pre and post middleware to log dispatched actions and new representations of state.
//pre middleware takes an observable of actions, returning an observable
export const actionLogger = action => {
return action.do(a => console.log('DISPATCHED ACTION:', a));
}
//post middleware takes an observable of state, returning observable
export const stateLogger = state => {
return state.do(s => console.log('NEW STATE:', s));
}bootstrap(App, [
provideStore({people, partyFilter}),
usePreMiddleware(actionLogger),
usePostMiddleware(stateLogger)
]);(Work Along | Completed Lesson)
Note: Middleware has been remove in ngrx/store v2. Please see the meta-reducers section for how to create similar functionality.
When creating custom middleware you may find the need to utilize your Angular services. Store comes with a helper function, 13 createMiddleware, to make this process easy. The createMiddleware function takes a factory function, accepting any number of dependecies supplied as a second paramater. Behind the scenes, Store handles the Angular 2 provider setup, creating the proper tokens and injecting the declared Angular depencies into your middleware factory function. You can then utilize these dependencies as you wish to produce the desired result.
In this example we are going to create a LocalStorageService, wrapping the local storage API for use in the party planner application. This service will then be injected into our custom localStorageMiddleware, which accepts a state key to keep in sync with local storage. All that is left to do is apply this as postMiddleware on application bootstrap and any state updates to the predefined section will be reflected in local storage.
import {Injectable} from 'angular2/core';
//simple service wrapping local storage
@Injectable()
export class LocalStorageService {
setItem(key, value){
localStorage.setItem(key, JSON.stringify(value));
}
getItem(key){
return JSON.parse(localStorage.getItem(key));
}
}/*
create middleware with a dependency on the localStorageService
basic example, accept state key to sync with local storage
*/
export const localStorageMiddleware = key => createMiddleware(localStorageService => {
return state => {
//sync specified state slice with local storage
return state.do(state => localStorageService.setItem(key, state[key]));
}
}, [LocalStorageService]);13 createMiddleware(useFactory: (...deps: any[]) => Middleware, deps?: any[]): Provider
(Work Along | Completed Lesson)
Note: Middleware has been remove in ngrx/store v2. Please see the meta-reducers section for how to create similar functionality.
There are times when you will want to provide initial state to your reducers, outside of initial state supplied as a default function parameter. In these scenarios, store provides an INITIAL_STATE token which can be imported an overridden in order update state accordingly on application bootstrap. Behind the scenes, store will first check if the data has been initialized under the INITIAL_STATE token and if so, use this as the default initial state for the appropriate reducer.
To demonstrate this, we will expand upon the previous example and rehydrate the state collected in local store when the application is close or the page is refreshed. To handle this, we expose a function that accepts a key to rehydrate, returning a Angular provider for INITIAL_STATE, returning our data from local storage as the value. Now, when store asks for INITIAL_STATE, Angular will return our rehydrated data.
import {provide, Provider} from 'angular2/core';
import {INITIAL_STATE} from '@ngrx/store';
export const rehydrateState = key => {
//override initial state token for use in store
return provide(INITIAL_STATE, {
useValue: { [key]: JSON.parse(localStorage.getItem(key))};
});
};bootstrap(App, [
LocalStorageService,
provideStore({people, partyFilter}),
usePreMiddleware(actionLogger),
usePostMiddleware(stateLogger, localStorageMiddleware('people')),
rehydrateState('people')
]);(Work Along | Completed Lesson)
One of the many advantages to a single, immutable state tree is the ease of implementation for generally tricky features like undo/redo. Since the progression of application state is fully tracable through snapshots of store, the ability to walk back through these snap shots becomes trivial. A popular method for implementing this functionality is through meta-reducers.
Despite the ominous sound, meta-reducers are actually quite simple in theory and implementation. To create a meta-reducer, you wrap a current reducer in a parent reducer, delegating the majority of actions through the wrapped reducer as normal, stepping in only when defined meta actions (such as undo/redo) are dispatched. How does this look in practice? Let's see by creating a reset feature for our party planning application, allowing the user to start from scratch if they want to enter all new data.
To encapsulate this functionality we create a factory function, accepting any reducer to wrap, returning our reset reducer. When the reset reducer is initialize we grab the initial state of the wrapped reducer, saving it for later use. All that is left is to listen for a specific reset action to be dispatched. If RESET_STATE is not dispatched, the action is passed to the wrapped reducer and state returned as normal. When RESET_STATE is triggered the stored initial state is returned instead of the result of invoking the wrapped reducer. Undo/redo can be handled similarly, keeping track of previous actions in local state.
export const RESET_STATE = 'RESET_STATE';
const INIT = '__NOT_A_REAL_ACTION__';
export const reset = reducer => {
let initialState = reducer(undefined, {type: INIT})
return function (state, action) {
//if reset action is fired, return initial state
if(action.type === RESET_STATE){
return initialState;
}
//calculate next state based on action
let nextState = reducer(state, action);
//return nextState as normal when not reset action
return nextState;
}
} bootstrap(App, [
//wrap people in reset meta-reducer
provideStore({people: reset(people), partyFilter})
]);It is worth noting that the store root reducer is itself a meta-reducer, created behind the scenes when calling provideStore (by 14 combineReducers). For each dispatched action, the root reducer invokes each application reducer with the previous state and current action, returning an object map of [reducer]: state[reducer].
export function combineReducers(reducers: any): Reducer<any> {
const reducerKeys = Object.keys(reducers);
const finalReducers = {};
for (let i = 0; i < reducerKeys.length; i++) {
const key = reducerKeys[i];
if (typeof reducers[key] === 'function') {
finalReducers[key] = reducers[key];
}
}
const finalReducerKeys = Object.keys(finalReducers);
return function combination(state = {}, action) {
let hasChanged = false;
const nextState = {};
for (let i = 0; i < finalReducerKeys.length; i++) {
const key = finalReducerKeys[i];
const reducer = finalReducers[key];
const previousStateForKey = state[key];
const nextStateForKey = reducer(previousStateForKey, action);
nextState[key] = nextStateForKey;
hasChanged = hasChanged || nextStateForKey !== previousStateForKey;
}
return hasChanged ? nextState : state;
};
}This is a living document that will be expanded upon consistently as @ngrx/store develops over time. Keep checking back for additional examples, explanations, and best practices!
Excellent RxJS and Redux related Egghead.io videos and courses:
- Step-by-Step Async JavaScript with RxJS - John Lindquist
- RxJS Beyond The Basics - Creating Observables From Scratch - André Staltz
- Rx Lessons from Ben Lesh - Ben Lesh
- Introduction to Reactive Programming - André Staltz
- Getting Started With Redux - Dan Abramov
- Asynchronous Programming - The End of the Loop - Jafar Husain
- Using the Async Pipe in Angular 2 - Brian Troncone
For additional examples, explanations, and resources for RxJS check out my new site at http://learnrxjs.io/!