Alternative to Events which got removed in Ionic 5

events.ts

import {Injectable} from '@angular/core';
import {Subject, Subscription} from 'rxjs';

/**
 * A custom Events service just like Ionic 3 Events https://ionicframework.com/docs/v3/api/util/Events/ which got removed in Ionic 5.
 *
 * @author Shashank Agrawal
 */
@Injectable({
    providedIn: 'root'
})
export class Events {

    private channels: { [key: string]: Subject<any>; } = {};

    /**
     * Subscribe to a topic and provide a single handler/observer.
     * @param topic The name of the topic to subscribe to.
     * @param observer The observer or callback function to listen when changes are published.
     *
     * @returns Subscription from which you can unsubscribe to release memory resources and to prevent memory leak.
     */
    subscribe(topic: string, observer: (_: any) => void): Subscription {
        if (!this.channels[topic]) {
            // You can also use ReplaySubject with one concequence
            this.channels[topic] = new Subject<any>();
        }

        return this.channels[topic].subscribe(observer);
    }

    /**
     * Publish some data to the subscribers of the given topic.
     * @param topic The name of the topic to emit data to.
     * @param data data in any format to pass on.
     */
    publish(topic: string, data?: any): void {
        const subject = this.channels[topic];
        if (!subject) {
            // Or you can create a new subject for future subscribers
            return;
        }

        subject.next(data);
    }

    /**
     * When you are sure that you are done with the topic and the subscribers no longer needs to listen to a particular topic, you can
     * destroy the observable of the topic using this method.
     * @param topic The name of the topic to destroy.
     */
    destroy(topic: string): null {
        const subject = this.channels[topic];
        if (!subject) {
            return;
        }

        subject.complete();
        delete this.channels[topic];
    }
}

upgrading-examples.md

1. Change the imports

Before

import {Events} from 'ionic-angular';

After

import {Events} from '../your/path/to/service/events';

2. Changes in the subscribe method

Before

events.subscribe('user:created', (user, time) => {
    console.log('Welcome', user, 'at', time);
});

After

this.events.subscribe('user:created', (data: any) => {
    console.log('Welcome', data.user, 'at', data.time);
});

3. Changes in the publish method

Before

this.events.publish('user:created', someUserInstance, Date.now());

After

this.events.publish('foo:user:logged-out', {
    user: someUserInstance,
    time: new Date()
});

4. To Unsubscribe

const subscription = this.events.subscribe('user:foo:created', (data: any) => {
    // your logic
});

Once you are done, you can do this-

subscription.unsubscribe();

Thanks for the update. I actually see the issue. Your code must be publishing to the topic first and letter subscribing while the original publisher might have removed by that time.

If you look closely, I have already left two comments in the code to either use ReplySubject (for scenario 1) or Or you can create a new subject for future subscribers (for another scenario).

Now, again based on the actual clarity of your requirement, you either have to change the code accordingly. Let me know if you need to fix and use it.

Okay, so to confirm, I need to publish to the topic first before the subscription? I would think that the subscription needs to take place first in order to listen for any publish events?

That's the catch, you don't have to worry about what to do first if you address two comments in my code-

1. Instead of new Subject() use new ReplaySubject(1) (note- for future subscribers, this will always give you the last value).
2. If your code is publishing first before any subscriber, change the method of publish to-

    publish(topic: string, data?: any): void {
        if (!this.channels[topic]) {
            this.channels[topic] = new ReplaySubject<any>();

        }

        this.channels[topic].next(data);
    }

Let me know if this helps!

Thanks! I will give it a bash!

Hi, Does someone perhaps know where I can find documentation on Unit testing this?

@grantdevon it's a Gist, a code sample; just a snippet of code, not a repo with tests, etc.
If you want unit tests, you're going to have to write them. :-)

Thanks @johnwargo for clarifying it for me.