React Native Bridging Cheatsheet

0-bridging-react-native-cheatsheet.md

Bridging Native & React for iOS & Android

js -> native

• Simple Example
• Callbacks
• Promises
• Properties

native -> js

• Events
• Events (for UI components)

Additional Resources

• Argument Types (iOS)
• Argument Types (Android)

callbacks.md

Callbacks (getting a response from js)

React

// index.ios.js
videoPlayer.seekTo(100, (error, someData) => {
  if (error) {
    console.error(error)
  } else {
    console.log(someData)
  }
})

// index.android.js
videoPlayer.seekTo(
  100, 
  (msg) => {
    console.error(msg)
  }, 
  (someData) => {
    console.log(someData)
  }
)

iOS

docs

RCT_EXPORT_METHOD(seekTo:(double)time callback:(RCTResponseSenderBlock)callback)
{
  NSArray *someData;
  callback(@[[NSNull null], someData]); // (error, someData) in js
}

Android

docs

@ReactMethod
public void seekTo(double time, Callback errorCallback, Callback successCallback) {
  try {
    successCallback.invoke(someData);
  } catch (Exception e) {
    errorCallback.invoke(e.getMessage());
  }
}

Go to Top

events-ui.md

Events (for UI components)

Keep in mind:

• Events share namespace (so come up with an app-wide naming convention)

React

// VideoPlayer.js

class VideoPlayer extends React.PureComponent {
  _onEnd = (event) => {
    if (!this.props.onEnd) {
      return;
    }
    this.props.onEnd(event.nativeEvent)
  }
  render() {
    // Re-assign onEnd to the private _onEnd and store it in `nativeProps`
    const nativeProps = {
      ...this.props,
      onEnd: this._onEnd,
    }
    return (
      <RCTVideo
        {...nativeProps}
      />
    )
  }
}

const RCTVideo = requireNativeComponent('RCTVideo', VideoPlayer)

VideoPlayer.propTypes = {
  /**
   *  Callback that is called when the current player item ends.
   */
  onEnd: PropTypes.func,
}

iOS

docs

Notes: Bubbling events are like DOM events so that a parent component can capture an event fired by its child. Generally these are UI-related, like "the user touched this box". Direct events are not bubbled and are intended for more abstract events like "this image failed to load".

// VideoPlayer.h
#import <UIKit/UIKit.h>
#import <React/RCTView.h>

@interface VideoPlayer : UIView

// or RCTBubblingEventBlock
@property (nonatomic, copy) RCTDirectEventBlock onEnd;

@end

// VideoPlayerManager.m (inherits from RCTViewManager)
@implementation VideoPlayerManager

RCT_EXPORT_MODULE()

RCT_EXPORT_VIEW_PROPERTY(onEnd, RCTDirectEventBlock)

- (UIView *)view
{
  ...
}

/* 
 * `VideoPlayerManager` acts as the delegate of all of the `VideoPlayer` views. This is just one
 * pattern and it's perfectly fine to call `onEnd` from the `VideoPlayer` directly.
 */
- (void)onEnd:(VideoPlayer *)videoPlayer
{
  if (!videoPlayer.onEnd) {
    return;
  }
  videoPlayer.onEnd(@{ @"some-data" : @1 });
}

@end

Android

docs

example

Define a custom event mapping by overriding getExportedCustomDirectEventTypeConstants in the manager class:

// VideoPlayerManager.java
@Override
public @Nullable Map getExportedCustomDirectEventTypeConstants() {
    return MapBuilder.of(
            "onEnd",
            MapBuilder.of("registrationName", "onEnd")
    );
}

Dispatch the event:

// VideoPlayerView.java
private void dispatchOnEnd() {
  WritableMap event = Arguments.createMap();
  ...
  reactContext.getJSModule(RCTEventEmitter.class).receiveEvent(
    getId(),
    "onEnd",
    event
  );
}

Go to Top

events.md

Events

Keep in mind:

• Events share namespace (so come up with an app-wide naming convention)

React

import { NativeEventEmitter, NativeModules } from 'react-native'
const videoPlayer = NativeModules.VideoPlayer
const videoPlayerEmitter = new NativeEventEmitter(VideoPlayer)
const subscription = videoPlayerEmitter.addListener('video-progress', (data) => console.log(data.progress))

// Don't forget to unsubscribe, typically in `componentWillUnmount`
subscription.remove()

iOS

docs

Important Note: Don't send events if there are no listeners attached. You will get a warning about spending unneccessary resources. Override -startObserving and -stopObserving like in the example below:

// VideoPlayer.h
#import <React/RCTBridgeModule.h>
#import <React/RCTEventEmitter.h>

@interface CalendarManager : RCTEventEmitter <RCTBridgeModule>

@end

// VideoPlayer.m
#import "VideoPlayer.h"

@implementation VideoPlayer
{
  BOOL _hasListeners;
}

RCT_EXPORT_MODULE();

- (NSArray<NSString *> *)supportedEvents
{
  return @[@"video-progress"];
}

// Will be called when this module's first listener is added.
- (void)startObserving
{
  _hasListeners = YES;
}

// Will be called when this module's last listener is removed, or on dealloc.
- (void)stopObserving
{
  _hasListeners = NO;
}

- (void)onVideoProgress
{
  CGFloat progress = ...;
  if (_hasListeners) {
    [self sendEventWithName:@"video-progress" body:@{ @"progress": @(progress) }];
  }
}

@end

Android

docs

example

private void onVideoProgress() {
  WriteableMap params = Arguments.createMap();
  ...
  reactContext.getJSModule(DeviceEventManagerModule.RCTDeviceEventEmitter.class).emit("video-progress", params);
}

Go to Top

promises.md

Promises

React

async function loadVideo() {
  try {
    var videoLoaded = await VideoPlayer.loadVideo();
    this.setState(videoLoaded: videoLoaded)
  } catch (e) {
    console.error(e)
  }
}

loadVideo()

iOS

docs

RCT_REMAP_METHOD(loadVideo, loadVideoWithResolver:(RCTPromiseResolveBlock)resolve rejecter:(RCTPromiseRejectBlock)reject)
{
  BOOL videoLoaded = ...; // could be any data type listed under https://facebook.github.io/react-native/docs/native-modules-ios.html#argument-types
  if (videoLoaded) {
    resolve(videoLoaded);
  } else {
    NSError *error = ...
    reject(@"error", @"error description", error);
  }
}

Android

docs

@ReactMethod loadVideo(Promise promise) {
  try {
    Boolean videoLoaded = ...; // could be any data type listed under https://facebook.github.io/react-native/docs/native-modules-android.html#argument-types
    if (videoLoaded) {
      promise.resolve(videoLoaded);
    }
  } catch (Exception e) {
    promise.reject(ERROR, e);
  }
}

Go to Top

properties.md

Properties (for UI)

React

render() {
  <VideoPlayer loop={false} />
}

VideoPlayer.propTypes = {
  loop: PropTypes.bool,
}

VideoPlayer.defaultProps = {
  loop: false,
}

iOS

docs

// BMEVideoManager.m
RCT_EXPORT_VIEW_PROPERTY(loop, BOOL);

Android

docs

@ReactProp(name = "loop")
public void setLoop(Boolean loop) {
  ...
}

Go to Top

simple-example.md

Simple one-off example

React

import { NativeModules } from 'react-native'
const videoPlayer = NativeModules.VideoPlayer
videoPlayer.seekTo(100)

iOS

Keep in mind:

• RCT_EXPORT_METHOD exposes the name of the method up to the first colon in js land.
• When many methods share the same name, use RCT_REMAP_METHOD to define a different name for js and map it to the native method. ie. RCT_REMAP_METHOD(differentMethodName, methodName:(BOOL)arg1 arg2:(BOOL)arg2).

docs

// VideoPlayer.h
#import <React/RCTBridgeModule.h>

@interface VideoPlayer : NSObject <RCTBridgeModule>
@end

@implementation VideoPlayer
RCT_EXPORT_MODULE(); // or RCT_EXPLORT_MODULE(AwesomeVideoPlayer) -- custom name

RCT_EXPORT_METHOD(seekTo:(double)time)
{
  // seek to time
}
@end

Android

docs

// VideoPlayer.java
package com.beme.react

import com.facebook.react.bridge.NativeModule;
import com.facebook.react.bridge.ReactApplicationContext;
import com.facebook.react.bridge.ReactContext;
import com.facebook.react.bridge.ReactContextBaseJavaModule;
import com.facebook.react.bridge.ReactMethod;

public class VideoModule extends ReactContextBaseJavaModule {
  public VideoModule(ReactApplicationContext reactContext) {
    super(reactContext)
  }

  @Override
  public String getName() {
    return "VideoPlayer";
  }

  @ReactMethod
  public void seekTo(double time) {
    // seek to time
  }
}

Register the module

// VideoPackage.java
package com.beme.react

import com.facebook.react.ReactPackage;
import com.facebook.react.bridge.JavaScriptModule;
import com.facebook.react.bridge.NativeModule;
import com.facebook.react.bridge.ReactApplicationContext;
import com.facebook.react.uimanager.ViewManager;

public class VideoPackage implements ReactPackage {
  @Override
  public List<Class<? extends JavaScriptModule>> createJSModules() {
    return Collections.emptyList();
  }

  @Override
  public List<ViewManager> createViewManagers(ReactApplicationContext reactContext) {
    return Collections.emptyList();
  }

  @Override
  public List<NativeModule> createNativeModules(
                              ReactApplicationContext reactContext) {
    List<NativeModule> modules = new ArrayList<>();
    modules.add(new VideoModule(reactContext));
    return modules;
  }
}

// MainApplication.java
protected List<ReactPackage> getPackages() {
    return Arrays.<ReactPackage>asList(
            new MainReactPackage(),
            new VideoPackage()); // <-- Add this line with your package name.
}

Go to Top

Thank you for this!!! I'm still trying to understand why we need to use RCT_REMAP_METHOD for promises - can someone explain that in more detail for me?