React-Native系列Android——Touch事件原理及狀態效果

Native原生相比於Hybrid或H5最大優點是具有流暢和復雜的交互效果，觸摸事件便是其中重要一項，包括點擊（Click）、長按（LongClick）、手勢（gesture）等。

以最簡單常見的點擊（Click）為例，Native組件可以自定義selector，使得被點擊的組件具有動態效果，Android 5.0以上甚至可以有漣漪效果（Material Design）。而這些在Hybrid或H5中很難實現，很多時候區分它們與原生最簡單的方法就是檢驗點擊交互效果。

React-Native的強大之處在於實現了較為全面的Touch事件機制，雖然仍略有缺陷，但相比於Hybrid或H5的體驗而言，已經足足提高了一大截，下面分析講解一下其實現原理，和具體使用方式。

1、Touch事件機制

如果閱讀過React-Native源碼的話，應該了解React-Native頁面的UI根視圖是ReactRootView，包路徑是：com.facebook.react.ReactRootView，它是FramLayout的一個子類。

首先，來看一下ReactActivity這個頁面基類，ReactRootView是如何作為React-Native的根視圖被初始化及添加的。

public abstract class ReactActivity extends Activity implements DefaultHardwareBackBtnHandler {

...

  protected ReactRootView createRootView() {
    return new ReactRootView(this);
  }

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    if (getUseDeveloperSupport() && Build.VERSION.SDK_INT >= 23) {
      // Get permission to show redbox in dev builds.
      if (!Settings.canDrawOverlays(this)) {
        Intent serviceIntent = new Intent(Settings.ACTION_MANAGE_OVERLAY_PERMISSION);
        startActivity(serviceIntent);
        FLog.w(ReactConstants.TAG, REDBOX_PERMISSION_MESSAGE);
        Toast.makeText(this, REDBOX_PERMISSION_MESSAGE, Toast.LENGTH_LONG).show();
      }
    }

    mReactInstanceManager = createReactInstanceManager();
    ReactRootView mReactRootView = createRootView();
    mReactRootView.startReactApplication(mReactInstanceManager, getMainComponentName(), getLaunchOptions());
    setContentView(mReactRootView);
  }

...

}

在ReactActivity的onCreate這個生命周期裡，直接實列化，然後作為當前Window的ContentView，也就可以認為其是所有React-Native組件的根視圖。

熟悉Android觸摸事件機制的，應該知道視圖樹中，觸摸事件是逐級傳遞的，每個視圖（View）中有兩個接收和處理Touch事件的方法，分別是onInterceptTouchEvent和onTouchEvent，這兩個方法的區別為：

onInterceptTouchEvent的傳遞層級是由父視圖向子視圖，顧名思義，通常用作事件攔截。
onTouchEvent的傳遞層級是由子視圖向父視圖，通常用作事件處理。

我們來看一下ReactRootView的事件接收和處理。

public class ReactRootView extends SizeMonitoringFrameLayout implements RootView {
   ...

     @Override
  public boolean onInterceptTouchEvent(MotionEvent ev) {
    handleTouchEvent(ev);
    return super.onInterceptTouchEvent(ev);
  }

  @Override
  public boolean onTouchEvent(MotionEvent ev) {
    handleTouchEvent(ev);
    super.onTouchEvent(ev);
    // In case when there is no children interested in handling touch event, we return true from
    // the root view in order to receive subsequent events related to that gesture
    return true;
  }

   ...
}

很明顯，這裡onInterceptTouchEvent和onTouchEvent的處理都是全部交給handleTouchEvent方法統一處理的。

我們再繼續看一下handleTouchEvent方法。

public class ReactRootView extends SizeMonitoringFrameLayout implements RootView {
   ...

    /**
   * Main catalyst view is responsible for collecting and sending touch events to JS. This method
   * reacts for an incoming android native touch events ({@link MotionEvent}) and calls into
   * {@link com.facebook.react.uimanager.events.EventDispatcher} when appropriate.
   * It uses {@link com.facebook.react.uimanager.TouchTargetManagerHelper#findTouchTargetView}
   * helper method for figuring out a react view ID in the case of ACTION_DOWN
   * event (when the gesture starts).
   */
  private void handleTouchEvent(MotionEvent ev) {

   ...

   int action = ev.getAction() & MotionEvent.ACTION_MASK;
    ReactContext reactContext = mReactInstanceManager.getCurrentReactContext();
    EventDispatcher eventDispatcher = reactContext.getNativeModule(UIManagerModule.class)
        .getEventDispatcher();
    if (action == MotionEvent.ACTION_DOWN) {

    ...

    mTargetTag = TouchTargetHelper.findTargetTagAndCoordinatesForTouch(
          ev.getX(),
          ev.getY(),
          this,
          mTargetCoordinates);    

    eventDispatcher.dispatchEvent(
          TouchEvent.obtain(
              mTargetTag,
              SystemClock.uptimeMillis(),
              TouchEventType.START,
              ev,
              mTargetCoordinates[0],
              mTargetCoordinates[1]));
    } else if (action == MotionEvent.ACTION_UP) {
      // End of the gesture. We reset target tag to -1 and expect no further event associated with
      // this gesture.
      eventDispatcher.dispatchEvent(
          TouchEvent.obtain(
              mTargetTag,
              SystemClock.uptimeMillis(),
              TouchEventType.END,
              ev,
              mTargetCoordinates[0],
              mTargetCoordinates[1]));
      mTargetTag = -1;
    } else if (action == MotionEvent.ACTION_MOVE) {
      // Update pointer position for current gesture
      eventDispatcher.dispatchEvent(
          TouchEvent.obtain(
              mTargetTag,
              SystemClock.uptimeMillis(),
              TouchEventType.MOVE,
              ev,
              mTargetCoordinates[0],
              mTargetCoordinates[1]));
    } else if (action == MotionEvent.ACTION_POINTER_DOWN) {
      // New pointer goes down, this can only happen after ACTION_DOWN is sent for the first pointer
      eventDispatcher.dispatchEvent(
          TouchEvent.obtain(
              mTargetTag,
              SystemClock.uptimeMillis(),
              TouchEventType.START,
              ev,
              mTargetCoordinates[0],
              mTargetCoordinates[1]));
    } else if (action == MotionEvent.ACTION_POINTER_UP) {
      // Exactly onw of the pointers goes up
      eventDispatcher.dispatchEvent(
          TouchEvent.obtain(
              mTargetTag,
              SystemClock.uptimeMillis(),
              TouchEventType.END,
              ev,
              mTargetCoordinates[0],
              mTargetCoordinates[1]));
    } else if (action == MotionEvent.ACTION_CANCEL) {
      dispatchCancelEvent(ev);
      mTargetTag = -1;
    }
  }
   ...
}

代碼不是很多，也很好理解。先來看一下注釋，意思是ReactRootView 負責收集和發送事件給JS，當原生觸摸事件響應時通過EventDispatcher類發送，並且在Down事件時通過TouchTargetManagerHelper查找具體被觸摸的子View。這裡一語道破了觸摸事件的核心原理：

所有React組件的觸摸事件都是由ReactRootView統一處理，將具體被觸摸組件和具體觸摸事件發送給Javascript。其中隱藏的一層意思是：React組件自身不用處理觸摸事件。

這個很關鍵，而具體被處理的觸摸事件有以下6種，分別是ACTION_DOWN、ACTION_UP、ACTION_MOVE、ACTION_POINTER_DOWN、ACTION_POINTER_UP、ACTION_CANCEL，已經包含了幾乎所有的手勢動作。

2、Touch事件接收者

接下來，看一下ACTION_DOWN事件時，是如何定位消費Touch事件的React組件的呢？以下圖為例：

如果黃色的點表示被觸摸的事件點，由於Touch事件是由ReactRootView根節點開始攔截，所以從ReactRootView開始遍歷視圖樹，遍歷順序如下：

1、ReactViewGroup<喎?/kf/ware/vc/" target="_blank" class="keylink">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"brush:java;"> /** * Class responsible for identifying which react view should handle a given {@link MotionEvent}. * It uses the event coordinates to traverse the view hierarchy and return a suitable view. */ public class TouchTargetHelper { ... /** * Find touch event target view within the provided container given the coordinates provided * via {@link MotionEvent}. * * @param eventX the X screen coordinate of the touch location * @param eventY the Y screen coordinate of the touch location * @param viewGroup the container view to traverse * @param viewCoords an out parameter that will return the X,Y value in the target view * @return the react tag ID of the child view that should handle the event */ public static int findTargetTagAndCoordinatesForTouch( float eventX, float eventY, ViewGroup viewGroup, float[] viewCoords) { UiThreadUtil.assertOnUiThread(); int targetTag = viewGroup.getId(); // Store eventCoords in array so that they are modified to be relative to the targetView found. viewCoords[0] = eventX; viewCoords[1] = eventY; View nativeTargetView = findTouchTargetView(viewCoords, viewGroup); if (nativeTargetView != null) { View reactTargetView = findClosestReactAncestor(nativeTargetView); if (reactTargetView != null) { targetTag = getTouchTargetForView(reactTargetView, viewCoords[0], viewCoords[1]); } } return targetTag; } ... }

具體有三層邏輯：findTouchTargetView、findClosestReactAncestor、getTouchTargetForView，最終是要返回目標View的ID，代碼我們一一來看。

/**
   * Returns the touch target View that is either viewGroup or one if its descendants.
   * This is a recursive DFS since view the entire tree must be parsed until the target is found.
   * If the search does not backtrack, it is possible to follow a branch that cannot be a target
   * (because of pointerEvents). For example, if both C and E can be the target of an event:
   * A (pointerEvents: auto) - B (pointerEvents: box-none) - C (pointerEvents: none)
   *  \ D (pointerEvents: auto)  - E (pointerEvents: auto)
   * If the search goes down the first branch, it would return A as the target, which is incorrect.
   * NB: This modifies the eventCoords to always be relative to the current viewGroup. When the
   * method returns, it will contain the eventCoords relative to the targetView found.
   */
  private static View findTouchTargetView(float[] eventCoords, ViewGroup viewGroup) {
    int childrenCount = viewGroup.getChildCount();
    for (int i = childrenCount - 1; i >= 0; i--) {
      View child = viewGroup.getChildAt(i);
      PointF childPoint = mTempPoint;
      if (isTransformedTouchPointInView(eventCoords[0], eventCoords[1], viewGroup, child, childPoint)) {
        // If it is contained within the child View, the childPoint value will contain the view
        // coordinates relative to the child
        // We need to store the existing X,Y for the viewGroup away as it is possible this child
        // will not actually be the target and so we restore them if not
        float restoreX = eventCoords[0];
        float restoreY = eventCoords[1];
        eventCoords[0] = childPoint.x;
        eventCoords[1] = childPoint.y;
        View targetView = findTouchTargetViewWithPointerEvents(eventCoords, child);
        if (targetView != null) {
          return targetView;
        }
        eventCoords[0] = restoreX;
        eventCoords[1] = restoreY;
      }
    }
    return viewGroup;
}

/**
   * Returns the touch target View of the event given, or null if neither the given View nor any of
   * its descendants are the touch target.
   */
  private static @Nullable View findTouchTargetViewWithPointerEvents(
      float eventCoords[], View view) {
    PointerEvents pointerEvents = view instanceof ReactPointerEventsView ?
        ((ReactPointerEventsView) view).getPointerEvents() : PointerEvents.AUTO;
    if (pointerEvents == PointerEvents.NONE) {
        return null;
    } else if (pointerEvents == PointerEvents.BOX_ONLY) {
        return view;
    } else if (pointerEvents == PointerEvents.BOX_NONE) {
      if (view instanceof ViewGroup) {
        View targetView = findTouchTargetView(eventCoords, (ViewGroup) view);
        if (targetView != view) {
          return targetView;
        }
        ...
      }
      return null;

    } else if (pointerEvents == PointerEvents.AUTO) {
      // Either this view or one of its children is the target
      if (view instanceof ViewGroup) {
        return findTouchTargetView(eventCoords, (ViewGroup) view);
      }
      return view;

    } else {
      throw new JSApplicationIllegalArgumentException(
          "Unknown pointer event type: " + pointerEvents.toString());
    }
  }

  private static View findClosestReactAncestor(View view) {
    while (view != null && view.getId() <= 0) {
      view = (View) view.getParent();
    }
    return view;
  }

  private static int getTouchTargetForView(View targetView, float eventX, float eventY) {
    if (targetView instanceof ReactCompoundView) {
      // Use coordinates relative to the view, which have been already computed by
      // {@link #findTouchTargetView()}.
      return ((ReactCompoundView) targetView).reactTagForTouch(eventX, eventY);
    }
    return targetView.getId();
  }

public class EventDispatcher implements LifecycleEventListener {

     ...

     private final ReactApplicationContext mReactContext;
     private @Nullable RCTEventEmitter mRCTEventEmitter;
     private volatile @Nullable ScheduleDispatchFrameCallback mCurrentFrameCallback;

     public EventDispatcher(ReactApplicationContext  reactContext) {
      mReactContext = reactContext;
      mReactContext.addLifecycleEventListener(this);
    }

  @Override
  public void onHostResume() {
    UiThreadUtil.assertOnUiThread();
    Assertions.assumeCondition(mCurrentFrameCallback == null);

    if (mRCTEventEmitter == null) {
      mRCTEventEmitter = mReactContext.getJSModule(RCTEventEmitter.class);
    }

    mCurrentFrameCallback = new ScheduleDispatchFrameCallback();
    ReactChoreographer.getInstance().postFrameCallback(ReactChoreographer.CallbackType.TIMERS_EVENTS, mCurrentFrameCallback);
  }

     ...

}

private class ScheduleDispatchFrameCallback implements Choreographer.FrameCallback {

    private boolean mShouldStop = false;

    @Override
    public void doFrame(long frameTimeNanos) {
      UiThreadUtil.assertOnUiThread();

      if (mShouldStop) {
        return;
      }

     ...

      try {
        moveStagedEventsToDispatchQueue();

        if (!mHasDispatchScheduled) {
          mHasDispatchScheduled = true;
          ...
          mReactContext.runOnJSQueueThread(mDispatchEventsRunnable);
        }

        ReactChoreographer.getInstance()
            .postFrameCallback(ReactChoreographer.CallbackType.TIMERS_EVENTS, this);
      } finally {
        Systrace.endSection(Systrace.TRACE_TAG_REACT_JAVA_BRIDGE);
      }
    }

    public void stop() {
      mShouldStop = true;
    }
  }

 private class DispatchEventsRunnable implements Runnable {

    @Override
    public void run() {
      ...
      try {
        ...
        mHasDispatchScheduled = false;
        mHasDispatchScheduledCount++;
        ...
        synchronized (mEventsToDispatchLock) {
          ...
          for (int eventIdx = 0; eventIdx < mEventsToDispatchSize; eventIdx++) {
            Event event = mEventsToDispatch[eventIdx];
            // Event can be null if it has been coalesced into another event.
            if (event == null) {
              continue;
            }
            ...
            event.dispatch(mRCTEventEmitter);
            event.dispose();
          }
          clearEventsToDispatch();
          mEventCookieToLastEventIdx.clear();
        }
      } finally {
        Systrace.endSection(Systrace.TRACE_TAG_REACT_JAVA_BRIDGE);
      }
    }
  }

function onClick(){
   var ToastAndroid = require('ToastAndroid')
   ToastAndroid.show('Awesome, Clicking!', ToastAndroid.SHORT);
}

class MyProject extends Component {
  render() {
    return (
      
        Click Me!
      
    );
  }
}

function onClick(){
   var ToastAndroid = require('ToastAndroid')
   ToastAndroid.show('Awesome, Clicking!', ToastAndroid.SHORT);
}

class MyProject extends Component {
  render() {
    return (
      
        
            Click Me!
        
      
    );
  }
}

function onClick(){
   var ToastAndroid = require('ToastAndroid')
   ToastAndroid.show('Awesome, Clicking!', ToastAndroid.SHORT);
}

class MyProject extends Component {
  render() {
    return (
      
        
            Click Me!
        
      
    );
  }
}

function onClick(){
   var ToastAndroid = require('ToastAndroid')
   ToastAndroid.show('Awesome, Clicking!', ToastAndroid.SHORT);
}

class MyProject extends Component {
  render() {
    return (
      
        
            
                Click Me!
            
        
      
    );
  }
}

function onClick(){
   var ToastAndroid = require('ToastAndroid')
   ToastAndroid.show('Awesome, Clicking!', ToastAndroid.SHORT);
}

class MyProject extends Component {
  render() {
    return (
      
        
            Click Me!
        
      
    );
  }
}