JSTouchDispatcher.java

/**
 * Copyright (c) 2015-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 */

package com.facebook.react.uimanager;

import android.view.MotionEvent;
import android.view.ViewGroup;

import com.facebook.common.logging.FLog;
import com.facebook.infer.annotation.Assertions;
import com.facebook.react.common.ReactConstants;
import com.facebook.react.uimanager.events.EventDispatcher;
import com.facebook.react.uimanager.events.TouchEvent;
import com.facebook.react.uimanager.events.TouchEventCoalescingKeyHelper;
import com.facebook.react.uimanager.events.TouchEventType;

/**
 * JSTouchDispatcher handles dispatching touches to JS from RootViews.  If you implement RootView
 * you need to call handleTouchEvent from onTouchEvent and onInterceptTouchEvent.  It will correctly
 * find the right view to handle the touch and also dispatch the appropriate event to JS
 */
public class JSTouchDispatcher {

  private int mTargetTag = -1;
  private final float[] mTargetCoordinates = new float[2];
  private boolean mChildIsHandlingNativeGesture = false;
  private final ViewGroup mRootViewGroup;
  private final TouchEventCoalescingKeyHelper mTouchEventCoalescingKeyHelper =
    new TouchEventCoalescingKeyHelper();

  public JSTouchDispatcher(ViewGroup viewGroup) {
    mRootViewGroup = viewGroup;
  }

  public void onChildStartedNativeGesture(MotionEvent androidEvent, EventDispatcher eventDispatcher) {
    if (mChildIsHandlingNativeGesture) {
      // This means we previously had another child start handling this native gesture and now a
      // different native parent of that child has decided to intercept the touch stream and handle
      // the gesture itself. Example where this can happen: HorizontalScrollView in a ScrollView.
      return;
    }

    dispatchCancelEvent(androidEvent, eventDispatcher);
    mChildIsHandlingNativeGesture = true;
    mTargetTag = -1;
  }

  /**
   * 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.TouchTargetHelper#findTouchTargetView}
   * helper method for figuring out a react view ID in the case of ACTION_DOWN
   * event (when the gesture starts).
   */
  public void handleTouchEvent(MotionEvent ev, EventDispatcher eventDispatcher) {
    int action = ev.getAction() & MotionEvent.ACTION_MASK;
    if (action == MotionEvent.ACTION_DOWN) {
      if (mTargetTag != -1) {
        FLog.e(
          ReactConstants.TAG,
          "Got DOWN touch before receiving UP or CANCEL from last gesture");
      }

      // First event for this gesture. We expect tag to be set to -1, and we use helper method
      // {@link #findTargetTagForTouch} to find react view ID that will be responsible for handling
      // this gesture
      mChildIsHandlingNativeGesture = false;
      mTargetTag = TouchTargetHelper.findTargetTagAndCoordinatesForTouch(
        ev.getX(),
        ev.getY(),
        mRootViewGroup,
        mTargetCoordinates,
        null);
      eventDispatcher.dispatchEvent(
        TouchEvent.obtain(
          mTargetTag,
          TouchEventType.START,
          ev,
          mTargetCoordinates[0],
          mTargetCoordinates[1],
          mTouchEventCoalescingKeyHelper));
    } else if (mChildIsHandlingNativeGesture) {
      // If the touch was intercepted by a child, we've already sent a cancel event to JS for this
      // gesture, so we shouldn't send any more touches related to it.
      return;
    } else if (mTargetTag == -1) {
      // All the subsequent action types are expected to be called after ACTION_DOWN thus target
      // is supposed to be set for them.
      FLog.e(
        ReactConstants.TAG,
        "Unexpected state: received touch event but didn't get starting ACTION_DOWN for this " +
          "gesture before");
    } 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,
          TouchEventType.END,
          ev,
          mTargetCoordinates[0],
          mTargetCoordinates[1],
          mTouchEventCoalescingKeyHelper));
      mTargetTag = -1;
    } else if (action == MotionEvent.ACTION_MOVE) {
      // Update pointer position for current gesture
      eventDispatcher.dispatchEvent(
        TouchEvent.obtain(
          mTargetTag,
          TouchEventType.MOVE,
          ev,
          mTargetCoordinates[0],
          mTargetCoordinates[1],
          mTouchEventCoalescingKeyHelper));
    } 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,
          TouchEventType.START,
          ev,
          mTargetCoordinates[0],
          mTargetCoordinates[1],
          mTouchEventCoalescingKeyHelper));
    } else if (action == MotionEvent.ACTION_POINTER_UP) {
      // Exactly onw of the pointers goes up
      eventDispatcher.dispatchEvent(
        TouchEvent.obtain(
          mTargetTag,
          TouchEventType.END,
          ev,
          mTargetCoordinates[0],
          mTargetCoordinates[1],
          mTouchEventCoalescingKeyHelper));
    } else if (action == MotionEvent.ACTION_CANCEL) {
      if (mTouchEventCoalescingKeyHelper.hasCoalescingKey(ev.getDownTime())) {
        dispatchCancelEvent(ev, eventDispatcher);
      } else {
        FLog.e(
          ReactConstants.TAG,
          "Received an ACTION_CANCEL touch event for which we have no corresponding ACTION_DOWN"
        );
      }
      mTargetTag = -1;
    } else {
      FLog.w(
        ReactConstants.TAG,
        "Warning : touch event was ignored. Action=" + action + " Target=" + mTargetTag);
    }
  }

  private void dispatchCancelEvent(MotionEvent androidEvent, EventDispatcher eventDispatcher) {
    // This means the gesture has already ended, via some other CANCEL or UP event. This is not
    // expected to happen very often as it would mean some child View has decided to intercept the
    // touch stream and start a native gesture only upon receiving the UP/CANCEL event.
    if (mTargetTag == -1) {
      FLog.w(
        ReactConstants.TAG,
        "Can't cancel already finished gesture. Is a child View trying to start a gesture from " +
          "an UP/CANCEL event?");
      return;
    }

    Assertions.assertCondition(
      !mChildIsHandlingNativeGesture,
      "Expected to not have already sent a cancel for this gesture");
    Assertions.assertNotNull(eventDispatcher).dispatchEvent(
      TouchEvent.obtain(
        mTargetTag,
        TouchEventType.CANCEL,
        androidEvent,
        mTargetCoordinates[0],
        mTargetCoordinates[1],
        mTouchEventCoalescingKeyHelper));
  }
}