react-native/ReactAndroid/src/main/java/com/facebook/react/uimanager/TouchTargetHelper.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 javax.annotation.Nullable;

import android.graphics.Matrix;
import android.graphics.PointF;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;

import com.facebook.react.bridge.JSApplicationIllegalArgumentException;
import com.facebook.react.bridge.UiThreadUtil;

/**
 * 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 {

  private static final float[] mEventCoords = new float[2];
  private static final PointF mTempPoint = new PointF();
  private static final float[] mMatrixTransformCoords = new float[2];
  private static final Matrix mInverseMatrix = new Matrix();

  /**
   * 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
   * @return the react tag ID of the child view that should handle the event
   */
  public static int findTargetTagForTouch(
      float eventX,
      float eventY,
      ViewGroup viewGroup) {
    return findTargetTagAndCoordinatesForTouch(eventX, eventY, viewGroup, mEventCoords);
  }

  /**
   * 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;
  }

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

  /**
   * 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 whether the touch point is within the child View
   * It is transform aware and will invert the transform Matrix to find the true local points
   * This code is taken from {@link ViewGroup#isTransformedTouchPointInView()}
   */
  private static boolean isTransformedTouchPointInView(
      float x,
      float y,
      ViewGroup parent,
      View child,
      PointF outLocalPoint) {
    float localX = x + parent.getScrollX() - child.getLeft();
    float localY = y + parent.getScrollY() - child.getTop();
    Matrix matrix = child.getMatrix();
    if (!matrix.isIdentity()) {
      float[] localXY = mMatrixTransformCoords;
      localXY[0] = localX;
      localXY[1] = localY;
      Matrix inverseMatrix = mInverseMatrix;
      matrix.invert(inverseMatrix);
      inverseMatrix.mapPoints(localXY);
      localX = localXY[0];
      localY = localXY[1];
    }
    if ((localX >= 0 && localX < (child.getRight() - child.getLeft()))
        && (localY >= 0 && localY < (child.getBottom() - child.getTop()))) {
      outLocalPoint.set(localX, localY);
      return true;
    }
    return false;
  }


  /**
   * 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) {
      // This view and its children can't be the target
      return null;

    } else if (pointerEvents == PointerEvents.BOX_ONLY) {
      // This view is the target, its children don't matter
      return view;

    } else if (pointerEvents == PointerEvents.BOX_NONE) {
      // This view can't be the target, but its children might
      if (view instanceof ViewGroup) {
        View targetView = findTouchTargetView(eventCoords, (ViewGroup) view);
        return targetView != view ? targetView : null;
      }
      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 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();
  }

}