Android 深入探究自定义view之事件的分发机制与处理详解

时间：2022-11-17 10:15:14|栏目：Android代码|点击：次

题引

事件只有一个，多个人想要处理，处理的对象不是我们想给的对象就是事件冲突。

在这里插入图片描述

如上图，RecyclerView 的父布局是ViewPager，左右滑动时没问题，上下滑动时RecyclerView好像没收到滑动事件一样，无法达到我们预期的效果。我们的触摸被封装成MotionEvent事件传递，在多个层级中它是如何传递的呢？又是根据什么来确定哪个view处理这个事件的呢，咱们抽丝剥茧一步步揭开她的面纱！

Activity对事件的分发过程

追溯本源，寻找事件分发的开始。

当一个点击操作发生时，事件最先传递给当前的Activity，由Activity的dispatchTouchEvent进行分发

    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }
        return onTouchEvent(ev);
    }

这里的getWindow返回的window类只有一个实现，PhoneWindow

	private DecorView mDecor

    public boolean superDispatchTouchEvent(MotionEvent event) {
        return mDecor.superDispatchTouchEvent(event);
    }

我们继续看 DecorView的superDispatchTouchEvent方法实现

    public boolean superDispatchTouchEvent(MotionEvent event) {
        return super.dispatchTouchEvent(event);
    }

DecorView 继承于 ViewGroup。此时应该理解了，Activity 的事件分发交给了 DecorView 处理，而 DecorView 又是什么

在这里插入图片描述

DecorView是activity窗口的根视图，是一个FrameLayout，DecorView内部又分为两部分，一部分是ActionBar，另一部分是ContentParent，即activity在setContentView对应的布局。如此一来，事件分发从系统层面开始向我们写的布局分发事件！

事件分发是一个递归的过程，主要涉及三个函数

dispatchTouchEvent
onInterceptTouchEvent
onTouchEvent

三者关系

public boolean dispatchTouchEvent(MotionEvent ev){
	boolean result = false;
	if(onInterceptTouchEvent(ev)){	//	如果拦截则交给自己的 onTouchEvent 处理事件
		result = onTouchEvent(ev);
	}else{
		//	如果不拦截，交给子布局分发，这是个层层递归过程
		result = chlid.dispatchTouchEvent(ev);
	}
	return result;
}

直接撸源码是一件很痛苦的事情，多种可能的发生让源码可读性很差。下面我们会从某一种特定逻辑下分析，这样会清晰很多。每次只分析一种情境！

父布局拦截的分发处理过程

父布局拦截我们分两步，ACTION_DOWN、ACTION_MOVE

ACTION_DOWN 事件

进入 ViewGroup 的 dispatchTouchEvent 方法内

   if (actionMasked == MotionEvent.ACTION_DOWN) {
       // Throw away all previous state when starting a new touch gesture.
       // The framework may have dropped the up or cancel event for the previous gesture
       // due to an app switch, ANR, or some other state change.
       cancelAndClearTouchTargets(ev);
       resetTouchState();
   }

因为是 ACTION_DOWN 事件，先清空状态，一个是TouchTarget的状态，一个是 mGroupFlags。这个用不到继续走

// Check for interception.
final boolean intercepted;
//	因为是第一次过来	 mFirstTouchTarget = null ，且是 ACTION_DOWN 事件，走入 if 内
if (actionMasked == MotionEvent.ACTION_DOWN
         || mFirstTouchTarget != null) {
     //	咱们走的是父布局拦截事件，子布局用尚方宝剑，disallowIntercept =false
     final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
     if (!disallowIntercept) {
     	//	咱们在这拦截， intercepted = true
         intercepted = onInterceptTouchEvent(ev);
         ev.setAction(action); // restore action in case it was changed
     } else {
         intercepted = false;
     }
 } else {
     // There are no touch targets and this action is not an initial down
     // so this view group continues to intercept touches.
     intercepted = true;
 }

代码注释的比较全，这边主要是判断本view是否拦截，如果拦截 intercepted = true 。所以后面的遍历子view分发都进不去

//	intercepted = true , 进不去
if (!canceled && !intercepted) {
	//	这是一个遍历子 view 接盘的故事
	for (int i = childrenCount - 1; i >= 0; i--) {
	}
}

一直往下走

// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
    // No touch targets so treat this as an ordinary view.
    handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS);
}

符合这个条件，深入 dispatchTransformedTouchEvent 函数，第三个参数是 null

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
         View child, int desiredPointerIdBits) {
     
     final boolean handled;
     if (child == null) {
      	//	===== 执行位置 ====
          handled = super.dispatchTouchEvent(transformedEvent);
      } else {
          final float offsetX = mScrollX - child.mLeft;
          final float offsetY = mScrollY - child.mTop;
          transformedEvent.offsetLocation(offsetX, offsetY);
          if (! child.hasIdentityMatrix()) {
              transformedEvent.transform(child.getInverseMatrix());
          }

          handled = child.dispatchTouchEvent(transformedEvent);
      }
}

第三个参数传的是 null ，即 child = null 。调用 super 的 dispatchTouchEvent 。ViewGroup 的 super 即是 View。

handled = View.dispatchTouchEvent(event);

深入 View 的 dispatchTouchEvent 方法，主要处理逻辑是下面两段代码

ListenerInfo li = mListenerInfo;
 if (li != null && li.mOnTouchListener != null
         && (mViewFlags & ENABLED_MASK) == ENABLED
         && li.mOnTouchListener.onTouch(this, event)) {
     result = true;
 }

 if (!result && onTouchEvent(event)) {
     result = true;
 }

我们可以得出结论：onTouch 比 onTouchEvent 优先级高，如果onTouch 拦截事件则 onTouchEvent 无法接到事件。这也是为什么我们在onTouch方法返回true后onClick事件失效的原因。onTouchEvent 的逻辑比较简单，此处不做分析

这里要说明一点，事件分发机制的分发其实有两种含义。一是事件在不同view之间的分发，父布局到子布局的分发；二是事件在view中对不同监听的分发，onTouch、onClick、onLongClick 在分发时也是有顺序的。

到这里父布局拦截的DOWN事件算结束了，下面是MOVE事件，继DOWN后的滑动，这是个连续的过程

ACTION_MOVE 事件

手指点击后开始滑动，继续分发move事件

final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
        || mFirstTouchTarget != null) {
    final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
    if (!disallowIntercept) {
        intercepted = onInterceptTouchEvent(ev);
        ev.setAction(action); // restore action in case it was changed
    } else {
        intercepted = false;
    }
} else {
    // There are no touch targets and this action is not an initial down
    // so this view group continues to intercept touches.
    intercepted = true;
}

actionMasked = ACTION_MOVE ，mFirstTouchTarget = null ，直接走 else 模块，即 intercepted = true。

//	intercepted = true , 进不去
if (!canceled && !intercepted) {
	for (int i = childrenCount - 1; i >= 0; i--) {
	}
}

同样的分发子view的模块我们依旧进不去

// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
    // No touch targets so treat this as an ordinary view.
    handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS);
}

到这里就跟之前的逻辑完全一样了，至此父布局的拦截过程结束
总结：

ViewGroup 的 onTouchEvent 方法直接调用父类（View）的实现
父布局一旦拦截Down事件，后续的move事件都直接由父布局执行

这么分析的好处是咱们的状态是确定的，分析代码不会有太多可能性搞乱逻辑，下面是父布局不拦截的情况下事件分发

父布局不拦截时的分发处理过程

父布局不拦截，咱们按照正常流程走一遍，还是按上面那个思路，先 down 后 move

ACTION_DOWN

进入 GroupView 的 dispatchTouchEvent 方法后依旧西先是清空状态，然后判断当前布局是否拦截

final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
        || mFirstTouchTarget != null) {
    final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
    if (!disallowIntercept) {
        intercepted = onInterceptTouchEvent(ev);
        ev.setAction(action); // restore action in case it was changed
    } else {
        intercepted = false;
    }
} else {
    // There are no touch targets and this action is not an initial down
    // so this view group continues to intercept touches.
    intercepted = true;
}

咱们的设定是不拦截，所以 intercepted = false。下面是遍历子view的代码

final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
    final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
    //	逆序拿到一个 child ，即从最上层的子view开始往内层遍历
    final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
    
    //	判断触点的位置是否在view的范围之内或者view是否在播放动画，如果都不满足则直接遍历下一个
    if (!child.canReceivePointerEvents()|| !isTransformedTouchPointInView(x, y, child, null)) {
        continue;
    }

    newTouchTarget = getTouchTarget(child);
    if (newTouchTarget != null) {
        // Child is already receiving touch within its bounds.
        // Give it the new pointer in addition to the ones it is handling.
        newTouchTarget.pointerIdBits |= idBitsToAssign;
        break;
    }
    resetCancelNextUpFlag(child);
    //	dispatchTransformedTouchEvent 函数是处理分发的函数，父布局处理用的也是这个
    //	如果子view消费了事件则给标志位赋值，并 break 结束循环，如果没有消费则继续循环寻找分发
    if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {					注释1
        // Child wants to receive touch within its bounds.
        mLastTouchDownTime = ev.getDownTime();
        if (preorderedList != null) {
            // childIndex points into presorted list, find original index
            for (int j = 0; j < childrenCount; j++) {
                if (children[childIndex] == mChildren[j]) {
                    mLastTouchDownIndex = j;
                    break;
                }
            }
        } else {
            mLastTouchDownIndex = childIndex;
        }
        mLastTouchDownX = ev.getX();
        mLastTouchDownY = ev.getY();
        //	如果子view消费了事件则给 alreadyDispatchedToNewTouchTarget  和 mFirstTouchTarget 赋值
        //	保存 child
        newTouchTarget = addTouchTarget(child, idBitsToAssign);							注释2
        alreadyDispatchedToNewTouchTarget = true;
        break;
    }

    // The accessibility focus didn't handle the event, so clear
    // the flag and do a normal dispatch to all children.
    ev.setTargetAccessibilityFocus(false);
}

分析上面干了啥

从最上层的子view开始往内层遍历
判断当前的view在位置上是否满足触点位置
调用 dispatchTransformedTouchEvent 判断是否子view消费了事件

如果消费了事件则记录 mFirstTouchTarget 和标志位，并跳出循环

如果没有没有消费事件则继续循环

注释1的逻辑 dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
         View child, int desiredPointerIdBits) {
     
     final boolean handled;
     if (child == null) {
          handled = super.dispatchTouchEvent(transformedEvent);
      } else {
          final float offsetX = mScrollX - child.mLeft;
          final float offsetY = mScrollY - child.mTop;
          transformedEvent.offsetLocation(offsetX, offsetY);
          if (! child.hasIdentityMatrix()) {
              transformedEvent.transform(child.getInverseMatrix());
          }
          //	===== 执行位置 ====
          handled = child.dispatchTouchEvent(transformedEvent);
      }
}

这次过来 child != null ，调用的是 child.dispatchTouchEvent(event) 。child 可能是View，也可能是 ViewGroup。如果是 ViewGroup 又是一个递归的过程。层层的递归返回 handled 告诉父布局是否消费了事件！

再看注释2的逻辑

 private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
     final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
     //	此时 mFirstTouchTarget = null
     target.next = mFirstTouchTarget;
     mFirstTouchTarget = target;
     return target;
 }

给 mFirstTouchTarget 赋值，下次 move 事件过来时 mFirstTouchTarget 就是有值的了！！即

target.next = null
mFirstTouchTarget = newTouchTarget
保存 child 在 target 中

至此 ACTION_DOWN 事件结束

ACTION_MOVE

继上面点击后开始滑动

if (actionMasked == MotionEvent.ACTION_DOWN) {
      cancelAndClearTouchTargets(ev);
      resetTouchState();
  }

move事件不会重置，继续走

 if (actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null)

记得down事件中给mFirstTouchTarget 赋过值嘛，虽然不是down事件依旧可以进入此方法。也就是说这里依旧会判断父布局是否要拦截子view，这里也是以后咱们处理事件冲突的重点。当前的逻辑是不拦截，所以 intercepted = false

if (actionMasked == MotionEvent.ACTION_DOWN
       || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
        || actionMasked == MotionEvent.ACTION_HOVER_MOVE)

只有ACTION_DOWN事件才会进行分发，所以不会进入遍历子view的逻辑代码！MOVE事件不会分发事件！

//	mFirstTouchTarget  有值，走else模块
if (mFirstTouchTarget == null) {
    // No touch targets so treat this as an ordinary view.
    handled = dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS);
} else {
    // Dispatch to touch targets, excluding the new touch target if we already
    // dispatched to it.  Cancel touch targets if necessary.
    TouchTarget predecessor = null;
    TouchTarget target = mFirstTouchTarget;
    while (target != null) {
        final TouchTarget next = target.next;
        //	alreadyDispatchedToNewTouchTarget  是 false
        if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
            handled = true;
        } else {
        	//	此处的结果是 false
            final boolean cancelChild = resetCancelNextUpFlag(target.child)|| intercepted;
            //	在这里被分发处理 child就是我们要分发的对象
            if (dispatchTransformedTouchEvent(ev, cancelChild,target.child, target.pointerIdBits)) {
                handled = true;
            }
            if (cancelChild) {
                if (predecessor == null) {
                    mFirstTouchTarget = next;
                } else {
                    predecessor.next = next;
                }
                target.recycle();
                target = next;
                continue;
            }
        }
        predecessor = target;
        target = next;
    }
}

alreadyDispatchedToNewTouchTarget 在每次进来时都会重置为 false ，最后又会调用 dispatchTransformedTouchEvent 处理分发

if (child == null) {
    handled = super.dispatchTouchEvent(transformedEvent);
} else {
    final float offsetX = mScrollX - child.mLeft;
    final float offsetY = mScrollY - child.mTop;
    transformedEvent.offsetLocation(offsetX, offsetY);
    if (! child.hasIdentityMatrix()) {
        transformedEvent.transform(child.getInverseMatrix());
    }
	//	递归调用它来分发
    handled = child.dispatchTouchEvent(transformedEvent);
}

至此move事件也结束，做个总结

DOWN 事件是事件分发，寻找接盘的 child 并保存在 mFirstTouchTarget 中
MOVE 事件虽然不需要遍历寻找接盘的view，但还可以被ViewGroup拦截的（比如ViewPager包裹着RecyclerView，DOWN事件时被RecyclerView拦截，横向滑动时被抛弃，这时候ViewPager是可以拦截横向滑动接盘的）

解决冲突方案

滑动冲突解决方案有两种：内部拦截、外部拦截。顾名思义，内部拦截是在子View中写逻辑拦截，外部拦截则是从父布局下手解决问题

都以ViewPager包裹RecyclerView滑动冲突为例

外部拦截

public class BadViewPager extends ViewPager {

    private int mLastX, mLastY;

    public BadViewPager(@NonNull Context context) {
        super(context);
    }

    public BadViewPager(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    // 外部拦截法：父容器处理冲突
    // 我想要把事件分发给谁就分发给谁
    @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {

        int x = (int) event.getX();
        int y = (int) event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN: {
                mLastX = (int) event.getX();
                mLastY = (int) event.getY();
                break;
            }
            case MotionEvent.ACTION_MOVE: {
                int deltaX = x - mLastX;
                int deltaY = y - mLastY;
                if (Math.abs(deltaX) > Math.abs(deltaY)) {	//	横向滑动时拦截
                    return true;
                }
                break;
            }
            case MotionEvent.ACTION_UP: {
                break;
            }
            default:
                break;
        }

        return super.onInterceptTouchEvent(event);

    }
}

内部拦截

ViewPager 代码

public class BadViewPager extends ViewPager {

    private int mLastX, mLastY;

    public BadViewPager(@NonNull Context context) {
        super(context);
    }

    public BadViewPager(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }
    @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {
        if (event.getAction() == MotionEvent.ACTION_DOWN){
            super.onInterceptTouchEvent(event);
            //	此处是重点
            return false;
        }
        return true;
    }
}

RecyclerView 代码

public class MyListView extends ListView {

    public MyListView(Context context) {
        super(context);
    }

    public MyListView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    // 内部拦截法：子view处理事件冲突
    private int mLastX, mLastY;

    @Override
    public boolean dispatchTouchEvent(MotionEvent event) {
        int x = (int) event.getX();
        int y = (int) event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN: {
                getParent().requestDisallowInterceptTouchEvent(true);
                break;
            }
            case MotionEvent.ACTION_MOVE: {
                int deltaX = x - mLastX;
                int deltaY = y - mLastY;
                if (Math.abs(deltaX) > Math.abs(deltaY)) {
                    getParent().requestDisallowInterceptTouchEvent(false);
                }
                break;
            }
            case MotionEvent.ACTION_UP: {
                break;

            }
            default:
                break;
        }

        mLastX = x;
        mLastY = y;
        return super.dispatchTouchEvent(event);
    }
}

此处一定要注意，父布局在 ACTION_DOWN 时一定要返回false。原因如下：

当分发DOWN事件时，执行了 resetTouchState(); 函数

    private void resetTouchState() {
        clearTouchTargets();
        resetCancelNextUpFlag(this);
        mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
        mNestedScrollAxes = SCROLL_AXIS_NONE;
    }

mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT

在判断父布局拦截时

final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
   intercepted = onInterceptTouchEvent(ev);
   ev.setAction(action); // restore action in case it was changed
} else {
    intercepted = false;
}

即 mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT & FLAG_DISALLOW_INTERCEPT ！= 0 ==》false
使用 if 语句永远是true，在这里ViewPager会拦截事件，所以RecyclerView无法上下滑动。所以内部拦截时要修改父布局的 onInterceptTouchEvent 函数！

上一篇：Android自定义View之渐变色折线图的实现

栏目：Android代码

下一篇：RecyclerView仿应用列表实现网格布局

本文标题：Android 深入探究自定义view之事件的分发机制与处理详解

本文地址：http://www.codeinn.net/misctech/219215.html

更多Android代码