前言

面试得时候经常会被问到Android事件分发机制，如下：
用户得点击，Activity Window和DecorView最先谁获取到事件？
答案：Activity最先接触到，传递模式由Activity->Window->DecorView->ViewGroup,显而易见这是明显得责任链模式。

//底层传感器接收到信号之后传到activity
 public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }

/*调用window传递事件，如果superDispatchTouchEvent
返回True则直接返回True不走Activity的onTouchEvent方法*/

        if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }
        return onTouchEvent(ev);
    }

Window处理事件

//Window中直接调用DecorView传递事件
 @Override
    public boolean superDispatchTouchEvent(MotionEvent event) {
        return mDecor.superDispatchTouchEvent(event);
    }

Decor处理事件

//从这里看出DecorView继承得是FrameLayout，
public class DecorView extends FrameLayout

Decor调用父类superDispatchTouchEvent方法进入ViewGroup得dispatchTouchEvent方法

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

ViewGroup中是怎么处理的呢？

首先介绍三个方法，onInterceptTouchEvent（事件拦截），onTouchEvent（事件处理）和dispatchTouchEvent（事件分发）

（一）onInterceptTouchEvent方法

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

为什么返回true之后，子view不向下传递了呢？

拦截的情况

拦截时候进入拦截方法，遍历子view传入dispatchTransformedTouchEvent方法中

 if (!canceled && !intercepted)
/****/
//此时child不为空
 for (int i = childrenCount - 1; i >= 0; i--) {
//此处调用dispatchTransformedTouchEvent方法传入child
    if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                              
  }
}

进入dispatchTransformedTouchEvent方法中

 private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
            View child, int desiredPointerIdBits) {
        final boolean handled;

        // Canceling motions is a special case.  We don't need to perform any transformations
        // or filtering.  The important part is the action, not the contents.
        final int oldAction = event.getAction();
        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
            event.setAction(MotionEvent.ACTION_CANCEL);
//此处child不为空，直接调用子view dispatchTouchEvent方法
            if (child == null) {
                handled = super.dispatchTouchEvent(event);
            } else {
                handled = child.dispatchTouchEvent(event);
            }
            event.setAction(oldAction);
            return handled;
        }
}

所以只有再不拦截的时候，才会向子类传递事件调用child.dispatchTouchEvent(event);

不拦截的情况

1.当用户第一次点击进入dispatchTransformedTouchEvent方法

 // 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);
            }

2.调用父类dispatchTouchEvent方法

  // Perform any necessary transformations and dispatch.
//当child等于null时候调用父类（View）的dispatchTouchEvent
        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());
            }

            handled = child.dispatchTouchEvent(transformedEvent);
        }

3父类View中的dispatchTouchEvent方法内部

 //noinspection SimplifiableIfStatement
            ListenerInfo li = mListenerInfo;
//如果mOnTouchListener 不等于空，执行onTouch回调
            if (li != null && li.mOnTouchListener != null
                    && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {
                result = true;
            }
//如果result等于true，那么不走onTouchEvent方法
            if (!result && onTouchEvent(event)) {
                result = true;
            }

所以再OnTouchListener设置之后不走onTouchEvent方法

事件分发机制解析（一）