Android事件的分发机制详解

在分析Android事件分发机制前，明确android的两大基础控件类型:View和ViewGroup。View即普通的控件，没有子布局的，如Button、TextView. ViewGroup继承自View，表示可以有子控件，如Linearlayout、Listview这些。今天我们先来了解View的事件分发机制。
先看下代码，非常简单，只有一个Button，分别给它注册了OnClick和OnTouch的点击事件。

btn.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        Log.i("Tag", "This is button onClick event");
      }
    });
    btn.setOnTouchListener(new View.OnTouchListener() {
      @Override
      public boolean onTouch(View v, MotionEvent event) {
        Log.i("Tag", "This is button onTouch action" + event.getAction());
        return false;
      }
    });

运行一下项目，结果如下：
 I/Tag: This is button onTouch action0
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action1
 I/Tag: This is button onClick event 
可以看到，onTouch是有先于onClick执行的，因此事件的传递顺序是先onTouch，在到OnClick。具体为什么这样，下面会通过源码来说明。这时，我们可能注意到了，onTouch的方法是有返回值，这里是返回false，我们将它改为true再运行一次，结果如下：
 I/Tag: This is button onTouch action0
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action1

对比两次结果，我们发现onClick方法不再执行，为什么会这样，下面我将通过源码给大家一步步理清这个思路。
查看源码时，首先要知道所有View类型控件事件入口都是dispatchTouchEvent()，所以我们直接进入到View这个类里面的dispatchTouchEvent（）方法看一下。

public boolean dispatchTouchEvent(MotionEvent event) {
    // If the event should be handled by accessibility focus first.
    if (event.isTargetAccessibilityFocus()) {
      // We don't have focus or no virtual descendant has it, do not handle the event.
      if (!isAccessibilityFocusedViewOrHost()) {
        return false;
      }
      // We have focus and got the event, then use normal event dispatch.
      event.setTargetAccessibilityFocus(false);
    }
    boolean result = false;
    if (mInputEventConsistencyVerifier != null) {
      mInputEventConsistencyVerifier.onTouchEvent(event, 0);
    }
    final int actionMasked = event.getActionMasked();
    if (actionMasked == MotionEvent.ACTION_DOWN) {
      // Defensive cleanup for new gesture
      stopNestedScroll();
    }
    if (onFilterTouchEventForSecurity(event)) {
      //noinspection SimplifiableIfStatement
      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;
      }
    }
    if (!result && mInputEventConsistencyVerifier != null) {
      mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
    }
    // Clean up after nested scrolls if this is the end of a gesture;
    // also cancel it if we tried an ACTION_DOWN but we didn't want the rest
    // of the gesture.
    if (actionMasked == MotionEvent.ACTION_UP ||
        actionMasked == MotionEvent.ACTION_CANCEL ||
        (actionMasked == MotionEvent.ACTION_DOWN && !result)) {
      stopNestedScroll();
    }
    return result;
  }

从源码第25行处可以看到，mOnTouchListener.onTouch()的方法首先被执行，如果li != null && li.mOnTouchListener != null&& (mViewFlags & ENABLED_MASK) == ENABLED&& li.mOnTouchListener.onTouch(this, event)都为真的话，result赋值为true，否则就执行onTouchEvent(event)方法。

从上面可以看到要符合条件有四个，
 1、ListenerInfo li，它是view中的一个静态类，里面定义view的事件的监听等等，所以有涉及到view的事件，ListenerInfo都会被实例化，因此li不为null
 2、mOnTouchiListener是在setOnTouchListener方法里面赋值的，只要touch事件被注册，mOnTouchiListener一定不会null
 3、 (mViewFlags & ENABLED_MASK) == ENABLED，是判断当前点击的控件是否是enable的，button默认为enable，这个条件也恒定为true，
 4、重点来了，li.mOnTouchListener.onTouch(this, event)就是回调控件onTouch方法，当这个条件也为true时，result=true,onTouchEvent（event）将不会被执行。如果onTouch返回false，就会再执行onTouchEvent(event)方法。
我们接着再进入到onTouchEvent方法查看源码。

public boolean onTouchEvent(MotionEvent event) {
    final float x = event.getX();
    final float y = event.getY();
    final int viewFlags = mViewFlags;
    final int action = event.getAction();
    if ((viewFlags & ENABLED_MASK) == DISABLED) {
      if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
        setPressed(false);
      }
      // A disabled view that is clickable still consumes the touch
      // events, it just doesn't respond to them.
      return (((viewFlags & CLICKABLE) == CLICKABLE
          || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
          || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);
    }
    if (mTouchDelegate != null) {
      if (mTouchDelegate.onTouchEvent(event)) {
        return true;
      }
    }
    if (((viewFlags & CLICKABLE) == CLICKABLE ||
        (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
        (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
      switch (action) {
        case MotionEvent.ACTION_UP:
          boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
          if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
            // take focus if we don't have it already and we should in
            // touch mode.
            boolean focusTaken = false;
            if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
              focusTaken = requestFocus();
            }
            if (prepressed) {
              // The button is being released before we actually
              // showed it as pressed. Make it show the pressed
              // state now (before scheduling the click) to ensure
              // the user sees it.
              setPressed(true, x, y);
            }
            if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
              // This is a tap, so remove the longpress check
              removeLongPressCallback();
              // Only perform take click actions if we were in the pressed state
              if (!focusTaken) {
                // Use a Runnable and post this rather than calling
                // performClick directly. This lets other visual state
                // of the view update before click actions start.
                if (mPerformClick == null) {
                  mPerformClick = new PerformClick();
                }
                if (!post(mPerformClick)) {
                  performClick();
                }
              }
            }
            if (mUnsetPressedState == null) {
              mUnsetPressedState = new UnsetPressedState();
            }
            if (prepressed) {
              postDelayed(mUnsetPressedState,
                  ViewConfiguration.getPressedStateDuration());
            } else if (!post(mUnsetPressedState)) {
              // If the post failed, unpress right now
              mUnsetPressedState.run();
            }
            removeTapCallback();
          }
          mIgnoreNextUpEvent = false;
          break;
        case MotionEvent.ACTION_DOWN:
          mHasPerformedLongPress = false;
          if (performButtonActionOnTouchDown(event)) {
            break;
          }
          // Walk up the hierarchy to determine if we're inside a scrolling container.
          boolean isInScrollingContainer = isInScrollingContainer();
          // For views inside a scrolling container, delay the pressed feedback for
          // a short period in case this is a scroll.
          if (isInScrollingContainer) {
            mPrivateFlags |= PFLAG_PREPRESSED;
            if (mPendingCheckForTap == null) {
              mPendingCheckForTap = new CheckForTap();
            }
            mPendingCheckForTap.x = event.getX();
            mPendingCheckForTap.y = event.getY();
            postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
          } else {
            // Not inside a scrolling container, so show the feedback right away
            setPressed(true, x, y);
            checkForLongClick(0);
          }
          break;
        case MotionEvent.ACTION_CANCEL:
          setPressed(false);
          removeTapCallback();
          removeLongPressCallback();
          mInContextButtonPress = false;
          mHasPerformedLongPress = false;
          mIgnoreNextUpEvent = false;
          break;
        case MotionEvent.ACTION_MOVE:
          drawableHotspotChanged(x, y);
          // Be lenient about moving outside of buttons
          if (!pointInView(x, y, mTouchSlop)) {
            // Outside button
            removeTapCallback();
            if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
              // Remove any future long press/tap checks
              removeLongPressCallback();
              setPressed(false);
            }
          }
          break;
      }
      return true;
    }
    return false;
  }

从源码的21行我们可以看出，该控件可点击就会进入到switch判断中，当我们触发了手指离开的实际，则会进入到MotionEvent.ACTION_UP这个case当中。我们接着往下看，在源码的50行，调用到了mPerformClick()方法，我们继续进入到这个方法的源码看看。

public boolean performClick() {
    final boolean result;
    final ListenerInfo li = mListenerInfo;
    if (li != null && li.mOnClickListener != null) {
      playSoundEffect(SoundEffectConstants.CLICK);
      li.mOnClickListener.onClick(this);
      result = true;
    } else {
      result = false;
    }
    sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
    return result;
  }

现在我们可以看到，只要ListenerInfo和mOnClickListener不为null就会调用onClick这个方法，之前说过，只要有监听事件，ListenerInfo就不为null，带mOnClickListener又是在哪里赋值呢？我们再继续看下它的源码。

public void setOnClickListener(@Nullable OnClickListener l) {
    if (!isClickable()) {
      setClickable(true);
    }
    getListenerInfo().mOnClickListener = l;
  }

看到这里一切就清楚了，当我们调用setOnClickListener方法来给按钮注册一个点击事件时，就会给mOnClickListener赋值。整个分发事件的顺序是onTouch()-->onTouchEvent(event)-->performClick()-->OnClick()。
 现在我们可以解决之前的问题。
1、onTouch方法是优先于OnClick，所以是执行了onTouch，再执行onClick。 
2、无论是dispatchTouchEvent还是onTouchEvent，如果返回true表示这个事件已经被消费、处理了，不再往下传了。在dispathTouchEvent的源码里可以看到，如果onTouchEvent返回了true，那么它也返回true。如果dispatchTouchEvent在执行onTouch监听的时候，onTouch返回了true，那么它也返回true，这个事件提前被onTouch消费掉了。就不再执行onTouchEvent了，更别说onClick监听了。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持我们。