android事件分发机制的实现原理

android中的事件处理，以及解决滑动冲突问题都离不开事件分发机制，android中的事件流，即MotionEvent都会经历一个从分发，拦截到处理的一个过程。即dispatchTouchEvent(),onInterceptEvent()到onTouchEvent()的一个过程，在dispatchTouchEvent()负责了事件的分发过程，在dispatchTouchEvent()中会调用onInterceptEvent()与onTouchEvent()，如果onInterceptEvent()返回true，那么会调用到当前view的onTouchEvent()方法，如果不拦截，事件就会下发到子view的dispatchTouchEvent()中进行同样的操作。本文将带领大家从源码角度来分析android是如何进行事件分发的。

android中的事件分发流程最先从activity的dispatchTouchEvent()开始:

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

这里调用了getWindow().superDispatchTouchEvent(ev)，这里可以看出activity将MotionEvent传寄给了Window。而Window是一个抽象类，superDispatchTouchEvent()也是一个抽象方法，这里用到的是window的子类phoneWindow。

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

从这里可以看出，event事件被传到了DecorView,也就是我们的顶层view.我们继续跟踪：

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

这里调用到了父类的dispatchTouchEvent()方法，而DecorView是继承自FrameLayout，FrameLayout继承了ViewGroup,所以这里会调用到ViewGroup的dispatchTouchEvent()方法。

所以整个事件流从activity开始，传递到window，最后再到我们的view(viewGroup也是继承自view)中，而view才是我们整个事件处理的核心阶段。

我们来看一下viewGroup的dispatchTouchEvent()中的实现：

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();    }

这是dispatchTouchEvent()开始时截取的一段代码，我们来看一下，首先，当我们手指按下view时，会调用到resetTouchState()方法，在resetTouchState()中：

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

我们继续跟踪clearTouchTargets()方法：

private void clearTouchTargets() {  TouchTarget target = mFirstTouchTarget;  if (target != null) {    do {      TouchTarget next = target.next;      target.recycle();      target = next;    } while (target != null);    mFirstTouchTarget = null;  }}

在clearTouchTargets()方法中，我们最终将mFirstTouchTarget赋值为null，我们继续回到dispatchTouchEvent()中，接着执行了下段代码：

// Check for interception.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;    }

当view被按下或mFirstTouchTarget != null 的时候，从前面可以知道，当每次view被按下时，也就是重新开始一次事件流的处理时，mFirstTouchTarget都会被设置成null，一会我们看mFirstTouchTarget是什么时候被赋值的。

从disallowIntercept属性我们大概能猜到是用来判断是否需要坐拦截处理，而我们知道可以通过调用父view的requestDisallowInterceptTouchEvent(true)可以让我们的父view不能对事件进行拦截，我们先来看看requestDisallowInterceptTouchEvent()方法中的实现：

@Overridepublic void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {  if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {    // We're already in this state, assume our ancestors are too    return;  }  if (disallowIntercept) {    mGroupFlags |= FLAG_DISALLOW_INTERCEPT;  } else {    mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;  }  // Pass it up to our parent  if (mParent != null) {    mParent.requestDisallowInterceptTouchEvent(disallowIntercept);  }}

这里也是通过设置标志位做判断处理，所以这里是通过改变mGroupFlags标志，然后在dispatchTouchEvent()刚发中变更disallowIntercept的值判断是否拦截，当为true时，即需要拦截，这个时候便会跳过onInterceptTouchEvent()拦截判断，并标记为不拦截，即intercepted = false，我们继续看viewGroup的onInterceptTouchEvent()处理：

public boolean onInterceptTouchEvent(MotionEvent ev) {  if (ev.isFromSource(InputDevice.SOURCE_MOUSE)      && ev.getAction() == MotionEvent.ACTION_DOWN      && ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)      && isOnScrollbarThumb(ev.getX(), ev.getY())) {    return true;  }  return false;}

即默认情况下，只有在ACTION_DOWN时，viewGroup才会表现为拦截。

我们继续往下看：

final int childrenCount = mChildrenCount;if (newTouchTarget == null && childrenCount != 0) {   final float x = ev.getX(actionIndex);   final float y = ev.getY(actionIndex);   // Find a child that can receive the event.   // Scan children from front to back.   final ArrayList preorderedList = buildTouchDispatchChildList();   final boolean customOrder = preorderedList == null              && isChildrenDrawingOrderEnabled();   final View[] children = mChildren;   for (int i = childrenCount - 1; i >= 0; i--) {      final int childIndex = getAndVerifyPreorderedIndex(                childrenCount, i, customOrder);            final View child = getAndVerifyPreorderedView(                preorderedList, children, childIndex);            // If there is a view that has accessibility focus we want it            // to get the event first and if not handled we will perform a            // normal dispatch. We may do a double iteration but this is            // safer given the timeframe.            if (childWithAccessibilityFocus != null) {              if (childWithAccessibilityFocus != child) {                continue;              }              childWithAccessibilityFocus = null;              i = childrenCount - 1;            }            if (!canViewReceivePointerEvents(child)                || !isTransformedTouchPointInView(x, y, child, null)) {              ev.setTargetAccessibilityFocus(false);              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);            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {              // 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();              newTouchTarget = addTouchTarget(child, idBitsToAssign);              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);          }          if (preorderedList != null) preorderedList.clear();        }

这段代码首先会通过一个循环去遍历所有的子view，最终会调用到dispatchTransformedTouchEvent()方法，我们继续看dispatchTransformedTouchEvent()的实现：

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);    if (child == null) {      handled = super.dispatchTouchEvent(event);    } else {      handled = child.dispatchTouchEvent(event);    }    event.setAction(oldAction);    return handled;  }  // Calculate the number of pointers to deliver.  final int oldPointerIdBits = event.getPointerIdBits();  final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;  // If for some reason we ended up in an inconsistent state where it looks like we  // might produce a motion event with no pointers in it, then drop the event.  if (newPointerIdBits == 0) {    return false;  }  // If the number of pointers is the same and we don't need to perform any fancy  // irreversible transformations, then we can reuse the motion event for this  // dispatch as long as we are careful to revert any changes we make.  // Otherwise we need to make a copy.  final MotionEvent transformedEvent;  if (newPointerIdBits == oldPointerIdBits) {    if (child == null || child.hasIdentityMatrix()) {      if (child == null) {        handled = super.dispatchTouchEvent(event);      } else {        final float offsetX = mScrollX - child.mLeft;        final float offsetY = mScrollY - child.mTop;        event.offsetLocation(offsetX, offsetY);        handled = child.dispatchTouchEvent(event);        event.offsetLocation(-offsetX, -offsetY);      }      return handled;    }    transformedEvent = MotionEvent.obtain(event);  } else {    transformedEvent = event.split(newPointerIdBits);  }  // Perform any necessary transformations and dispatch.  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);  }  // Done.  transformedEvent.recycle();  return handled;}

这段代码就比较明显了，如果child不为null，始终会调用到child.dispatchTouchEvent();否则调用super.dispatchTouchEvent();

如果child不为null时，事件就会向下传递，如果子view处理了事件，即dispatchTransformedTouchEvent()即返回true。继续向下执行到addTouchTarget()方法，我们继续看addTouchTarget()方法的执行结果：

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

这个时候我们发现mFirstTouchTarget又出现了，这时候会给mFirstTouchTarget重新赋值，即mFirstTouchTarget不为null。也就是说，如果事件被当前view或子view消费了，那么在接下来的ACTION_MOVE或ACTION_UP事件中，mFirstTouchTarget就不为null。但如果我们继承了该viewGroup，并在onInterceptTouchEvent()的ACTION_MOVE中拦截了事件，那么后续事件将不会下发，将由该viewGroup直接处理，从下面代码我们可以得到：

// 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;        if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {          handled = true;        } else {          final boolean cancelChild = resetCancelNextUpFlag(target.child)              || intercepted;          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;      }

当存在子view并且事件被子view消费时，即在ACTION_DOWN阶段mFirstTouchTarget会被赋值，即在接下来的ACTION_MOVE事件中，由于intercepted为true，所以将ACTION_CANCEL 事件传递过去，从dispatchTransformedTouchEvent()中可以看到：

if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {    event.setAction(MotionEvent.ACTION_CANCEL);    if (child == null) {      handled = super.dispatchTouchEvent(event);    } else {      handled = child.dispatchTouchEvent(event);    }    event.setAction(oldAction);    return handled;  }

并将mFirstTouchTarget 最终赋值为 next，而此时mFirstTouchTarget位于TouchTarget链表尾部，所以mFirstTouchTarget会赋值为null，那么接下来的事件将不会进入到onInterceptTouchEvent()中。也就会直接交由该view处理。

如果我们没有进行事件的拦截，而是交由子view去处理，由于ViewGroup的onInterceptTouchEvent()默认并不会拦截除了ACTION_DOWN以外的事件，所以后续事件将继续交由子view去处理，如果存在子view且事件位于子view内部区域的话。

所以无论是否进行拦截，事件流都会交由view的dispatchTouchEvent()中进行处理，我们接下来跟踪一下view中的dispatchTouchEvent()处理过程：

if (actionMasked == MotionEvent.ACTION_DOWN) {    // Defensive cleanup for new gesture    stopNestedScroll();  }  if (onFilterTouchEventForSecurity(event)) {    if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {      result = true;    }    //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;    }  }

当被按下时，即ACTION_DOWN时，view会停止内部的滚动，如果view没有被覆盖或遮挡时，首先会进行mListenerInfo是否为空的判断，我们看下mListenerInfo是在哪里初始化的：

ListenerInfo getListenerInfo() {  if (mListenerInfo != null) {    return mListenerInfo;  }  mListenerInfo = new ListenerInfo();  return mListenerInfo;}

这里可以看出，mListenerInfo一般不会是null，知道在我们使用它时调用过这段代码，而当view被加入window中的时候，会调用下面这段代码，从注释中也可以看出来：

/** * Add a listener for attach state changes. * * This listener will be called whenever this view is attached or detached * from a window. Remove the listener using * {@link #removeOnAttachStateChangeListener(OnAttachStateChangeListener)}. * * @param listener Listener to attach * @see #removeOnAttachStateChangeListener(OnAttachStateChangeListener) */public void addOnAttachStateChangeListener(OnAttachStateChangeListener listener) {  ListenerInfo li = getListenerInfo();  if (li.mOnAttachStateChangeListeners == null) {    li.mOnAttachStateChangeListeners        = new CopyOnWriteArrayList();  }  li.mOnAttachStateChangeListeners.add(listener);}

到这里我们就知道，mListenerInfo一开始就是被初始化好了的，所以li不可能为null，li.mOnTouchListener != null即当设置了TouchListener时不为null，并且view是enabled状态，一般情况view都是enable的。这个时候会调用到onTouch()事件，当onTouch()返回true时，这个时候result会赋值true。而当result为true时，onTouchEvent()将不会被调用。

从这里可以看出，onTouch()会优先onTouchEvent()调用；
当view设置touch监听并返回true时，那么它的onTouchEvent()将被屏蔽。否则会调用onTouchEvent()处理。

那么让我们继续来看看onTouchEvent()中的事件处理：

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);  }

首先，当view状态是DISABLED时，只要view是CLICKABLE或LONG_CLICKABLE或CONTEXT_CLICKABLE，都会返回true,而button默认是CLICKABLE的，textview默认不是CLICKABLE的，而view一般默认都不是LONG_CLICKABLE的。

我们继续向下看：

if (mTouchDelegate != null) {    if (mTouchDelegate.onTouchEvent(event)) {      return true;    }  }

如果有代理事件，仍然会返回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, x, y);        }        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;  }

当view是CLICKABLE或LONG_CLICKABLE或CONTEXT_CLICKABLE状态时，当手指抬起时，如果设置了click监听，最终会调用到performClick()，触发click()事件。这点从performClick()方法中可以看出：

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;}

从这里我们也可以得出，click事件会在onTouchEvent()中被调用，如果view设置了onTouch()监听并返回true，那么click事件也会被屏蔽掉，不过我们可以在onTouch()中通过调用view的performClick()继续执行click()事件，这个就看我们的业务中的需求了。

从这里我们可以看出，如果事件没有被当前view或子view处理，即返回false，那么事件就会交由外层view继续处理，直到被消费。

如果事件一直没有被处理，会最终传递到Activity的onTouchEvent()中。

到这里我们总结一下：

事件是从Activity->Window->View(ViewGroup)的一个传递流程；

如果事件没有被中途拦截，那么它会一直传到最内层的view控件；

如果事件被某一层拦截，那么事件将不会向下传递，交由该view处理。如果该view消费了事件，那么接下来的事件也会交由该view处理；如果该view没有消费该事件，那么事件会交由外层view处理，...并最终调用到activity的onTouchEvent()中，除非某一层消费了该事件；

一个事件只能交由一个view处理；

DispatchTouchEvent()总是会被调用，而且最先被调用，onInterceptTouchEvent()和onTouchEvent()在DispatchTouchEvent()内部调用；

子view不能干扰ViewGroup对ACTION_DOWN事件的处理；

子view可以通过requestDisallowInterceptTouchEvent(true)控制父view不对事件进行拦截，跳过onInterceptTouchEvent()方法的执行。

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脚本之家。

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