Android源码分析-点击事件派发机制

转载请注明出处：http://blog.csdn.net/singwhatiwanna/article/details/17339857

概述

一直想写篇关于Android事件派发机制的文章，却一直没写，这两天刚好是周末，有时间了，想想写一篇吧，不然总是只停留在会用的层次上但是无法了解其内部机制。我用的是4.4源码，打开看看，挺复杂的，尤其是事件是怎么从Activity派发出来的，太费解了。了解Windows消息机制的人会发现，觉得Android的事件派发机制和Windows的消息派发机制挺像的，其实这是一种典型的消息“冒泡”机制，很多平台采用这个机制，消息最先到达最底层View，然后它先进行判断是不是它所需要的，否则就将消息传递给它的子View，这样一来，消息就从水底的气泡一样向上浮了一点距离，以此类推，气泡达到顶部和空气接触，破了（消息被处理了），当然也有气泡浮出到顶层了，还没破（消息无人处理），这个消息将由系统来处理，对于Android来说，会由Activity来处理。

Android点击事件的派发机制

1. 从Activity传递到底层View

点击事件用MotionEvent来表示，当一个点击操作发生时，事件最先传递给当前Activity，由Activity的dispatchTouchEvent来进行事件派发，具体的工作是由Activity内部的Window来完成的，Window会将事件传递给decor view，decor view一般就是当前界面的底层容器（即setContentView所设置的View），通过Activity.getWindow.getDecorView（）可以获得。另外，看下面代码的的时候，主要看我注释的地方，代码很多很复杂，我无法一一说明，但是我注释的地方都是关键点，是博主仔细读代码总结出来的。

源码解读：

事件是由哪里传递给Activity的，这个我还不清楚，但是不要紧，我们从activity开始分析，已经足够我们了解它的内部实现了。

Code：Activity#dispatchTouchEvent

/**
 * Called to process touch screen events.  You can override this to
 * intercept all touch screen events before they are dispatched to the
 * window.  Be sure to call this implementation for touch screen events
 * that should be handled normally.
 *
 * @param ev The touch screen event.
 *
 * @return boolean Return true if this event was consumed.
 */
public boolean dispatchTouchEvent(MotionEvent ev) {
    if (ev.getAction() == MotionEvent.ACTION_DOWN) {
        //这个函数其实是个空函数，啥也没干，如果你没重写的话，不用关心
        onUserInteraction();
    }
    //这里事件开始交给Activity所附属的Window进行派发，如果返回true，整个事件循环就结束了
    //返回false意味着事件没人处理，所有人的onTouchEvent都返回了false，那么Activity就要来做最后的收场。
    if (getWindow().superDispatchTouchEvent(ev)) {
        return true;
    }
    //这里，Activity来收场了，Activity的onTouchEvent被调用
    return onTouchEvent(ev);
}

Window是如何将事件传递给ViewGroup的

Code：Window#superDispatchTouchEvent

/**
 * Used by custom windows, such as Dialog, to pass the touch screen event
 * further down the view hierarchy. Application developers should
 * not need to implement or call this.
 *
 */
public abstract boolean superDispatchTouchEvent(MotionEvent event);

这竟然是一个抽象函数，还注明了应用开发者不要实现它或者调用它，这是什么情况？再看看如下类的说明，大意是说：这个类可以控制顶级View的外观和行为策略，而且还说这个类的唯一一个实现位于android.policy.PhoneWindow，当你要实例化这个Window类的时候，你并不知道它的细节，因为这个类会被重构，只有一个工厂方法可以使用。好吧，还是很模糊啊，不太懂，不过我们可以看一下android.policy.PhoneWindow这个类，尽管实例化的时候此类会被重构，但是重构而已，功能是类似的。

Abstract base class for a top-level window look and behavior policy. An instance of this class should be used as the top-level view added to the window manager. It provides standard UI policies such as a background, title area, default key processing, etc.

The only existing implementation of this abstract class is android.policy.PhoneWindow, which you should instantiate when needing a Window. Eventually that class will be refactored and a factory method added for creating Window instances without knowing about a particular implementation. 

Code：PhoneWindow#superDispatchTouchEvent

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

这个逻辑很清晰了，PhoneWindow将事件传递给DecorView了，这个DecorView是啥呢，请看下面

private final class DecorView extends FrameLayout implements RootViewSurfaceTaker

// This is the top-level view of the window, containing the window decor.
private DecorView mDecor;

@Override
public final View getDecorView() {
    if (mDecor == null) {
        installDecor();
    }
    return mDecor;
}

顺便说一下，平时Window用的最多的就是((ViewGroup)getWindow().getDecorView().findViewById(android.R.id.content)).getChildAt(0)即通过Activity来得到内部的View。这个mDecor显然就是getWindow().getDecorView()返回的View，而我们通过setContentView设置的View是它的一个子View。目前事件传递到了DecorView 这里，由于DecorView 继承自FrameLayout且是我们的父View，所以最终事件会传递给我们的View，原因先不管了，换句话来说，事件肯定会传递到我们的View，不然我们的应用如何响应点击事件呢。不过这不是我们的重点，重点是事件到了我们的View以后应该如何传递，这是对我们更有用的。从这里开始，事件已经传递到我们的顶级View了，注意：顶级View实际上是最底层View，也叫根View。

2.底层View对事件的分发过程

点击事件到底层View（一般是一个ViewGroup）以后，会调用ViewGroup的dispatchTouchEvent方法，然后的逻辑是这样的：如果底层ViewGroup拦截事件即onInterceptTouchEvent返回true，则事件由ViewGroup处理，这个时候，如果ViewGroup的mOnTouchListener被设置，则会onTouch会被调用，否则，onTouchEvent会被调用，也就是说，如果都提供的话，onTouch会屏蔽掉onTouchEvent。在onTouchEvent中，如果设置了mOnClickListener，则onClick会被调用。如果顶层ViewGroup不拦截事件，则事件会传递给它的在点击事件链上的子View，这个时候，子View的dispatchTouchEvent会被调用，到此为止，事件已经从最底层View传递给了上一层View，接下来的行为和其底层View一致，如此循环，完成整个事件派发。另外要说明的是，ViewGroup默认是不拦截点击事件的，其onInterceptTouchEvent返回false。

源码解读：

Code：ViewGroup#dispatchTouchEvent

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    if (mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
    }

    boolean handled = false;
    if (onFilterTouchEventForSecurity(ev)) {
        final int action = ev.getAction();
        final int actionMasked = action & MotionEvent.ACTION_MASK;

        // Handle an initial down.
        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();
        }

        // Check for interception.
        final boolean intercepted;
        if (actionMasked == MotionEvent.ACTION_DOWN
                || mFirstTouchTarget != null) {
            final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
            if (!disallowIntercept) {
          //这里判断是否拦截点击事件，如果拦截，则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;
        }

        // Check for cancelation.
        final boolean canceled = resetCancelNextUpFlag(this)
                || actionMasked == MotionEvent.ACTION_CANCEL;

        // Update list of touch targets for pointer down, if needed.
        final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
        TouchTarget newTouchTarget = null;
        boolean alreadyDispatchedToNewTouchTarget = false;
         //这里面一大堆是派发事件到子View，如果intercepted是true，则直接跳过
        if (!canceled && !intercepted) {
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                final int actionIndex = ev.getActionIndex(); // always 0 for down
                final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
                        : TouchTarget.ALL_POINTER_IDS;

                // Clean up earlier touch targets for this pointer id in case they
                // have become out of sync.
                removePointersFromTouchTargets(idBitsToAssign);

                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 View[] children = mChildren;

                    final boolean customOrder = isChildrenDrawingOrderEnabled();
                    for (int i = childrenCount - 1; i >= 0; i--) {
                        final int childIndex = customOrder ?
                                getChildDrawingOrder(childrenCount, i) : i;
                        final View child = children[childIndex];
                        if (!canViewReceivePointerEvents(child)
                                || !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);
                        if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                            // Child wants to receive touch within its bounds.
                            mLastTouchDownTime = ev.getDownTime();
                            mLastTouchDownIndex = childIndex;
                            mLastTouchDownX = ev.getX();
                            mLastTouchDownY = ev.getY();
                            //注意下面两句，如果有子View处理了点击事件，则newTouchTarget会被赋值，
                            //同时alreadyDispatchedToNewTouchTarget也会为true，这两个变量是直接影响下面的代码逻辑的。
                            newTouchTarget = addTouchTarget(child, idBitsToAssign);
                            alreadyDispatchedToNewTouchTarget = true;
                            break;
                        }
                    }
                }

                if (newTouchTarget == null && mFirstTouchTarget != null) {
                    // Did not find a child to receive the event.
                    // Assign the pointer to the least recently added target.
                    newTouchTarget = mFirstTouchTarget;
                    while (newTouchTarget.next != null) {
                        newTouchTarget = newTouchTarget.next;
                    }
                    newTouchTarget.pointerIdBits |= idBitsToAssign;
                }
            }
        }

        // Dispatch to touch targets.
     //这里如果当前ViewGroup拦截了事件，或者其子View的onTouchEvent都返回了false，则事件会由ViewGroup处理
        if (mFirstTouchTarget == null) {
            // No touch targets so treat this as an ordinary view.
          //这里就是ViewGroup对点击事件的处理
            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;
                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;
            }
        }

        // Update list of touch targets for pointer up or cancel, if needed.
        if (canceled
                || actionMasked == MotionEvent.ACTION_UP
                || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
            resetTouchState();
        } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
            final int actionIndex = ev.getActionIndex();
            final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
            removePointersFromTouchTargets(idBitsToRemove);
        }
    }

    if (!handled && mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
    }
    return handled;
}

下面再看ViewGroup对点击事件的处理

Code：ViewGroup#dispatchTransformedTouchEvent

/**
 * Transforms a motion event into the coordinate space of a particular child view,
 * filters out irrelevant pointer ids, and overrides its action if necessary.
 * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
 */
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) {
      //这里就是ViewGroup对点击事件的处理，其调用了View的dispatchTouchEvent方法
            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;
}

再看

Code：View#dispatchTouchEvent

/**
  * Pass the touch screen motion event down to the target view, or this
  * view if it is the target.
  *
  * @param event The motion event to be dispatched.
  * @return True if the event was handled by the view, false otherwise.
  */
 public boolean dispatchTouchEvent(MotionEvent event) {
     if (mInputEventConsistencyVerifier != null) {
         mInputEventConsistencyVerifier.onTouchEvent(event, 0);
     }

     if (onFilterTouchEventForSecurity(event)) {
         //noinspection SimplifiableIfStatement
         ListenerInfo li = mListenerInfo;
         if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
                 && li.mOnTouchListener.onTouch(this, event)) {
             return true;
         }

         if (onTouchEvent(event)) {
             return true;
         }
     }

     if (mInputEventConsistencyVerifier != null) {
         mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
     }
     return false;
 }

这段代码比较简单，View对事件的处理是这样的：如果设置了OnTouchListener就调用onTouch，否则就直接调用onTouchEvent，而onClick是在onTouchEvent内部通过performClick触发的。简单来说，事件如果被ViewGroup拦截或者子View的onTouchEvent都返回了false，则事件最终由ViewGroup处理。

3.无人处理的点击事件

如果一个点击事件，子View的onTouchEvent返回了false，则父View的onTouchEvent会被直接调用，以此类推。如果所有的View都不处理，则最终会由Activity来处理，这个时候，Activity的onTouchEvent会被调用。这个问题已经在1和2中做了说明。