Android 事件分发机制

转载自：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。不然总是仅仅停留在会用的层次上可是无法了解其内部机制。

我用的是4.4源代码，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。认为Android的事件派发机制和Windows的消息派发机制挺像的，事实上这是一种典型的消息“冒泡”机制，非常多平台採用这个机制，消息最先到达最底层View，然后它先进行推断是不是它所须要的，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传递究竟层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，详细的工作是由Activity内部的Window来完毕的，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的父容器），通过Activity.getWindow.getDecorView（）能够获得。另外。看以下代码的的时候。主要看我凝视的地方。代码非常多非常复杂，我无法一一说明。可是我凝视的地方都是关键点，是博主细致读代码总结出来的。

源代码解读：

事件是由哪里传递给Activity的。这个我还不清楚。可是不要紧。我们从activity開始分析，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) {
            //这个函数事实上是个空函数。啥也没干，假设你没重写的话，不用关心
            onUserInteraction();
        }
        //这里事件開始交给Activity所附属的Window进行派发，假设返回true。整个事件循环就结束了
        //返回false意味着事件没人处理。全部人的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是怎样将事件传递给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);

这居然是一个抽象函数，还注明了应用开发人员不要实现它或者调用它。这是什么情况？再看看例如以下类的说明，大意是说：这个类能够控制顶级View的外观和行为策略。并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，由于这个类会被重构，仅仅有一个工厂方法能够使用。

好吧。还是非常模糊啊，不太懂，只是我们能够看一下android.policy.PhoneWindow这个类。虽然实例化的时候此类会被重构，可是重构而已，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);
    }

这个逻辑非常清晰了。PhoneWindow将事件传递给DecorView了。这个DecorView是啥呢，请看以下

    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。

眼下事件传递到了DecorView 这里。因为DecorView 继承自FrameLayout且是我们的父View。所以终于事件会传递给我们的View，原因先无论了。换句话来说，事件肯定会传递到我们的View，不然我们的应用怎样响应点击事件呢。

只是这不是我们的重点，重点是事件到了我们的View以后应该怎样传递。这是对我们更实用的。从这里開始。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

点击事件究竟层View（通常是一个ViewGroup）以后，会调用ViewGroup的dispatchTouchEvent方法，然后的逻辑是这种：假设底层ViewGroup拦截事件即onInterceptTouchEvent返回true。则事件由ViewGroup处理。这个时候。假设ViewGroup的mOnTouchListener被设置，则会onTouch会被调用，否则，onTouchEvent会被调用。也就是说，假设都提供的话，onTouch会屏蔽掉onTouchEvent。在onTouchEvent中。假设设置了mOnClickListener，则onClick会被调用。

假设顶层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，完毕整个事件派发。另外要说明的是，ViewGroup默认是不拦截点击事件的，其onInterceptTouchEvent返回false。

源代码解读：

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) {
		            //这里推断是否拦截点击事件，假设拦截。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，假设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();
                                //注意以下两句。假设有子View处理了点击事件。则newTouchTarget会被赋值。
                                //同一时候alreadyDispatchedToNewTouchTarget也会为true。这两个变量是直接影响以下的代码逻辑的。
                                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.
	        //这里假设当前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;
    }

以下再看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;
    }

这段代码比較简单，View对事件的处理是这种：假设设置了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。事件假设被ViewGroup拦截或者子View的onTouchEvent都返回了false，则事件终于由ViewGroup处理。

3.无人处理的点击事件

假设一个点击事件。子View的onTouchEvent返回了false，则父View的onTouchEvent会被直接调用，以此类推。假设全部的View都不处理，则终于会由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。