Android源码分析-点击事件派发机制

长平狐 发布于 2013/12/25 17:20
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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。


原文链接:http://blog.csdn.net/singwhatiwanna/article/details/17339857
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