Android事件分发机制

最近在学习Android事件分发机制，也参考了网上许多文章。特别是Android事件分发机制详解：史上最全面、最易懂这篇文章，分析的特别全面、详细，所以在此文章基础上对分发机制做个总结。

1、activity事件分发机制

在activity中，事件分发是从dispatchTouchEvent开始的，这个方法实现也很简单，代码如下：

    public boolean dispatchTouchEvent(MotionEvent ev) {        // 监听手指按下事件        if (ev.getAction() == MotionEvent.ACTION_DOWN) {            onUserInteraction();        }        //交给PhoneWindow去处理，如果返回true则代表已经消费事件，否则activity自己处理事件。        if (getWindow().superDispatchTouchEvent(ev)) {            return true;        }        return onTouchEvent(ev);    }

这里有一个很有意思的方法onUserInteraction，它是一个空实现，但它可以用来实现屏保功能且当点击Home、Back、Recent 时会调用栈顶activity的onUserInteraction方法。然后调用getWindow().superDispatchTouchEvent(ev)将事件交给DecorView处理，DecorView继承自FrameLayout，所以就将事件交给了ViewGroup处理。

来自于Android事件分发机制详解：史上最全面、最易懂

2、ViewGroup事件分发机制

来看ViewGroup中dispatchTouchEvent的实现。

    @Override    public boolean dispatchTouchEvent(MotionEvent ev) {        ...        //默认返回false        boolean handled = false;        if (onFilterTouchEventForSecurity(ev)) {            final int action = ev.getAction();            final int actionMasked = action & MotionEvent.ACTION_MASK;            //当手指按下时            if (actionMasked == MotionEvent.ACTION_DOWN) {                  //在开始新的触摸手势时丢弃所有先前的状态。 由于应用程序切换，ANR或某些其他状态更改，framework可能已经删除了先前手势的up或cancel事件。                cancelAndClearTouchTargets(ev);                resetTouchState();            }            //检查是否拦截事件            final boolean intercepted;            //监听手指按下、移动、抬起等事件，如果mFirstTouchTarget为null则可以认为没有子View响应事件了            if (actionMasked == MotionEvent.ACTION_DOWN                    || mFirstTouchTarget != null) {                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;                if (!disallowIntercept) {                    //调用onInterceptTouchEvent来判断是否需要拦截事件，默认是返回false                    intercepted = onInterceptTouchEvent(ev);                    ev.setAction(action); // restore action in case it was changed                } else {                    intercepted = false;                }            } else {                //当前ViewGroup消费事件                intercepted = true;            }            ...            if (!canceled && !intercepted) {                ...                if (actionMasked == MotionEvent.ACTION_DOWN                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {                    ...                    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.                        //拿到处于重叠状态的所有View并倒序排列，因为我们是要响应最上面的View                        final ArrayList<View> preorderedList = buildTouchDispatchChildList();                        final boolean customOrder = preorderedList == null                                && isChildrenDrawingOrderEnabled();                        final View[] children = mChildren;                        //倒序遍历ViewGroup的所有子View                        for (int i = childrenCount - 1; i >= 0; i--) {                            ...                            //进行事件分发                            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();                                //找到对应的子View，这里之所以是一个链表，是因为有时候会多手指触发事件                                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();                    }                    ...                }            }            //mFirstTouchTarget == null可以理解为没有子View响应事件，需要当前ViewGroup自己处理事件            if (mFirstTouchTarget == null) {                //进行事件分发，传递的子View为null，则代表需要当前ViewGroup来处理事件                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 {                        //判断子View是否需要取消事件                        final boolean cancelChild = resetCancelNextUpFlag(target.child)                                || intercepted;                        //进行事件分发                        if (dispatchTransformedTouchEvent(ev, cancelChild,                                target.child, target.pointerIdBits)) {                            handled = true;                        }                        //子View取消事件                        if (cancelChild) {                            if (predecessor == null) {                                mFirstTouchTarget = next;                            } else {                                predecessor.next = next;                            }                            target.recycle();                            target = next;                            continue;                        }                    }                    predecessor = target;                    target = next;                }            }            ...        }        ...        return handled;    }

从上面代码可以看出，按下事件是无法被拦截的，其他后续事件都有可能被拦截，当不拦截的时候，都会调用onInterceptTouchEvent方法。当拦截事件时，会给子View传一个取消的事件，且将mFirstTouchTarget设置为null，后续的事件就都不会调用onInterceptTouchEvent。buildTouchDispatchChildList这个方法需要注意一下，它主要处理View叠加的问题，具体实现是遍历所有子View然后将这个列表倒序返回，这是为什么尼？如果多个View叠加在一起的话，按照正常逻辑应该响应最下面的那个子View，但实际上应该响应的是最上面的子View，所以需要倒序。最后都是调用dispatchTransformedTouchEvent来进行事件分发，先来看看这个方法的实现。

    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;    }

可以看出dispatchTransformedTouchEvent的实现逻辑很简单，就是将事件传递给当前ViewGroup来处理或者让子View继续进行分发，如果要处理事件就返回true，否则返回false。细心一点可以发现mFirstTouchTarget是一个单链表，那这里为什么要使用单链表尼？先来看看TouchTarget这个类，在这个类的开始部分有一段注释，大意就是最多支持32点在屏幕上触发事件，这也就是mFirstTouchTarget是一个单链表的原因了（如果是单点触摸则该链表只有一个元素）。

    /* Describes a touched view and the ids of the pointers that it has captured.     *     * This code assumes that pointer ids are always in the range 0..31 such that     * it can use a bitfield to track which pointer ids are present.     * As it happens, the lower layers of the input dispatch pipeline also use the     * same trick so the assumption should be safe here...     */    private static final class TouchTarget {        private static final int MAX_RECYCLED = 32;        private static final Object sRecycleLock = new Object[0];        private static TouchTarget sRecycleBin;        private static int sRecycledCount;        public static final int ALL_POINTER_IDS = -1; // all ones        // The touched child view.        public View child;        // The combined bit mask of pointer ids for all pointers captured by the target.        public int pointerIdBits;        // The next target in the target list.        public TouchTarget next;        private TouchTarget() {        }        public static TouchTarget obtain(@NonNull View child, int pointerIdBits) {            if (child == null) {                throw new IllegalArgumentException("child must be non-null");            }            final TouchTarget target;            synchronized (sRecycleLock) {                if (sRecycleBin == null) {                    target = new TouchTarget();                } else {                    target = sRecycleBin;                    sRecycleBin = target.next;                     sRecycledCount--;                    target.next = null;                }            }            target.child = child;            target.pointerIdBits = pointerIdBits;            return target;        }        public void recycle() {            if (child == null) {                throw new IllegalStateException("already recycled once");            }            synchronized (sRecycleLock) {                if (sRecycledCount < MAX_RECYCLED) {                    next = sRecycleBin;                    sRecycleBin = this;                    sRecycledCount += 1;                } else {                    next = null;                }                child = null;            }        }    }

到此ViewGroup的事件分发就分析完毕了，一般在使用过程中都会ViewGroup里放ViewGroup这样嵌套，但流程都是不变的。

来自于Android事件分发机制详解：史上最全面、最易懂

3、View事件事件分发机制

View的事件分发主要是dispatchTouchEvent里实现。先来看一下该方法的具体实现

    public boolean dispatchTouchEvent(MotionEvent event) {        ...        boolean result = false;        ...        if (onFilterTouchEventForSecurity(event)) {            if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {                result = true;            }            //noinspection SimplifiableIfStatement            ListenerInfo li = mListenerInfo;            //从这里判断可以看出onTouch在onTouchEvent之前调用且如果onTouch返回了true，onTouchEvent就不会执行            if (li != null && li.mOnTouchListener != null                    && (mViewFlags & ENABLED_MASK) == ENABLED                    && li.mOnTouchListener.onTouch(this, event)) {                result = true;            }            if (!result && onTouchEvent(event)) {                result = true;            }        }        ...        return result;    }

View的事件分发实现还是比较简单的，没有ViewGroup那么复杂，从上面可以看出onTouch方法在onTouchEvent之前调用且如果onTouch返回true则onTouchEvent就不会执行。

public boolean onTouchEvent(MotionEvent event) {        ...        if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {            switch (action) {                case MotionEvent.ACTION_UP:                    ...                    if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {                        ...                        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();                                }                                //点击事件                                if (!post(mPerformClick)) {                                    performClick();                                }                            }                        }                        ...                        removeTapCallback();                    }                    mIgnoreNextUpEvent = false;                    break;                 ...                //当ViewGroup拦截事件时，会传递给子View一个ACTION_CANCEL事件                case MotionEvent.ACTION_CANCEL:                    ...                    break;            }            return true;        }        return false;    }

从上面可以看出点击事件及长按事件优先级别是最低的，如果子类重写了onTouchEvent则该View的点击、长按等事件就无效了，需要在子类里调用主动调用super.onTouchEvent或者performClick。

来自于Android事件分发机制详解：史上最全面、最易懂

到此事件分发机制就总结完毕了，最后来一张总的流程图。

参考

Android事件分发机制详解：史上最全面、最易懂

1、activity事件分发机制

2、ViewGroup事件分发机制

3、View事件事件分发机制

参考

更多相关文章

随机推荐