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