Android事件分发机制源码分析
前面写过一篇文章，说了下事件分发机制的方法和大致流程，本文尝试从源码的角度一层一层的看下分发机制。
源码的查看：https://www.androidos.net.cn/sourcecode（可能是我下的源码有问题，部分方法我是在线查看的）
Activity的事件分发机制
我们从activity的dispatchTouchEvent方法进行源码分析：
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
点进去看到，onUserInteraction是个空方法，这里是用来实现屏保功能的，当activity位于栈顶时，触屏点击home、menu、back会触发。
public void onUserInteraction() {
}
然后我们来看第二个判断
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
继续往下看，
public abstract boolean superDispatchTouchEvent(MotionEvent event);
这玩意是个抽象方法，对应的Window也是个抽象类，我们能找到PhoneWindow，
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
然后我们在看下这个mDecor是啥，
找到这个类
public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks
他是继承FrameLayout的，也就是mDecor.SuperDispatchTouchEvent即等同于Viegroup的分发机制。（即事件从activity传递到了viewgroup）
再看最后一个onTouch方法，
public boolean onTouchEvent(MotionEvent event) {
if (mWindow.shouldCloseOnTouch(this, event)) {
finish();
return true;
}
return false;
}
这里默认返回false，就是不处理，向下层分发，那我们来看下这个判断里面。
public boolean shouldCloseOnTouch(Context context, MotionEvent event) {
final boolean isOutside =
event.getAction() == MotionEvent.ACTION_DOWN && isOutOfBounds(context, event)
|| event.getAction() == MotionEvent.ACTION_OUTSIDE;
if (mCloseOnTouchOutside && peekDecorView() != null && isOutside) {
return true;
}
return false;
}
这里主要判断是否在边界外，在即消费事件，返回true，分发结束，反之返回false，activity层的分发也结束，扔给viewgroup继续分发，直到被消费。
viewgroup的分发机制
我们从ViewGroup的dispatchTouchEvent方法进行源码分析：
if (!canceled && !intercepted) {
// If the event is targeting accessibility focus we give it to the
// view that has accessibility focus and if it does not handle it
// we clear the flag and dispatch the event to all children as usual.
// We are looking up the accessibility focused host to avoid keeping
// state since these events are very rare.
View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
? findChildWithAccessibilityFocus() : null;
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 ArrayList<View> preorderedList = buildTouchDispatchChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(
preorderedList, children, childIndex);
代码很长，挑一点看看（不完整，感兴趣的自己查看），就是判断事件没有取消也没有被拦截，然后给viewgroup内的子view进行遍历，继续看重点
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();
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
我们给这个dispatchTransformedTouchEvent点进去，
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;
}
这里面进行了判断，事件传递给了view（即child）进行分发，或者给他的上一层viewgroup进行分发，直到事件分发结束，即被消费。
回过头来，我们看拦截的判断
final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
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;
}
这里面的事件被拦截即 mFirsTouchTarget！=null不成立，即不拦截，mFirsTouchTarget！=null，disallowIntercept表示是否允许被拦截，是可以用代码来控制的，经过判断，允许被拦截再调用拦截的方法，
public boolean onInterceptTouchEvent(MotionEvent ev) {
if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
&& ev.getAction() == MotionEvent.ACTION_DOWN
&& ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
&& isOnScrollbarThumb(ev.getX(), ev.getY())) {
return true;
}
return false;
}
这里面进行多个判断，默认返回false，即不拦截。拦截的要求大致上，这个事件是来自使用者的输入、是down事件、这个是按钮、点击的事件只在viewgroup内。一旦拦截了，它就会向它的父类分发，也就是view的分发，但执行ViewGroup的onTouch() ->> onTouchEvent() ->> performClick（） ->> onClick()，即自己处理该事件，事件不会往下传递（具体请参考View事件的分发机制中的View.dispatchTouchEvent（））。（可以理解为虽然是个viewgroup，但里面并没有子view，所以事件分发相当于view的事件分发），这个地方要注意。
View的事件分发机制
我们从View的dispatchTouchEvent方法进行源码分析：（源码可能下的有点问题，我们在线看下）
boolean result = false;
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
final int actionMasked = event.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Defensive cleanup for new gesture
stopNestedScroll();
}
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
}
它返回的是result，即决定是否继续分发，返回true，即事件被消费，false即要调用onTouch方法。返回true有三种情况
1. view是可点击的且handleScrollBarDragging
看下源码（太长，看下什么时候返回true）
* @return true if the event was handled as a scroll bar dragging, false otherwise.
*/
protected boolean handleScrollBarDragging(MotionEvent event) {
翻译：如果作为一个滚动条拖动事件处理返回true
2. view可点击且onTouch事件不为空
3. onTouch事件返回true
下面看下onTouchEvent方法
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
switch (action) {
case MotionEvent.ACTION_UP:
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
if ((viewFlags & TOOLTIP) == TOOLTIP) {
handleTooltipUp();
}
if (!clickable) {
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
}
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
// take focus if we don't have it already and we should in
// touch mode.
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}
即view可点击进入switch判断具体DOWN、UP等事件，重点看下这个
public boolean performClick() {
final boolean result;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
notifyEnterOrExitForAutoFillIfNeeded(true);
return result;
}
根据代码我们知道只要我们通过setOnClickListener（）为控件View注册1个点击事件，那么就会给mOnClickListener变量赋值（即不为空），则会往下回调onClick（） & performClick（）返回true。即调用onTouch事件要调用performClick事件，当这些执行完才能执行我们常见的onClick事件，至此，事件分发结束。
核心结论
事件逐层分发，判断是否拦截，拦截就本层消费，否则向下分发，直至被消费。
写在最后
---------------------
作者：月薪低于10k不改名
来源：CSDN
原文：https://blog.csdn.net/weixin_38703938/article/details/81906880
版权声明：本文为博主原创文章，转载请附上博文链接！
浙公网安备 33010602011771号