前言
上节我们已经说到事件最终传递给了DecorView,而DecorView是View的子类,到了这里，而这也到了我们最关心的部分，View及ViewGroup是怎么对事件进行处理的，接下来我将继续讲解这部分知识。
简述
在View和ViewGroup中和事件分发相关的方法为dispatchTouchEvent，onInterceptTouchEvent和onTouchEvent，下面我先简单的描述下着三个方法:
-
dispatchTouchEvent:用于传递和分发事件，一般不重写此方法，其返回值由当前View和其子View决定. -
onInterceptTouchEvent:在dispatchTouchEvent被调用，用于判断该ViewGroup是拦截当前事件或继续传递,在同一事件序列(即ACTION_DOWN至ACTION_UP)中，该方法不应该不重复调用, -
onTouchEvent:此方法用于处理事件,返回值为true时则处理此事件，否则返回false.
事件传递方式
当事件传递到dispatchTouchEvent的时候，先调用onInterceptTouchEvent判断是否消费该事件，如果消费该事件，本事件序列将不再传递给子View，而是传递给当前View的onTouchEvent方法，如果判断不消费该事件，则将事件继续往下传递，若子View不处理该事件，子View的传递方式依此类推，若最后一个View仍不处理该事件，事件将逐级返回，最后传递给Activity.
源码分析
首先我们先看下dispatchTouchEvent中的代码(由于代码量较大,这里将使用代码片段的方式):
// 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();
}
在上面的代码中，我们可以看出，当Event的Action为ACTION—DOWN的时候，会重置状态，接受新的事件序列;
// Check for interception.
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;
}
接着，在这里，我们看到当调用了onInterceptTouchEvent方法,接下来我们追踪到onInterceptTouchEvent中去看下.
public boolean onInterceptTouchEvent(MotionEvent ev) {
return false;
}
在这里,我们可以看到，此方法直接返回了false，也就是说，ViewGroup默认是不拦截事件的，这中设计我们也应该明白，因为ViewGroup是父级控件，如果拦截了事件的话，事件将如法继续往子控件分发(LinearLayout和RelativeLayout中也没有重写该方法,则可以体现这一思想）,如果我们在自定义View的时候需要处理是否拦截事件的时候，可以重写该方法，去判断是否拦截.接着，回到dispatchTouchEvent，我们继续追踪:
if (!canceled && !intercepted) {
// If the event is targeting accessiiblity 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 = buildOrderedChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = customOrder
? getChildDrawingOrder(childrenCount, i) : i;
final View child = (preorderedList == null)
? children[childIndex] : preorderedList.get(childIndex);
// If there is a view that has accessibility focus we want it
// to get the event first and if not handled we will perform a
// normal dispatch. We may do a double iteration but this is
// safer given the timeframe.
if (childWithAccessibilityFocus != null) {
if (childWithAccessibilityFocus != child) {
continue;
}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
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();
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;
}
// 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();
}
在这里，我们可以看到，遍历了下级View，然后判断子View是否能接收点击事件，然后调用dispatchTransformedTouchEvent方法.接着我们去查看下dispatchTransformedTouchEvent中的实现:
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);
}
在这里我们看到，这里将事件传递给了子View，接着，我们回到dispatchTouchEvent方法中:
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;
}
在这里，我们可以看到,当子View愿意接收事件时,则跳出循环，等等，难道不需要做点别的吗?当然是有的，在这里，会调用addTouchTarget
接下来，我们看下addTouchTarget方法：
private TouchTarget addTouchTarget(View child, int pointerIdBits) {
TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
target.next = mFirstTouchTarget;
mFirstTouchTarget = target;
return target;
}
在这里，将取出来出来的TouchTarget赋给了mFirstTouchTarget，并返回，赋值给newTouchTarget，接着，我们回到dispatchTouchEvent的方法中:
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;
}
在这里，我们看到，当newTouchTarget为null，也就是没有子View去接收事件的时候，会将事件派发给最近一次目标.接着:
// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
}
如上代码所示，如果没有接收的目标View的时候，则会继续调用dispatchTransformedTouchEvent方法，而且View传了一个null值，而根据dispatchTransformedTouchEvent的逻辑，在View为null的时候，则会调用super.dispatchTouchEvent即View的dispatchTouchEvent方法，接下来，我们就去看下
View中dispatchTouchEvent的源码:
//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;
}
在这里我们可以看出,这里会先判断是否有mOnTouchListener，如果与的话，则先将事件交给mOnTouchListener去处理，若OnTouchListener不愿意消费该事件，或者没有OnTouchListener时，则将事件传递给onTouchEvent，接着我们继续追踪onTouchEvent中的实现:
if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return (((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);
}
从上面的代码可以看出，当View的状态为DISABLED状态时，也是会消费事件的，只是没有什么效果，在这里不是我们关注的重点，接下来我们继续往下看:
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
如上代码所示，如果View设置了代理,事件将传递给代理，将会调用代理的onTouchEvent如果代理选择消费事件，则返回true，我们接着看:
if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
(viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
switch (action) {
case MotionEvent.ACTION_UP:
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();
}
if (prepressed) {
// The button is being released before we actually
// showed it as pressed. Make it show the pressed
// state now (before scheduling the click) to ensure
// the user sees it.
setPressed(true, x, y);
}
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();
}
if (!post(mPerformClick)) {
performClick();
}
}
}
if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}
if (prepressed) {
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_DOWN:
mHasPerformedLongPress = false;
if (performButtonActionOnTouchDown(event)) {
break;
}
// Walk up the hierarchy to determine if we're inside a scrolling container.
boolean isInScrollingContainer = isInScrollingContainer();
// For views inside a scrolling container, delay the pressed feedback for
// a short period in case this is a scroll.
if (isInScrollingContainer) {
mPrivateFlags |= PFLAG_PREPRESSED;
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPendingCheckForTap.x = event.getX();
mPendingCheckForTap.y = event.getY();
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true, x, y);
checkForLongClick(0);
}
break;
case MotionEvent.ACTION_CANCEL:
setPressed(false);
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_MOVE:
drawableHotspotChanged(x, y);
// Be lenient about moving outside of buttons
if (!pointInView(x, y, mTouchSlop)) {
// Outside button
removeTapCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
// Remove any future long press/tap checks
removeLongPressCallback();
setPressed(false);
}
}
break;
}
return true;
}
从上面的代码可以看出，只要View的CLICKABLE，LONG_CLICKABLE，CONTEXT_CLICKABLE有一个为true的时候，就消费该事件,接下来我们对看下具体实现:
ACTION_DOWN:
我们可以看到，在这里调用了performButtonActionOnTouchDown，这个方法是对鼠标点击事件的处理，在这里我们先不去关注它，接着：
-
mPrivateFlags |= PFLAG_PREPRESSED给mPrivateFlags添加一个PFLAG_PREPRESSED的flag； -
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout())检测Tap事件，ViewConfiguration.getTapTimeout()获取的值为100，到达延时时间后执行； - 我们看一下
CheckForTap中的实现:
private final class CheckForTap implements Runnable {
public float x;
public float y;
@Override
public void run() {
mPrivateFlags &= ~PFLAG_PREPRESSED;
setPressed(true, x, y);
checkForLongClick(ViewConfiguration.getTapTimeout());
}
}
这里取消了mPrivateFlags的PFLAG_PREPRESSED的设置，然后设置setPressed为true，更新点击状态，以便执行动画等，在checkForLongClick方法中，如果View支持长按，则检测是否为长按事件;
- 如果
View不在一个可滑动的ViewGroup中，则直接setPressed，在setPressed中会为mPrivateFlags设置一个PFLAG_PRESSED，然后执行checkForLongClick.
ACTION_MOVE:
- 如果手指移动到
View外部，就执行removeTapCallback方法:
private void removeTapCallback() {
if (mPendingCheckForTap != null) {
mPrivateFlags &= ~PFLAG_PREPRESSED;
removeCallbacks(mPendingCheckForTap);
}
}
这里将mPrivateFlags中的PFLAG_PREPRESSED移除，并将Tap事件的检测取消掉；
- 如果
mPrivateFlags重包含PFLAG_PRESSED，也就是说如果View是在不可滑动的父控件中时，则执行removeLongPressCallback方法，移除长按检测，并setPressed(false)，设置新的pressed状态.
ACTION_UP:
- 判断
mPrivateFlags是否包含PFLAG_PREPRESSED，当PFLAG_PREPRESSED包含``或者PFLAG_PRESSED时，都可以进入逻辑内部； - 如果在
prepressed为true，也就是Tap事件检测还有执行时，手指抬起，则执行setPressed去更新pressed状态为True； - 接着，如果还没有执行长按事件检测，则移除长按检测；
- 继续，如果
mPerformClick为null的话，则创建一个mPerformClick，然后post到队列中，如果失败的话，则执行performClick方法，到这一步，终于到了执行了Clic事件，如果设置有onclickListener，将会被调用； - 接着，如果
mUnsetPressedState为null的话，则创建一个UnsetPressedState，若prepressed为true，则延时执行mUnsetPressedState，否则将mUnsetPressedState直接post，如果失败则mUnsetPressedState.run()，也就是说，无论如何mUnsetPressedState都会执行，接下来我们看下其中的代码:
private final class UnsetPressedState implements Runnable {
@Override
public void run() {
setPressed(false);
}
}
这里，只执行了一个方法 setPressed(false)，在这个方法中，将会移除mPrivateFlags中的PFLAG_PRESSED，然后执行refreshDrawableState方法，并执行dispatchSetPressed将setPressed传递下去；
- 接着，会执行
removeTapCallback，移除TapCallback； - 最后会将
mIgnoreNextUpEvent状态置为false；
总结
至此，两节的事件分发已经告一段落，欢迎同学们指正，沟通，学习.
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