文章目录
一.概述
1.事件的分发概述
View对事件的分发即系统把MotionEvent事件传递给一个具体的View的过程。
这其中涉及三个重要方法：dispatchTouchEvent,onInterceptTouchEvent和onTouchEvent。
2.三个重要方法
3.基本传递过程
当一个点击事件产生后，一般顺序为先传递到Activity，在传到ViewGroup（这之前会传递到window），最终传到View。
二.事件分发的源码分析
1.Activity对事件的分发
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction(); //这是个空方法
}
if (getWindow().superDispatchTouchEvent(ev)) { //此处调用了Window类的抽象方法
return true;
}
return onTouchEvent(ev);
}
因为Window是抽象类，因此我们寻找它的实现类PhoneWindow，找到相应的superDispatchTouchEvent方法。
public boolean superDispatchTouchEvent(MotionEvent event) {
//Decor即是DecorView类，所以Window将事件传递给了DecorView
return mDecor.superDispatchTouchEvent(event);
}
再进入Decor的superDispatchTouchEvent方法，如下：
public boolean superDispatchTouchEvent(MotionEvent event) {
//因为Decor是FrameLayout的子类，FrameLayout是ViewGroup的子类，而FrameLayout中没有相应方法，因此下面一句代码调用了ViewGroup中的方法
//至此，事件就从Activity传递到了ViewGroup
return super.dispatchTouchEvent(event);
}
2.ViewGroup对事件的分发
ViewGroup对事件的分发可以用下面的伪代码表示：
public boolen dispatchTouchEvent（）{
boolen consume = false；
if(onInterceptTouchEvent){
consume = onTouchEvent(ev); //拦截事件，则调用当前viewGroup的onTouchEvent
}else{
consume = child.dispatchTouchEvent(); //否则，调用子view的dispatchTouchEvent，即事件传递给子view
}
return consume;
}
ViewGroup中的dispatchTouchEvent方法有点长，我们慢慢分析
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
// If the event targets the accessibility focused view and this is it, start
// normal event dispatch. Maybe a descendant is what will handle the click.
if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
ev.setTargetAccessibilityFocus(false);
}
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; //用来判断是否拦截事件
//mFirstTouchTarget :当有子元素成功处理事件后，mFirstTouchTarget会被赋值并指向子元素。
//mFirstTouchTarget!=null 即表示ViewGroup表示未拦截当前事件
if (actionMasked == MotionEvent.ACTION_DOWN //事件为“按下”
|| mFirstTouchTarget != null) {
//下面的disallowIntercept的值可由FLAG_DISALLOW_INTERCEPT更改
//更改方法：子View调用requestDisallowInterceptTouchEvent方法
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) { //允许ViewGroup拦截事件
intercepted = onInterceptTouchEvent(ev); //此方法中默认不拦截事件
ev.setAction(action); // restore action in case it was changed
} else { //不允许拦截事件
intercepted = false;
}
} else { //ViewGroup已经拦截过事件，则继续拦截事件
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
intercepted = true;
}
// If intercepted, start normal event dispatch. Also if there is already
// a view that is handling the gesture, do normal event dispatch.
//如果拦截，则正常分发事件。另外，如果已经有一个View正在处理事件，则执行正常事件分发。
if (intercepted || mFirstTouchTarget != null) {
ev.setTargetAccessibilityFocus(false);
}
// Check for cancelation. 检查取消事件，不做过多分析
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;
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;
//此处开始对子View进行遍历
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(
preorderedList, children, 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;
}
//子View不能收到事件或Touch的坐标不在其范围内，直接跳到下一个子View
if (!child.canReceivePointerEvents()
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
//获取child对应的Touch Target
newTouchTarget = getTouchTarget(child);
if (newTouchTarget != null) {
// Child is already receiving touch within its bounds. child在它的范围内已经获取了touch事件
// Give it the new pointer in addition to the ones it is handling. 除了它正在处理的那个，将新的指针赋给它
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
resetCancelNextUpFlag(child);
//在dispatchTransformedTouchEvent方法内部，如果child不为null，就会调用child的dispatchTouchEvent方法
//至此，事件就传递给了下一层View
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();
// 如果处理掉了事件，将此child添加到touch链的头部
//在addTouchTarget方法中更新了mFirstTouchTarget的值
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();
}
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;
}
}
}
// 非down事件直接从这里开始处理，不会走上面的一大堆寻找TouchTarget的逻辑
// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
//没有child处理事件，因此ViewGroup自己进行处理
// No touch targets so treat this as an ordinary view.
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.
//下面的代码作用是将事件分发给touch target。如果已经分发，则不分发给新的touch target。如果必要，则取消touch targets
TouchTarget predecessor = null;
TouchTarget target = mFirstTouchTarget;
//target是一个链表，下面开始遍历
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. 如果需要，更新touch targets的链表
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;
}
3.View对事件的分发
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;
//下面判断了该View是否设置了OnTouchListener
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
//如果该View设置OnTouchListener事件 ，则onTouchEvent就不会被调用，因此OnTouchListener比onTouchEvent优先级高
if (!result && onTouchEvent(event)) {
result = true;
}
}
...
return result;
}
下面逐步View的onTouchEvent方法中的代码：
final int viewFlags = mViewFlags;
final int action = event.getAction();
//判断是否可点击
final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;
if ((viewFlags & ENABLED_MASK) == DISABLED) {
//由下面的代码可见，不可点击的View仍然会消耗点击事件，尽管它不会对事件进行反应。
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return clickable;
}
下面是onTouchEvent对事件的具体处理：
//clickable表明该View可以点击(CLICKABLE,LONG_CLICKABLE为true)，它为true时
final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;
...
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
switch (action) {
case MotionEvent.ACTION_UP:
...
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
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();
}
if (!post(mPerformClick)) {
//以前会直接调用performClick方法，现在先调用下面的performClickInternal方法，
//是为了通知autofill manager，然后再在其中调用了performClick方法
performClickInternal();
}
}
}
...
}
break;
}
//clickable为true时都会返回true，表示消耗此事件
return true;
}
三.小结
事件从Activity经ViewGroup到View的传递过程可用如下图表示，黄框表示事件从此处传递给了下一级。
四.参考资料
《安卓开发艺术探索》
Android之View篇2————View的事件分发
Android事件分发机制详解：史上最全面、最易懂
