Android事件分发与责任链模式，
一、责任链模式
责任链模式是一种行为模式,为请求创建一个接收者的对象链.这样就避免,一个请求链接多个接收者的情况.进行外部解耦.类似于单向链表结构。
优点：
1. 降低耦合度。它将请求的发送者和接收者解耦。
2. 简化了对象。使得对象不需要知道链的结构。
3. 增强给对象指派职责的灵活性。通过改变链内的成员或者调动它们的次 序，允许动态地新增或者删除责任。
4. 增加新的请求处理类很方便。
缺点：
1. 不能保证请求一定被接收。
2. 系统性能将受到一定影响，而且在进行代码调试时不太方便，可能会造成循环调用。
3. 可能不容易观察运行时的特征，有碍于除错。
责任链的模式在事件分发场景方面的原理：
一般我们理解的事件分发的模式如下（传统模式）：
使用责任链模式直接将message丢到链中,让他们自己匹配.
二、Android 事件分发传递机制
1. View事件传递分发层级结构
a). 事件收集之后最先传递给 Activity， 然后依次向下传递，大致如下：
Activity －> PhoneWindow －> DecorView －> ViewGroup －> ... －> View
这样的事件分发机制逻辑非常清晰，可是，你是否注意到一个问题？如果最后分发到View，如果这个View也没有处理事件怎么办，就这样让事件浪费掉？当然不会啦。
b). 如果没有任何View消费掉事件，那么这个事件会按照反方向回传，最终传回给Activity，如果最后 Activity 也没有处理，本次事件才会被抛弃:
Activity <－ PhoneWindow <－ DecorView <－ ViewGroup <－ ... <－ View
可以看到，这是一个非常经典的责任链模式，如果我能处理就拦截下来自己干，如果自己不能处理或者不确定就交给责任链中下一个对象。 这种设计是非常精巧的，上层View既可以直接拦截该事件，自己处理，也可以先询问(分发给)子View，如果子View需要就交给子View处理，如果子View不需要还能继续交给上层View处理。既保证了事件的有序性，又非常的灵活。
View点击事件分发有三个关键流程方法:
1.dispatchTouchEvent：事件下发 --- View和ViewGroup都有的方法
2.onInterceptTouchEvent：拦截下发的事件,并交给自己OnTouchEvent处理处理 ---ViewGroup才有的方法
3.onTouchEvent：事件上报 --- View和ViewGroup都有的方法
以下是不同层级对事件的分发、拦截和消费的功能表：
可以看到 Activity 和 View 都是没有事件拦截的：
a). Activity 作为原始的事件分发者，如果 Activity 拦截了事件会导致整个屏幕都无法响应事件，这肯定不是我们想要的效果。
b). View最为事件传递的最末端，要么消费掉事件，要么不处理进行回传，根本没必要进行事件拦截。
下图是点击View，事件传递但是都没有被处理，生成的一个完整的事件分发流程图：
如果事件被View处理了，那么事件分发流程图应该如下：
如果事件被ViewGroup拦截处理了， 那么事件分发流程图应该如下：
从上面的流程，我们可以概括Android的事件分发机制为：责任链模式，事件层层传递，直到被消费。
三、Q&A
上面我们讲解了一下Android的事件分发机制，可能很多人会有疑惑，下面我们针对部分疑惑进行分析和说明：
1. 为什么 View 会有 dispatchTouchEvent ?
答：我们知道 View 可以注册很多事件监听器，例如：单击事件(onClick)、长按事件(onLongClick)、触摸事件(onTouch)，并且View自身也有 onTouchEvent 方法，那么问题来了，这么多与事件相关的方法应该由谁管理？毋庸置疑就是 dispatchTouchEvent，所以 View 也会有事件分发。
View的dispatchTouchEvent源码：
![]()
/**
* Pass the touch screen motion event down to the target view, or this
* view if it is the target.
*
* @param event The motion event to be dispatched.
* @return True if the event was handled by the view, false otherwise.
*/
public boolean dispatchTouchEvent(MotionEvent event) {
// If the event should be handled by accessibility focus first.
if (event.isTargetAccessibilityFocus()) {
// We don't have focus or no virtual descendant has it, do not handle the event.
if (!
isAccessibilityFocusedViewOrHost()) {
return false;
}
// We have focus and got the event, then use normal event dispatch.
event.setTargetAccessibilityFocus(
false);
}
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;
}
}
if (!result && mInputEventConsistencyVerifier !=
null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0
);
}
// Clean up after nested scrolls if this is the end of a gesture;
// also cancel it if we tried an ACTION_DOWN but we didn't want the rest
// of the gesture.
if (actionMasked == MotionEvent.ACTION_UP ||
actionMasked == MotionEvent.ACTION_CANCEL ||
(actionMasked == MotionEvent.ACTION_DOWN && !
result)) {
stopNestedScroll();
}
return result;
}
View Code
2. View事件分发时各个方法调用顺序是怎样的？
a). 单击事件(onClickListener) 需要两个两个事件(ACTION_DOWN 和 ACTION_UP )才能触发，如果先分配给onClick判断，等它判断完再交由其他相应时间显然是不合理的，会造成 View 无法响应其他事件，应该最后调用。(所以此调用顺序最后)
b). 长按事件(onLongClickListener) 同理，也是需要长时间等待才能出结果，肯定不能排到前面，但因为不需要ACTION_UP，应该排在 onClick 前面。(onLongClickListener > onClickListener)
c). 触摸事件(onTouchListener) 如果用户注册了触摸事件，说明用户要自己处理触摸事件了，这个应该排在最前面。(最前)
d). View自身处理(onTouchEvent) 提供了一种默认的处理方式，如果用户已经处理好了，也就不需要了，所以应该排在 onClickListener 后面。(onTouchListener > onClickListener)
所以事件的调度顺序应该是 onTouchListener > onTouchEvent > onLongClickListener > onClickListener。
3. ViewGroup 的事件分发流程又是如何的呢？
在默认的情况下 ViewGroup 事件分发流程是这样的。
a). 判断自身是否需要(询问 onInterceptTouchEvent 是否拦截)，如果需要，调用自己的 onTouchEvent。
b). 自身不需要或者不确定，则询问 ChildView ，一般来说是调用手指触摸位置的 ChildView。
c). 如果子 ChildView 不需要则调用自身的 onTouchEvent。
ViewGroup的dispatchTouchEvent源码：
![]()
@Override
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;
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;
}
// If intercepted, start normal event dispatch. Also if there is already
// a view that is handling the gesture, do normal event dispatch.
if (intercepted || mFirstTouchTarget !=
null) {
ev.setTargetAccessibilityFocus(false);
}
// Check for cancelation.
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;
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
;
}
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();
}
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;
}
}
}
// 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);
} 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 {
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.
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;
}
View Code
4. ViewGroup将事件分发给ChildView的机制
ViewGroup分发事件时会遍历 ChildView，如果手指触摸的点在 ChildView 区域内就分发给这个View。当 ChildView 重叠时，一般会分配给显示在最上面的 ChildView。
5. ViewGroup 和 ChildView 同时注册了事件监听器(onClick等)，哪个会执行?
事件优先给 ChildView，会被 ChildView消费掉，ViewGroup 不会响应。
附：参考资料：
1. Android事件传递机制分析
2. Android 事件分发机制详解
3. 安卓自定义View进阶-事件分发机制原理
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