Android事件分发与责任链模式
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2022-06-22 10:58:01
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 ......
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;
}
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;
}
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：