一阱驾、責(zé)任鏈模式
責(zé)任鏈模式是一種行為模式,為請(qǐng)求創(chuàng)建一個(gè)接收者的對(duì)象鏈.這樣就避免,一個(gè)請(qǐng)求鏈接多個(gè)接收者的情況.進(jìn)行外部解耦.類似于單向鏈表結(jié)構(gòu)攒驰。
優(yōu)點(diǎn)：
1. 降低耦合度。它將請(qǐng)求的發(fā)送者和接收者解耦。
2. 簡(jiǎn)化了對(duì)象瑟啃。使得對(duì)象不需要知道鏈的結(jié)構(gòu)。
3. 增強(qiáng)給對(duì)象指派職責(zé)的靈活性揩尸。通過(guò)改變鏈內(nèi)的成員或者調(diào)動(dòng)它們的次 序蛹屿，允許動(dòng)態(tài)地新增或者刪除責(zé)任。
4. 增加新的請(qǐng)求處理類很方便岩榆。
缺點(diǎn)：
1. 不能保證請(qǐng)求一定被接收错负。
2. 系統(tǒng)性能將受到一定影響，而且在進(jìn)行代碼調(diào)試時(shí)不太方便勇边，可能會(huì)造成循環(huán)調(diào)用犹撒。
3. 可能不容易觀察運(yùn)行時(shí)的特征，有礙于除錯(cuò)粒褒。
在Android事件分發(fā)機(jī)制是責(zé)任鏈模式最典型的應(yīng)用：
dispatchTouchEvent的,就是責(zé)任鏈中的將事件交給下一級(jí)處理的.
onInterceptTouchEvent ,就是責(zé)任鏈中,處理自己處理事務(wù)的方法.
onTouchEvent 是責(zé)任鏈中 事件上報(bào)的事件鏈识颊。
下面我們來(lái)通過(guò)Android事件分發(fā)機(jī)制來(lái)感悟一下責(zé)任鏈模式在Android中的應(yīng)用。
二奕坟、Android 事件分發(fā)傳遞機(jī)制
1. View事件傳遞分發(fā)層級(jí)結(jié)構(gòu)
a). 事件收集之后最先傳遞給 Activity祥款， 然后依次向下傳遞，大致如下：
Activity －> PhoneWindow －> DecorView －> ViewGroup －> ... －> View
這樣的事件分發(fā)機(jī)制邏輯非常清晰月杉，可是刃跛，你是否注意到一個(gè)問(wèn)題？如果最后分發(fā)到View沙合，如果這個(gè)View也沒(méi)有處理事件怎么辦，就這樣讓事件浪費(fèi)掉跌帐？當(dāng)然不會(huì)啦首懈。
b). 如果沒(méi)有任何View消費(fèi)掉事件，那么這個(gè)事件會(huì)按照反方向回傳谨敛，最終傳回給Activity究履，如果最后 Activity 也沒(méi)有處理，本次事件才會(huì)被拋棄:
Activity <－ PhoneWindow <－ DecorView <－ ViewGroup <－ ... <－ View
可以看到脸狸，這是一個(gè)非常經(jīng)典的責(zé)任鏈模式最仑，如果我能處理就攔截下來(lái)自己干，如果自己不能處理或者不確定就交給責(zé)任鏈中下一個(gè)對(duì)象炊甲。 這種設(shè)計(jì)是非常精巧的泥彤，上層View既可以直接攔截該事件，自己處理卿啡，也可以先詢問(wèn)(分發(fā)給)子View吟吝，如果子View需要就交給子View處理，如果子View不需要還能繼續(xù)交給上層View處理颈娜。既保證了事件的有序性剑逃，又非常的靈活浙宜。
View點(diǎn)擊事件分發(fā)有三個(gè)關(guān)鍵流程方法:
1.dispatchTouchEvent：事件下發(fā) --- View和ViewGroup都有的方法
2.onInterceptTouchEvent：攔截下發(fā)的事件,并交給自己OnTouchEvent處理處理 ---ViewGroup才有的方法
3.onTouchEvent：事件上報(bào) --- View和ViewGroup都有的方法
以下是不同層級(jí)對(duì)事件的分發(fā)、攔截和消費(fèi)的功能表：
可以看到 Activity 和 View 都是沒(méi)有事件攔截的：
a). Activity 作為原始的事件分發(fā)者蛹磺，如果 Activity 攔截了事件會(huì)導(dǎo)致整個(gè)屏幕都無(wú)法響應(yīng)事件粟瞬，這肯定不是我們想要的效果。
b). View最為事件傳遞的最末端萤捆，要么消費(fèi)掉事件裙品，要么不處理進(jìn)行回傳，根本沒(méi)必要進(jìn)行事件攔截鳖轰。
下圖是點(diǎn)擊View清酥，事件傳遞但是都沒(méi)有被處理，生成的一個(gè)完整的事件分發(fā)流程圖：
如果事件被View處理了蕴侣，那么事件分發(fā)流程圖應(yīng)該如下：
如果事件被ViewGroup攔截處理了焰轻， 那么事件分發(fā)流程圖應(yīng)該如下：
從上面的流程，我們可以概括Android的事件分發(fā)機(jī)制為：責(zé)任鏈模式昆雀，事件層層傳遞辱志，直到被消費(fèi)。
三狞膘、Q&A
上面我們講解了一下Android的事件分發(fā)機(jī)制揩懒，可能很多人會(huì)有疑惑，下面我們針對(duì)部分疑惑進(jìn)行分析和說(shuō)明：
1. 為什么 View 會(huì)有 dispatchTouchEvent ?
答：我們知道 View 可以注冊(cè)很多事件監(jiān)聽器挽封，例如：?jiǎn)螕羰录?onClick)已球、長(zhǎng)按事件(onLongClick)、觸摸事件(onTouch)辅愿，并且View自身也有 onTouchEvent 方法智亮，那么問(wèn)題來(lái)了，這么多與事件相關(guān)的方法應(yīng)該由誰(shuí)管理点待？毋庸置疑就是 dispatchTouchEvent阔蛉，所以 View 也會(huì)有事件分發(fā)。
View的dispatchTouchEvent源碼：
/**
* 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事件分發(fā)時(shí)各個(gè)方法調(diào)用順序是怎樣的癞埠？
a). 單擊事件(onClickListener) 需要兩個(gè)兩個(gè)事件(ACTION_DOWN 和 ACTION_UP )才能觸發(fā)状原，如果先分配給onClick判斷，等它判斷完再交由其他相應(yīng)時(shí)間顯然是不合理的苗踪，會(huì)造成 View 無(wú)法響應(yīng)其他事件颠区，應(yīng)該最后調(diào)用。(所以此調(diào)用順序最后)
b). 長(zhǎng)按事件(onLongClickListener) 同理通铲，也是需要長(zhǎng)時(shí)間等待才能出結(jié)果瓦呼，肯定不能排到前面，但因?yàn)椴恍枰狝CTION_UP，應(yīng)該排在 onClick 前面央串。(onLongClickListener > onClickListener)
c). 觸摸事件(onTouchListener) 如果用戶注冊(cè)了觸摸事件磨澡，說(shuō)明用戶要自己處理觸摸事件了，這個(gè)應(yīng)該排在最前面质和。(最前)
d). View自身處理(onTouchEvent) 提供了一種默認(rèn)的處理方式稳摄，如果用戶已經(jīng)處理好了，也就不需要了饲宿，所以應(yīng)該排在 onClickListener 后面厦酬。(onTouchListener > onClickListener)
所以事件的調(diào)度順序應(yīng)該是 onTouchListener > onTouchEvent > onLongClickListener > onClickListener。
3. ViewGroup 的事件分發(fā)流程又是如何的呢瘫想？
在默認(rèn)的情況下 ViewGroup 事件分發(fā)流程是這樣的仗阅。
a). 判斷自身是否需要(詢問(wèn) onInterceptTouchEvent 是否攔截)，如果需要国夜，調(diào)用自己的 onTouchEvent减噪。
b). 自身不需要或者不確定，則詢問(wèn) ChildView 车吹，一般來(lái)說(shuō)是調(diào)用手指觸摸位置的 ChildView筹裕。
c). 如果子 ChildView 不需要?jiǎng)t調(diào)用自身的 onTouchEvent。
ViewGroup的dispatchTouchEvent源碼：
@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;
}
可能看這么長(zhǎng)的代碼窄驹，我們會(huì)比較懵朝卒，下面我們用偽代碼來(lái)表示一下：
public boolean dispatchTouchEvent(MotionEvent ev) {
boolean consume = false;
if (onInterceptTouchEvent(ev)) {
consume = onTouchEvent(ev);
} else {
consume = child.dispatchTouchEvent(ev)
}
return consume;
}
這樣我們就能更直白的看懂View事件的傳遞機(jī)制了。
4. ViewGroup將事件分發(fā)給ChildView的機(jī)制
- ViewGroup分發(fā)事件時(shí)會(huì)遍歷 ChildView乐埠，如果手指觸摸的點(diǎn)在 ChildView 區(qū)域內(nèi)就分發(fā)給這個(gè)View抗斤。當(dāng) ChildView 重疊時(shí)，一般會(huì)分配給顯示在最上面的 ChildView丈咐。
- ViewGroup判斷是否需要攔截瑞眼，主要是根據(jù)onInterruptTouchEvent的返回值進(jìn)行判斷。
- 在Down事件中將touch事件分發(fā)給ChildView扯罐，如果有ChildView捕獲消費(fèi)了Down事件负拟，就會(huì)對(duì)mFirstTouchTarget進(jìn)行賦值烦衣。mFirstTouchTarget的作用就是記錄消費(fèi)事件的View歹河。
- 在ViewGroup的dispatchTouchEvent方法中，會(huì)根據(jù)mFirstTouchTarget 是否為 null花吟，決定是自己處理 touch 事件秸歧，還是分發(fā)給子 View。
- Down事件是touch事件序列的起點(diǎn)衅澈，決定了后續(xù)的事件由誰(shuí)來(lái)消費(fèi)處理键菱。Cancel事件的觸發(fā)場(chǎng)景為：父View先不攔截，但在MOVE事件中又重新攔截今布，此時(shí)子View會(huì)收到一個(gè)Cancel事件经备，
5. ViewGroup 和 ChildView 同時(shí)注冊(cè)了事件監(jiān)聽器(onClick等)拭抬，哪個(gè)會(huì)執(zhí)行?
事件優(yōu)先給 ChildView，會(huì)被 ChildView消費(fèi)掉侵蒙，ViewGroup 不會(huì)響應(yīng)造虎。
