前言
關(guān)于手勢(shì)分發(fā)的機(jī)制的講解券敌，網(wǎng)上的文章可以說(shuō)是一大堆唾戚。有些流程介紹的非常詳細(xì)，分析的也很精彩待诅，但是或許是本人記憶力不行的緣故叹坦，每次看完過(guò)段時(shí)間，又會(huì)遺忘掉一部分卑雁。有些則是潦草的描述過(guò)分發(fā)流程募书，給人一種非常空洞的感覺(jué)序厉。
概述
本篇文章不會(huì)花大量筆墨描述整個(gè)流程锐膜，只會(huì)找?guī)讉€(gè)關(guān)鍵的地方來(lái)解釋流程為什么會(huì)如此分發(fā)，那么我們首先回顧下整個(gè)分發(fā)流程(Activity-ViewGroup-View):
↑從上圖可以看出來(lái)ViewGroup里面的事件分發(fā)最為繁瑣弛房，所以今天這篇文章主要詳解ViewGroup里面的事件分發(fā)。
ViewGroup里面主要關(guān)注下面幾個(gè)參數(shù)及方法：
- intercepted:
是一個(gè)flag,代表是否攔截的標(biāo)記而柑，當(dāng)為true時(shí)文捶，代表父View已經(jīng)攔截手勢(shì)，子View將不會(huì)執(zhí)行dispatchTouchEvent方法 - mFirstTouchTarget:
顧名思義媒咳，指的是第一個(gè)接收到觸摸事件的目標(biāo)粹排，當(dāng)這個(gè)目標(biāo)不為空的時(shí)候，就代表當(dāng)前View的子View已經(jīng)消費(fèi)了事件涩澡，當(dāng)前View和父View將不再執(zhí)行onTouchEvent方法 - dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, View child, int desiredPointerIdBits):
當(dāng)?shù)谌齻€(gè)參數(shù)傳入的是child對(duì)象時(shí)顽耳，將會(huì)執(zhí)行child的dispatchTouchEvent，將事件分發(fā)到子View去妙同；當(dāng)傳入的是null是射富，執(zhí)行當(dāng)前View的super.dispatchTouchEvent然后執(zhí)行自己的onTouchEvent。換而言之粥帚，這個(gè)方法如果執(zhí)行自己的onTouchEvent胰耗，那么子View將不再執(zhí)行dispatchTouchEvent即事件被當(dāng)前View所攔截
我們需要關(guān)注以下幾個(gè)問(wèn)題：
- intercept什么時(shí)候?yàn)閠rue，什么時(shí)候不為true
- mFirstTouchTarget什么時(shí)候不為null芒涡，什么時(shí)候?yàn)閚ull
- dispatchTransformedTouchEvent什么時(shí)候傳child柴灯，什么時(shí)候傳null卖漫。
- 為什么View沒(méi)有消費(fèi)事件以后，View就無(wú)法接收后續(xù)事件
- 為什么View消費(fèi)事件以后赠群，所有的父級(jí)View的onTouchEvent將不會(huì)接收后續(xù)事件
(以下方法都在ViewGroup的dispatchTouchEvent方法里面)
第一個(gè)問(wèn)題：
// Check for interception.
final boolean intercepted;
//當(dāng)手勢(shì)是Down或者mFirstTouchTarget不為空時(shí)羊始，會(huì)進(jìn)入onInterceptTouchEvent事件
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;
}
直接跳過(guò)前面的幾十行代碼，來(lái)看上面這段代碼查描。從代碼可知intercepted默認(rèn)值是false店枣，要想intercepted為true，則只要滿足以下兩個(gè)條件之一即可:
- onInterceptTouchEvent返回true叹誉，即ViewGroup實(shí)現(xiàn)手勢(shì)攔截
- 手勢(shì)類型為Down以外的手勢(shì)鸯两，例如UP/MOVE等等，并且mFirstTouchTarget不為空的時(shí)候
這里我們先不管mFirstTouchTarget什么時(shí)候不為空长豁【疲總之，我們可以知道m(xù)FirstTouchTarget一旦不為空那么intercepted就會(huì)為true匠襟。
if (!canceled && !intercepted) {
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;
}
}
從上面這段代碼可知钝侠，一旦intercepted為true，那么就不會(huì)執(zhí)行dispatchTransformedTouchEvent這個(gè)方法
總結(jié)一下：mFirstTouchTarget不為空時(shí)酸舍，intercepted為false帅韧，從而會(huì)執(zhí)行dispatchTransformedTouchEvent
第二個(gè)問(wèn)題
if (!canceled && !intercepted) {
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;
}
}
/**
* Adds a touch target for specified child to the beginning of the list.
* Assumes the target child is not already present.
*/
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
target.next = mFirstTouchTarget;
mFirstTouchTarget = target;
return target;
}
還是這段代碼，當(dāng)intercepted為false的時(shí)候啃勉，才會(huì)進(jìn)入dispatchTransformedTouchEvent方法忽舟，只有當(dāng)這個(gè)方法為true的時(shí)候才會(huì)進(jìn)if條件里面，至于dispatchTransformedTouchEvent里面怎么處理先不管淮阐，而addTouchTarget方法就是設(shè)置mFirstTouchTarget叮阅。
總結(jié)一下：當(dāng)intercepted為false且dispatchTransformedTouchEvent為true的時(shí)候，mFirstTouchTarget賦值泣特。其他情況下浩姥，mFirstTouchTarget都為null
第三個(gè)問(wèn)題
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;
}
//代碼省略...
}
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);
}
}
總結(jié)一下：結(jié)合第二個(gè)問(wèn)題，可以看出mFirstTarget不為空時(shí)或者intercepted為true時(shí)状您，第三個(gè)參數(shù)傳child勒叠，當(dāng)mFirstTarget為空時(shí)，傳null膏孟。
第四個(gè)問(wèn)題
我們?cè)賮?lái)過(guò)一遍ViewGroup的dispatchTouchEvent方法
1.首先ViewGroup肯定是先接收Down事件眯分，一旦onInterceptTouchEvent沒(méi)有做攔截，那么intercepted必然為false骆莹。
那么
if (!canceled && !intercepted) {
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
}
}
這個(gè)條件一定會(huì)進(jìn)入颗搂，然后會(huì)執(zhí)行dispatchTransformedTouchEvent方法，此時(shí)傳入了child幕垦，那么會(huì)執(zhí)行子view的dispatchTouchEvent丢氢，然后執(zhí)行子View的onTouchEvent傅联，如果不做任何消費(fèi)，那么dispatchTransformedTouchEvent會(huì)返回false疚察，所以
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign))
這個(gè)條件進(jìn)不去蒸走，根據(jù)第二個(gè)問(wèn)題，那么mFirstTouchTarget就為null貌嫡。
2.然后后續(xù)事件進(jìn)入時(shí)比驻，根據(jù)第一個(gè)問(wèn)題可知intercepted為true(后續(xù)事件肯定不為DOWN事件)，再根據(jù)第二個(gè)問(wèn)題可知
if (!canceled && !intercepted)
這個(gè)if條件不會(huì)進(jìn)去岛抄，那么mFirstTouchTarget依然為null别惦，根據(jù)第三個(gè)問(wèn)題可知，會(huì)執(zhí)行dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS)方法夫椭，因?yàn)閭魅氲氖莕ull,所以會(huì)執(zhí)行super.dispatchTouchEvent掸掸，子View的dispatchTouchEvent將不會(huì)再執(zhí)行。因此子View將不會(huì)接收到后續(xù)事件了蹭秋。
第五個(gè)問(wèn)題
1.根據(jù)第四個(gè)問(wèn)題扰付，進(jìn)入的子View的onTouchEvent事件以后，如果onTouchEvent消費(fèi)事件仁讨，那么ViewGroup#dispatchTransformedTouchEvent會(huì)返回true羽莺，那么mFirstTouchTarget將會(huì)設(shè)置
2.然后后續(xù)事件進(jìn)入時(shí)，根據(jù)第一個(gè)問(wèn)題,可知intercepted為true并且此時(shí)父類已經(jīng)無(wú)法攔截事件洞豁。根據(jù)第二個(gè)問(wèn)題可知
if (!canceled && !intercepted)
這個(gè)if條件不會(huì)進(jìn)去盐固，但是此時(shí)mFirstTouchTarget已經(jīng)不為null，所以根據(jù)第三個(gè)問(wèn)題可知將會(huì)執(zhí)行dispatchTransformedTouchEvent(ev, cancelChild, target.child, target.pointerIdBits)方法族跛，傳入了child闰挡，所以會(huì)執(zhí)行子View的dispatchTouchEvent->onTouchEvent。而且父ViewGroup將不會(huì)再執(zhí)行super.dispatchTouchEvent->onTouchEvent礁哄。
總結(jié)
總的來(lái)說(shuō)，看事件消費(fèi)還是攔截主要看dispatchTransformedTouchEvent傳入的是child還是null溪北，而這個(gè)條件主要是靠mFirstTouchTarget和intercepted共同決定桐绒，只要掌握了這三個(gè)參數(shù)及方法，那么其他的手勢(shì)分發(fā)將沒(méi)有任何問(wèn)題
