View的事件分發(fā)和滑動沖突學(xué)習(xí)總結(jié)
前言
本文分兩個部分，第一部分會先過一遍事件分發(fā)機(jī)制的流程并做一些結(jié)論性的總結(jié),然后從源碼層面分析這些流程。第二部分會介紹滑動沖突問題的一些解決方案。查了比較多的資料,也有一些自己的看法,由于知識有限,差錯之處希望各位不吝指出。
View 的事件分發(fā)機(jī)制
簡介
當(dāng)一個MotionEvent產(chǎn)生了以后，系統(tǒng)需要把這個事件傳遞給一個具體的View,這個傳遞過程就是分發(fā)過程。這個過程由三個方法共同完成：
public boolean dispatchTouchEvent(MotionEvent ev)
這個方法用來進(jìn)行事件的分發(fā)，如果事件能夠傳遞給當(dāng)前View,那么這個方法就一定會被調(diào)用。他的返回結(jié)果受到當(dāng)前View的onTouchEvent和下級的dispatchTouchEvent的影響,表示的是當(dāng)前View(包括其子View)是否消耗這個事件。
public boolean onInterceptTouchEvent(MotionEvent event)
這個方法在該ViewGroup的dispatchTouchEvent方法中調(diào)用,用來判斷該View是否攔截某個事件，若攔截，那么該View將直接攔截與該事件同一事件序列的剩余事件，對這一事件序列不再調(diào)用onInterceptTouchEvent判斷是否攔截。返回值表示是否攔截當(dāng)前事件。這個方法在View的子類ViewGroup中而不在View中。
public boolean onTouchEvent(MotionEvent event)
在dispatchTouchEvent中調(diào)用,用來處理點(diǎn)擊事件,返回結(jié)果表示是否消耗當(dāng)前事件,如果不消耗,則不能接受同一事件序列內(nèi)的剩余事件。
圖解--來自Kelin
事件分發(fā)圖解
模擬流程的偽代碼
public boolean dispatchTouchEvent(MontionEvent ev){
boolean consume = false;
if(onInterceptTouchEvent(ev)){
consume = onTouchEvent(ev);
}else{
consume = child.dispatchTouchEvent(ev);
}
return consume;
}
結(jié)論
- 事件序列是指從手指接觸屏幕開始,到手指離開屏幕結(jié)束這個過程中產(chǎn)生的一系列事件,這個事件序列以down開始,中間有數(shù)量不定的move,最終以up結(jié)束。
- 某個ViewGroup被判定到攔截某一事件M,那么M所在序列中M之后的事件,都會被這個ViewGroup處理(如果事件能傳遞給它),而且這個ViewGroup不會再調(diào)用其onInterceptTouchEvent方法去判斷是否攔截,而是默認(rèn)攔截。也就是說,onInterceptTouchEvent這個方法并不會總是被調(diào)用。
- 如果某個View不消耗ACTION_DOWN事件,那么同一事件序列中的其他事件都不會再給它處理,而是給它的父元素處理。
- 如果View不消耗ACTION_DOWN以外的其他事件,那么這個點(diǎn)擊事件會消失,此時父元素的onTouchEvent不會被調(diào)用,并且當(dāng)前View可以持續(xù)接收到后續(xù)事件,消失的事件會傳遞給Activity處理。
- ViewGroup默認(rèn)不攔截任何事件。
- View沒有onInterceptTouchEvent方法,一旦有事件傳遞給它,它的onTouchEvent方法就會被調(diào)用。
- View的onTouchEvent默認(rèn)會消耗事件,除非它是不可點(diǎn)擊的(clickable和longClickable同時為false)。View的longClickable屬性默認(rèn)為false。
- 一個disable的View依然可能會消耗事件,只要clickable或longClickablec中有一個為true。但并不運(yùn)行onClickListener的onClick方法和onLongClickListener中的onLongClick方法甚至是onTouchListener中的onTouch方法。
- 事件的傳遞過程是由內(nèi)向外的,即事件總是先傳遞給父元素,然后再由父元素分發(fā)給子View,通過requestDisallowInterceptTouchEvent方法可以在子元素中干預(yù)父元素的分發(fā)過程,當(dāng)ACTION_DOWN事件除外。
分析
接下來就該上源碼了,首先是ViewGroup的dispatchTouch方法(分析在注釋里)
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;
//第一步,進(jìn)行初始化操作
/*判斷是否為ACTION_DOWN,如果是，
就將一些標(biāo)志位進(jìn)行重置等操作,包括
disallowIntercept,所以不能被不允許攔截
注意,在cancelAndClearTouchTargets(ev)中
會將mFirstTouchTarget設(shè)置為null*/
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();
}
// 第二步,檢查是否攔截
final boolean intercepted;
/* 判斷是否為ACTION_DOWN或mFirstTouchTarget是否為null
當(dāng)滿足其中一個的時候,進(jìn)入并調(diào)用onInterceptTouchEvent(ev)
來判斷是否攔截。這里需要注意的是,在ViewGroup的子View處理
事件成功的時候,mFirstTouchTarget會被賦值并指向子元素。
也就是說,當(dāng)事件不為ACTION_DOWN時,如果想調(diào)用
onInterceptTouchEvent(ev)判斷是否攔截,
就必須讓mFirstTouchTarget != null,
而這個條件必須是前一個事件沒有被攔截且
ACTION_DWON能被子View消耗(如果ACTION_Down不能被消耗,
則mFirstTouchTarget是不會被賦值的。而如果是其他事件不
被消耗,由于子View消耗ACTION_DOWN時對mFirstTouchTarget
賦了值,所以還是會進(jìn)入調(diào)用onInterceptTouchEvent)。*/
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
/* 如果子View調(diào)用了requestDisallowInterceptTouchEvent方法,
則disallowIntercept為true,那么除了ACTION_DOWN,其他事件都不允許被攔截*/
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 {
/*如果不滿足條件,則說明前一個事件被攔截,
那么剩下的同一序列的事件都會被攔截 */
// 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);
}
//第三步:檢查cancel
// Check for cancelation.
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
//第四步:事件分發(fā)
// 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 accessiiblity 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;
//處理ACTION_DOWN事件,如果子View處理成功,
//那么mFirstTouchTarget會被賦值
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) {
// 依據(jù)Touch坐標(biāo)尋找子View來接收Touch事件
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;
// 遍歷子View判斷哪個子View接受Touch事件
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.
/*mFirstTouchTarget == null有兩種可能,
一是ACTION_DOWN被攔截或沒有被處理,二是前一個事件被攔截。
不管是一還是二,當(dāng)前事件和同一序列后續(xù)事件都不會被子View處理*/
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
// 向子View傳遞一個cancel事件,
//dispatchTransformedTouchEvent()可以將事件分發(fā)給子View處理
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else {
//如果之前的事件沒有被攔截
// 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 {
//若當(dāng)前事件被攔截,cancelChild則為true
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
//如果cancelChild為true,那么向子View分發(fā)一個cancel事件
//從這里可以看出,如果攔截一個子View的事件,則會向
//它分發(fā)一個cancel事件使得它狀態(tài)重置
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
if (cancelChild) {
if (predecessor == null) {
//如果intercepted為true
//mFirstTouchTarget最后會被賦值為null
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
}
//處理ACTION_UP和ACTION_CANCEL,主要是還原狀態(tài)操作
// 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;
}
然后是View的onTouchEvent:
public boolean onTouchEvent(MotionEvent event) {
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
//判斷其屬性是否為DISABLED
if ((viewFlags & ENABLED_MASK) == DISABLED) {
//如果當(dāng)前事件為ACTION_UP且該View的狀態(tài)為Pressed
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
//清除掉Pressed狀態(tài)
setPressed(false);
}
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
//如果這個View是clickable或longClickable,
//則返回true,即消費(fèi)該事件
return (((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);
}
//如果設(shè)置有代理,那就執(zhí)行代理的onTouchEvent
//一般是由于View太小不好按,才會設(shè)置代理。
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
//按照邏輯的連貫性,接下來我們先看ACTION_DOWN,最后再看ACTION_UP
if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
(viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
switch (action) {
case MotionEvent.ACTION_UP:
/*檢測到ACTION_UP的時候,
不管是Pressed還是PrePressed狀態(tài),只要期間沒有
ACTION_MOVE,即Pressed和PrePressed狀態(tài)沒有被取消,
就可以執(zhí)行onClick方法,不同的是,由于PrePerssed
狀態(tài)還沒有被轉(zhuǎn)換為Pressed狀態(tài)的(mPendingCheckForTap
進(jìn)程未被執(zhí)行),所以在這里要setPressed(true, x, y);*/
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
// take focus if we don't have it already and we should in
// touch mode.
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}
if (prepressed) {
// The button is being released before we actually
// showed it as pressed. Make it show the pressed
// state now (before scheduling the click) to ensure
// the user sees it.
// 當(dāng)前并不是Pressed狀態(tài),所以在這里setPressed
setPressed(true, x, y);
}
//如果不是長按事件且下個ACTION_UP事件不被忽略
if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
// This is a tap, so remove the longpress check
/*移除等待長按的線程,這個線程做的事情其實(shí)就是
等待一段時間后調(diào)用longClick方法,如果你按下時間
足夠,那就會執(zhí)行這個方法。如果你中途移動或抬起,
那這個線程就會被停止*/
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)) {
//在performClick方法里面會判斷
//onClickListener是否為null,并執(zhí)行onClick方法
performClick();
}
}
}
//下面都是一些還原狀態(tài)的操作
if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}
if (prepressed) {
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_DOWN:
mHasPerformedLongPress = false;//初始化長按標(biāo)志為false
if (performButtonActionOnTouchDown(event)) {
break;
}
// Walk up the hierarchy to determine if we're inside a scrolling container.
boolean isInScrollingContainer = isInScrollingContainer();
// For views inside a scrolling container, delay the pressed feedback for
// a short period in case this is a scroll.
/* 如果是在可滑動的容器中,接到ACTION_DOWN事件時,
不能直接將View設(shè)置為Pressed狀態(tài),得先等一下
(讓手指保持當(dāng)前狀態(tài)115ms,即ViewConfiguration.getTapTimeout()),
這是為了避免將外部的滑動當(dāng)作點(diǎn)擊。如果不設(shè)置這個狀態(tài),
那么即使用戶想滑動,當(dāng)一碰到就會顯示Pressed的狀態(tài),這是
不合理的。在對ACTION_MOVE的處理我們也可以看到,
如果滑出了View的范圍,那這個PrePressed
狀態(tài)會被去除,如果不是在可滑動的容器中,則直接設(shè)置為
Pressed狀態(tài)。*/
if (isInScrollingContainer) {
mPrivateFlags |= PFLAG_PREPRESSED;
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPendingCheckForTap.x = event.getX();
mPendingCheckForTap.y = event.getY();
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true, x, y);
checkForLongClick(0, x, y);
}
break;
//處理取消點(diǎn)擊事件,將狀態(tài)還原
case MotionEvent.ACTION_CANCEL:
setPressed(false);
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
/*如果手指移出了View的范圍,則取消
“延遲115ms并設(shè)置為Pressed”
這一操作,也就是說如果在115ms
你的手指移動出這一范圍,就不算是Pressed。
如果已經(jīng)是Pressed狀態(tài),則進(jìn)一步把
"等待500ms,并設(shè)置為longPressed”
這一操作也取消了,并setPressed(false)。*/
case MotionEvent.ACTION_MOVE:
drawableHotspotChanged(x, y);
// Be lenient about moving outside of buttons
if (!pointInView(x, y, mTouchSlop)) {
// Outside button
removeTapCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
// Remove any future long press/tap checks
removeLongPressCallback();
setPressed(false);
}
}
break;
}
return true;
}
return false;
}
private final class CheckForTap implements Runnable {
public void run() {
/*取消mPrivateFlags的PREPRESSED，
然后設(shè)置PRESSED標(biāo)識，刷新背景，
如果View支持長按事件，則再發(fā)一個延時消息，檢測長按；*/
mPrivateFlags &= ~PREPRESSED;
mPrivateFlags |= PRESSED;
refreshDrawableState();
if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) {
postCheckForLongClick(ViewConfiguration.getTapTimeout());
}
}
}
class CheckForLongPress implements Runnable {
private int mOriginalWindowAttachCount;
public void run() {
// 1、如果此時設(shè)置了長按的回調(diào)，則執(zhí)行長按時的回調(diào)，且如果長按的回調(diào)返回true;才把mHasPerformedLongPress置為ture；
// 2、否則，如果沒有設(shè)置長按回調(diào)或者長按回調(diào)返回的是false；則mHasPerformedLongPress依然是false;
if (isPressed() && (mParent != null)
&& mOriginalWindowAttachCount == mWindowAttachCount) {
if (performLongClick()) {
mHasPerformedLongPress = true;
}
}
}
}
滑動沖突的處理
外部攔截法
讓在點(diǎn)擊事件都先經(jīng)過父容器的攔截處理,若父容器需要此事件就攔截,若不需要就不攔截。這個方法需要重寫父容器的onInterceptTouchEvent方法,在內(nèi)部做相應(yīng)的攔截即可,
偽代碼如下:
public boolean onInterceptTouchEvent(MotionEvent event){
boolean intercepted = false;
switch(event.getAction()){
case MotionEvent.ACTION_DOWN:
intercepted = false;
break;
case MotionEvent.ACTION_MOVE:
if(父容器需要當(dāng)前事件){
intercepted = false;
}else{
intercepted = false;
}
break;
case MotionEvent.ACTION_UP:
intercepted = false;
break;
default:
break;
}
return intercepted;
}
這里的ACTION_DOWN必須返回false,否則事件就沒法在傳遞給子元素了,而ACTION_UP在這里意義不大,但考慮到ACTION_UP如果被攔截,那子元素的onClick事件就無法觸發(fā),所以也讓它返回false。
內(nèi)部攔截法
內(nèi)部攔截罰是指父容器不攔截任何事件,所以的事件都傳遞給子元素,如果子元素選喲此事件就直接消耗掉,否則則由父容器來處理,這種方法需要配合requestDisallowInterceptTouchEvent方法才能正常工作。偽代碼如下:
public boolean dispatchTouchEvent(MotionEvent event){
switch(event.getAction()){
case MotionEvent.ACTION_DOWN:
parent.requestDisallowInterceptTouchEvent(true);
break;
case MotionEvent.ACTION_MOVE:
if(父容器需要此類點(diǎn)擊事件){
parent.requestDisallowIntercepTouchEvent(false);
}
break;
case MotionEvent.ACTION_UP:
break;
case default:
break;
}
return super.dispatchTouchEvent(event);
}
