研究了View的繪制流程，接下來不得不研究下View的Touch事件機制，只有掌握了這兩方面的知識育瓜，才能對View有全面的認識，Touch事件的入口在Activity中栽烂，代碼如下：
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
我們可以看到躏仇，它首先是由Window來處理的，如果Window不處理腺办，就會交給Activity#onTouchEvent來完成焰手，那么什么情況下會交給Activity來處理呢？我們看下這個方法怀喉，代碼如下：
/**
* Called when a touch screen event was not handled by any of the views
* under it. This is most useful to process touch events that happen
* outside of your window bounds, where there is no view to receive it.
*
* @param event The touch screen event being processed.
*
* @return Return true if you have consumed the event, false if you haven't.
* The default implementation always returns false.
*/
public boolean onTouchEvent(MotionEvent event) {
if (mWindow.shouldCloseOnTouch(this, event)) {
finish();
return true;
}
return false;
}
注釋里說明了书妻，這種情況是點擊的區(qū)域不在Window的范圍內(nèi)，并且會判斷是否需要關閉Activity躬拢。
接下來繼續(xù)看Window是如何處理Touch事件的躲履，這里的Window是PhoneWindow见间，代碼如下：
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
Touch事件最終交給了DecorView來完成，而DecorView又交給了父類ViewGroup來完成工猜，所以真正的入口在ViewGroup中米诉，代碼如下：
public boolean dispatchTouchEvent(MotionEvent ev) {
...
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
// Handle an initial down.
// 丟棄之前無用的TouchEvent
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;
// 如果是DOWN事件，或者有目標Target篷帅，是否攔截通過onInterceptTouchEvent判斷
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 {
// 直接攔截
// 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);
}
// 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;
// 如果沒有攔截史侣，就進行分配
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;
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);
...
// 判斷當前的child在不在之前接收事件的view列表里
newTouchTarget = getTouchTarget(child);
// 找到了接收事件的view，這里將跳出for循環(huán)
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;
}
// 這里就是說當前的child不在之前的列表里
resetCancelNextUpFlag(child);
// 如果這個child要攔截事件魏身，才會走下邊的if
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();
// 把這個view添加到之前的列表中
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();
}
// 這里意思是惊橱，沒有找到接收此事件的子View，那就讓最后添加進列表的view來處理
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) {
// TouchTarget這個鏈表是空的箭昵，也就是沒有child處理
// No touch targets so treat this as an ordinary view.
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else {
// 對所有的在TouchTarget鏈表中的child view李皇，一一進行分配
// 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) {
// 如果已經(jīng)由一個原先不在鏈表中的子view處理，就是handled完成了
handled = true;
} else {
循環(huán)查找能夠dispatch的view
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;
}
}
...
}
...
return handled;
}
以上流程相對比較復雜宙枷，總結(jié)下來就是掉房，先根據(jù)onInterceptTouchEvent的結(jié)果判斷當前的ViewGroup是否攔截，如果攔截了慰丛，就從維護的TouchTarget鏈表中尋找可以消費事件的child卓囚，如果不攔截，就遍歷所有的子view诅病，直到找到消費事件的子view哪亿，并添加到TouchTarget鏈表中。接下來我們看下dispatchTransformedTouchEvent的實現(xiàn)贤笆，代碼如下：
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
...
// Perform any necessary transformations and dispatch.
if (child == null) {
handled = super.dispatchTouchEvent(transformedEvent);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
transformedEvent.offsetLocation(offsetX, offsetY);
if (! child.hasIdentityMatrix()) {
transformedEvent.transform(child.getInverseMatrix());
}
handled = child.dispatchTouchEvent(transformedEvent);
}
// Done.
transformedEvent.recycle();
return handled;
}
這里如果child不存在蝇棉，就交給父級來處理，否則就交給child來處理芥永，但是它們調(diào)用的是同一個方法篡殷，這個方法屬于View，它的實現(xiàn)如下：
public boolean dispatchTouchEvent(MotionEvent event) {
...
boolean result = false;
...
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;
}
}
...
return result;
}
可以看到埋涧，如果我們設置了OnTouchListener板辽，就會調(diào)用它的onTouch方法，否則就會由View的onTouchEvent方法來完成棘催。這個方法的實現(xiàn)如下：
public boolean onTouchEvent(MotionEvent event) {
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
// 禁用之后的表現(xiàn)
if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return (((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
(viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
switch (action) {
case MotionEvent.ACTION_UP:
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.
setPressed(true, x, y);
}
// 如果不是長按事件劲弦，就是單擊事件
if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
// This is a tap, so remove the longpress check
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)) {
performClick();
}
}
}
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;
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.
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);
// 檢查是否長按事件
checkForLongClick(0, x, y);
}
break;
case MotionEvent.ACTION_CANCEL:
setPressed(false);
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
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;
}
這里可以看出醇坝，在ACTION_DOWN事件時會先檢查是否是長按邑跪，然后在ACTION_UP事件時，如果不是長按再確認是點擊事件，那么長按事件是何時觸發(fā)的呢画畅，我們看以下代碼：
private void checkForLongClick(int delayOffset, float x, float y) {
if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) {
mHasPerformedLongPress = false;
...
postDelayed(mPendingCheckForLongPress,
ViewConfiguration.getLongPressTimeout() - delayOffset);
}
}
private final class CheckForLongPress implements Runnable {
...
@Override
public void run() {
if (isPressed() && (mParent != null)
&& mOriginalWindowAttachCount == mWindowAttachCount) {
if (performLongClick(mX, mY)) {
mHasPerformedLongPress = true;
}
}
}
}
這里是發(fā)送了一個Delayed消息到Handler中贸毕，待取得Message之后就會調(diào)用performLongClick方法，這里也可以看到夜赵，長按的默認觸發(fā)時間是通過ViewConfiguration#getLongPressTimeout方法獲取的，值是500毫秒乡革。也就是說寇僧，只要長按，不需要ACTION_UP就能觸發(fā)事件沸版，這和我們使用手機時的體驗一致嘁傀。
這里我們看到了一堆函數(shù)，這些函數(shù)統(tǒng)統(tǒng)都有相同的返回值：一個boolean變量视粮。但是這個變量在不同的函數(shù)中代表的意義不同细办，產(chǎn)生的結(jié)果也不一樣。接下來蕾殴，我們分兩個部分笑撞，依次分析它們的區(qū)別，這兩個部分分別是關于DOWN事件和關于其他事件（如MOVE钓觉、UP）茴肥。
DOWN事件的傳遞
onInterceptTouchEvent
我們要首先分析這個方法，是因為它是ViewGroup特有的荡灾，對View而言并不需要瓤狐。這個方法只有在接收到的事件是DOWN，或著已經(jīng)維護有TouchTarget鏈表時才會觸發(fā)批幌，通俗說就是要么接收到了DOWN础锐，要么它有子View消費了DOWN。這時荧缘，ViewGroup才有決定是否要攔截此事件的選擇皆警，一旦攔截，就意味著之前不在TouchTarget鏈表里的子View不會再接收到任何事件截粗。
return true：攔截消息耀怜，不在TouchTarget鏈表里的子View不會接收到任何事件
return false：不攔截，將消息分發(fā)給子View來處理
dispatchTouchEvent
dispatch是分發(fā)的意思桐愉，在ViewGroup和View兩個類中的表現(xiàn)并不完全一致财破。對于ViewGroup，它的dispatch決定是自己消費還是交由子View消費从诲，而對于View而言則是自己消費左痢。而相同點在于，只要dispatch最終返回了false，也就是沒有消費俊性，就要交給上一層級來完成略步，最終如果到Activity還沒被消費，事件將被丟棄定页。
不過這里有一點需要注意的是趟薄，如果將事件分發(fā)給一個最初不在TouchTarget鏈表中的子View時，那么這個子View對DOWN事件的處理將決定它能否處理其他事件典徊。在上方分析ViewGroup#dispatchTouchEvent時杭煎，我們看到只有子View的dispatchTouchEvent返回為true時，才會把它加入到TouchTarget鏈表里卒落。
return true：表示告訴上一級羡铲，已消費
return false：表示沒有辦法處理消息，交由上一級來完成
onTouchEvent
這里是處理事件的具體地方了儡毕，設置OnTouchListener的道理也是一樣的也切，正如上面說到的，這里處理了DOWN事件后如果返回了false腰湾，就意味著它無法再處理UP雷恃、MOVE等事件。而返回false表示事件在此處并未終止费坊，還要把事件繼續(xù)傳遞下去褂萧，直到在某個View返回true為止，或者因沒有任何View消費而丟棄葵萎。
return true：消費事件导犹，終止事件的傳輸，并可以處理接下來的MOVE羡忘、UP等事件
return false：不消費事件谎痢，事件將繼續(xù)傳輸
MOVE、UP事件的傳遞
這些事件的傳輸和DOWN類似卷雕，但是最主要在于它受DOWN事件的影響节猿，如果DOWN事件返回false，也就是稱沒有消費漫雕，那么這個View就不能收到這些事件滨嘱。
Touch事件的機制，在遇見實際的問題時浸间，情況還是比較復雜的太雨，但是它的根本原理就是源碼里向我們展示的這一部分，以后分析具體的View時魁蒜，可能還會有更多的理解囊扳，這個知識需要不斷去體會吩翻，去嘗試，才能了然于心锥咸。
