都知道在Android中的事件主要包括三部分內(nèi)容：分發(fā)事件dispatchTouchEvent、攔截事件onInterceptTouchEvent、消費事件onTouchEvent。這幾乎是所有開發(fā)者都要面臨的問題，無論是解決一些事件沖突問題，還是自定義View，都會或多或少涉及到。由于其獨特的重要性，大多數(shù)面試的時候也基本會有所涉及，所以很好的掌握View的Touch事件傳遞顯得尤其重要。
1、Activity的dispatchTouchEvent
首先來看Activity的dispatchTouchEvent方法：
/**
* Called to process touch screen events. You can override this to
* intercept all touch screen events before they are dispatched to the
* window. Be sure to call this implementation for touch screen events
* that should be handled normally.
*
* @param ev The touch screen event.
*
* @return boolean Return true if this event was consumed.
*/
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
如果事件為按下狀態(tài)，則先調(diào)用onUserInteraction方法：
/**
* Called whenever a key, touch, or trackball event is dispatched to the
* activity. Implement this method if you wish to know that the user has
* interacted with the device in some way while your activity is running.
* This callback and {@link #onUserLeaveHint} are intended to help
* activities manage status bar notifications intelligently; specifically,
* for helping activities determine the proper time to cancel a notfication.
*
* <p>All calls to your activity's {@link #onUserLeaveHint} callback will
* be accompanied by calls to {@link #onUserInteraction}. This
* ensures that your activity will be told of relevant user activity such
* as pulling down the notification pane and touching an item there.
*
* <p>Note that this callback will be invoked for the touch down action
* that begins a touch gesture, but may not be invoked for the touch-moved
* and touch-up actions that follow.
*
* @see #onUserLeaveHint()
*/
public void onUserInteraction() {
}
該方法為空，從注釋可以知道，當(dāng)此activity在棧頂時，觸屏點擊按home、back、menu鍵等都會觸發(fā)此方法，一般會用于屏保。
接著調(diào)用了getWindow().superDispatchTouchEvent(ev)，即PhoneWindow類（Window的具體實現(xiàn)類）的superDispatchTouchEvent方法：
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
簡單調(diào)用了mDecor.superDispatchTouchEvent(event)，即DecorView的superDispatchTouchEvent方法：
public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}
我們知道DecorView繼承自FrameLayout，F(xiàn)rameLayout又繼承了ViewGroup，所以這里就是調(diào)用了ViewGroup的dispatchTouchEvent方法。
所以執(zhí)行g(shù)etWindow().superDispatchTouchEvent(ev)實際上是執(zhí)行了ViewGroup.dispatchTouchEvent(event)，這樣事件就從 Activity 傳遞到了 ViewGroup。這里后續(xù)會接著分析。
這里需要注意的是：
當(dāng)getWindow().superDispatchTouchEvent(ev)返回true時，即Activity的子View攔截了TouchEvent事件，那么接下來就不會再傳遞給Activity的onTouchEvent 方法，同時Activity的dispatchTouchEvent方法返回true；
反之返回false時，這個事件就交給Activity的onTouchEvent 方法來處理。
/**
* 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;
}
可以看到Activity的onTouchEvent 方法返回了false，也就意味著當(dāng)getWindow().superDispatchTouchEvent(ev)返回false時，Activity的dispatchTouchEvent方法默認返回false。
2、ViewGroup的dispatchTouchEvent
如果要很好掌握Touch事件處理，這部分要重點學(xué)習(xí)，而且不同Android版本的實現(xiàn)不一致，本文仍然使用最新的Android 7.1源碼，相比之前的源碼加入了更多的復(fù)雜邏輯操作，但是最基本的流程保持一致。
接下來直接分析ViewGroup的dispatchTouchEvent方法，這個方法代碼比較多，就分開幾段來做分析，首先來看下面這段源碼：
@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();
}
...
}
...
}
其中第一個if語句主要用于調(diào)試可直接忽略，后面的變量handled用于表示是否有view消費了該事件，接著調(diào)用了父類View的onFilterTouchEventForSecurity方法來判斷是否被其他窗口遮蓋，方法具體如下：
/**
* Filter the touch event to apply security policies.
*
* @param event The motion event to be filtered.
* @return True if the event should be dispatched, false if the event should be dropped.
*
* @see #getFilterTouchesWhenObscured
*/
public boolean onFilterTouchEventForSecurity(MotionEvent event) {
//noinspection RedundantIfStatement
if ((mViewFlags & FILTER_TOUCHES_WHEN_OBSCURED) != 0
&& (event.getFlags() & MotionEvent.FLAG_WINDOW_IS_OBSCURED) != 0) {
// Window is obscured, drop this touch.
return false;
}
return true;
}
如果被其他窗口遮擋，該方法返回false，表示需要過濾觸摸事件，就會跳過dispatchTouchEvent方法中的if語句代碼，直接退出dispatchTouchEvent方法并返回false，表示沒有View消費Touch事件；如果沒有被其他窗口遮擋，該方法返回true，進而繼續(xù)執(zhí)行if語句里面的代碼。
每一個事件都是由一個觸摸按下事件，一個觸摸抬起事件和N個觸摸滑動事件組成的，觸摸按下事件就是這里的ACTION_DOWN，其為一系列事件的開端。所以在ACTION_DOWN時進行一些初始化操作，分別調(diào)用了cancelAndClearTouchTargets方法和resetTouchState方法，用來清楚掉之前消費Touch事件的View信息，并重置觸摸狀態(tài)。
首先來看cancelAndClearTouchTargets方法：
/**
* Cancels and clears all touch targets.
*/
private void cancelAndClearTouchTargets(MotionEvent event) {
if (mFirstTouchTarget != null) {
boolean syntheticEvent = false;
if (event == null) {
final long now = SystemClock.uptimeMillis();
event = MotionEvent.obtain(now, now,
MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0);
event.setSource(InputDevice.SOURCE_TOUCHSCREEN);
syntheticEvent = true;
}
for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
resetCancelNextUpFlag(target.child);
dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits);
}
clearTouchTargets();
if (syntheticEvent) {
event.recycle();
}
}
}
首先判斷目標View，如果存在則進行統(tǒng)一清除操作。如果event為空，則將動作設(shè)為ACTION_CANCEL，接著用一個for循環(huán)不斷向下傳遞觸摸事件，然后再清除所有觸摸目標，最后在回收拷貝的對象。
接著再來看resetTouchState方法：
/**
* Resets all touch state in preparation for a new cycle.
*/
private void resetTouchState() {
clearTouchTargets();
resetCancelNextUpFlag(this);
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
mNestedScrollAxes = SCROLL_AXIS_NONE;
}
該方法非常簡單，就是重置了一些Touch標志位。
然后繼續(xù)回到dispatchTouchEvent方法，看第二個代碼塊：
// 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;
變量intercepted用來標記是否要攔截該Touch事件，true表示攔截，false表示不攔截。
接著一個if判斷語句，如果為ACTION_DOWN事件，此時還沒有找到消費Touch事件的View，所以mFirstTouchTarget為空；如果為ACTION_MOVE和ACTION_UP事件，當(dāng)前面的ACTION_DOWN事件找到了消費Touch事件的View則mFirstTouchTarget不為空。這兩種情況都可以執(zhí)行if里面的代碼塊。
變量disallowIntercept 用來標記是否允許攔截，默認為false，但是可以通過 requestDisallowInterceptTouchEvent方法來重置該變量的值。
如果允許攔截，則調(diào)用onInterceptTouchEvent方法，即我們熟知的攔截事件。該方法代碼如下：
/**
* Implement this method to intercept all touch screen motion events. This
* allows you to watch events as they are dispatched to your children, and
* take ownership of the current gesture at any point.
*
* <p>Using this function takes some care, as it has a fairly complicated
* interaction with {@link View#onTouchEvent(MotionEvent)
* View.onTouchEvent(MotionEvent)}, and using it requires implementing
* that method as well as this one in the correct way. Events will be
* received in the following order:
*
* <ol>
* <li> You will receive the down event here.
* <li> The down event will be handled either by a child of this view
* group, or given to your own onTouchEvent() method to handle; this means
* you should implement onTouchEvent() to return true, so you will
* continue to see the rest of the gesture (instead of looking for
* a parent view to handle it). Also, by returning true from
* onTouchEvent(), you will not receive any following
* events in onInterceptTouchEvent() and all touch processing must
* happen in onTouchEvent() like normal.
* <li> For as long as you return false from this function, each following
* event (up to and including the final up) will be delivered first here
* and then to the target's onTouchEvent().
* <li> If you return true from here, you will not receive any
* following events: the target view will receive the same event but
* with the action {@link MotionEvent#ACTION_CANCEL}, and all further
* events will be delivered to your onTouchEvent() method and no longer
* appear here.
* </ol>
*
* @param ev The motion event being dispatched down the hierarchy.
* @return Return true to steal motion events from the children and have
* them dispatched to this ViewGroup through onTouchEvent().
* The current target will receive an ACTION_CANCEL event, and no further
* messages will be delivered here.
*/
public boolean onInterceptTouchEvent(MotionEvent ev) {
if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
&& ev.getAction() == MotionEvent.ACTION_DOWN
&& ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
&& isOnScrollbarThumb(ev.getX(), ev.getY())) {
return true;
}
return false;
}
該方法是ViewGroup中特有的方法，用于表示是否攔截觸摸事件。返回為true的話則表示攔截事件，事件不在向子View中分發(fā)，若返回false的話，則表示不攔截事件，將繼續(xù)分發(fā)事件。
正常都是返回默認的false，但是一般我們在自定義ViewGroup中會重寫該方法，用于攔截事件的分發(fā)。當(dāng)我們在父ViewGroup重寫該方法返回為true執(zhí)行事件攔截的邏輯的時候，可以在子View中通過調(diào)用requestDisallowInterceptTouchEvent方法，重新設(shè)置父ViewGroup的onInterceptTouchEvent方法為false，不攔截對事件的分發(fā)邏輯。
這里也是我們在開發(fā)中接觸碰到的問題，所以需要好好理解一下，下面為requestDisallowInterceptTouchEvent方法的源碼：
@Override
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {
if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {
// We're already in this state, assume our ancestors are too
return;
}
if (disallowIntercept) {
mGroupFlags |= FLAG_DISALLOW_INTERCEPT;
} else {
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
}
// Pass it up to our parent
if (mParent != null) {
mParent.requestDisallowInterceptTouchEvent(disallowIntercept);
}
}
攔截事件判斷完成后，會接著調(diào)用resetCancelNextUpFlag方法來檢查當(dāng)前事件是否被取消。
/**
* Resets the cancel next up flag.
* Returns true if the flag was previously set.
*/
private static boolean resetCancelNextUpFlag(@NonNull View view) {
if ((view.mPrivateFlags & PFLAG_CANCEL_NEXT_UP_EVENT) != 0) {
view.mPrivateFlags &= ~PFLAG_CANCEL_NEXT_UP_EVENT;
return true;
}
return false;
}
繼續(xù)回到dispatchTouchEvent方法，看第三個代碼塊：
// 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;
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;
}
}
}
該段代碼首先是一個if判斷語句，如果事件沒有被取消，也沒有被攔截，就分發(fā)該事件。只有ACTION_DOWN事件才會執(zhí)行第二個if語句里面的代碼，對于ACTION_MOVE和ACTION_UP事件則直接傳給消費了ACTION_DOWN事件的目標View。
接著獲取該ViewGroup中子View的個數(shù)，得到該事件發(fā)生的位置，獲取子View的list集合preorderedList，再通過for循環(huán)倒序遍歷當(dāng)前ViewGroup的所有子視圖。
有一點值得注意的是，這里采用了倒序遍歷，這是由于preorderedList中的順序是按照addView或XML布局文件中的順序來的。如點擊的地方有兩個子View都包含點擊事件的坐標，那么后被添加到布局中的那個子view會先響應(yīng)事件，即點擊的時候最上層的那個組件先去響應(yīng)該事件。
在for循環(huán)中第一個if語句調(diào)用了canViewReceivePointerEvents(child)和isTransformedTouchPointInView(x, y, child, null)方法。
/**
* Returns true if a child view can receive pointer events.
* @hide
*/
private static boolean canViewReceivePointerEvents(@NonNull View child) {
return (child.mViewFlags & VISIBILITY_MASK) == VISIBLE
|| child.getAnimation() != null;
}
該方法用于判斷當(dāng)前視圖的狀態(tài)，只有其正在顯示或正在執(zhí)行動畫，才可以接受觸摸事件。
/**
* Returns true if a child view contains the specified point when transformed
* into its coordinate space.
* Child must not be null.
* @hide
*/
protected boolean isTransformedTouchPointInView(float x, float y, View child,
PointF outLocalPoint) {
final float[] point = getTempPoint();
point[0] = x;
point[1] = y;
transformPointToViewLocal(point, child);
final boolean isInView = child.pointInView(point[0], point[1]);
if (isInView && outLocalPoint != null) {
outLocalPoint.set(point[0], point[1]);
}
return isInView;
}
判斷視圖有scrollTo或scrollBy造成的滾動偏移也需要計算在內(nèi)，并判斷觸摸點是否在當(dāng)前子視圖內(nèi)。
從這兩個方法可知，如果當(dāng)前子View可以消費該ACTION_DOWN事件，并且該ACTION_DOWN事件發(fā)生的位置在當(dāng)前子View的范圍內(nèi)，則繼續(xù)執(zhí)行將ACTION_DOWN事件分發(fā)給它；否則continue判斷下一個子View可否接受該ACTION_DOWN事件。
然后代碼通過調(diào)用getTouchTarget方法去查找當(dāng)前子View是否在mFirstTouchTarget.next這條target鏈中的某一個targe中，如果在則返回這個target，否則返回null。緊接著用if判斷找到接收Touch事件的子View，即newTouchTarget，既然已經(jīng)找到則執(zhí)行break跳出for循環(huán)。
如果該子View還沒有消費掉該ACTION_DOWN事件，就直接調(diào)用dispatchTransformedTouchEvent方法將該ACTION_DOWN事件傳遞給該子View。
/**
* Transforms a motion event into the coordinate space of a particular child view,
* filters out irrelevant pointer ids, and overrides its action if necessary.
* If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
*/
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
// Canceling motions is a special case. We don't need to perform any transformations
// or filtering. The important part is the action, not the contents.
final int oldAction = event.getAction();
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
event.setAction(MotionEvent.ACTION_CANCEL);
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}
event.setAction(oldAction);
return handled;
}
// Calculate the number of pointers to deliver.
final int oldPointerIdBits = event.getPointerIdBits();
final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;
// If for some reason we ended up in an inconsistent state where it looks like we
// might produce a motion event with no pointers in it, then drop the event.
if (newPointerIdBits == 0) {
return false;
}
// If the number of pointers is the same and we don't need to perform any fancy
// irreversible transformations, then we can reuse the motion event for this
// dispatch as long as we are careful to revert any changes we make.
// Otherwise we need to make a copy.
final MotionEvent transformedEvent;
if (newPointerIdBits == oldPointerIdBits) {
if (child == null || child.hasIdentityMatrix()) {
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
event.offsetLocation(offsetX, offsetY);
handled = child.dispatchTouchEvent(event);
event.offsetLocation(-offsetX, -offsetY);
}
return handled;
}
transformedEvent = MotionEvent.obtain(event);
} else {
transformedEvent = event.split(newPointerIdBits);
}
// 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;
}
該方法是一個非常重要的方法，其主要包括三塊內(nèi)容，結(jié)構(gòu)雷同。而且會發(fā)現(xiàn)該方法中代碼為一個遞歸調(diào)用，若其子View是ViewGroup則重復(fù)執(zhí)行ViewGroup的dispatchTouchEvent方法，若其子View是View則執(zhí)行View的dispatchTouchEvent方法。
從最開始到這里，我們大概分析了一下事件分發(fā)流程，通過調(diào)用Activity的dispatchTouchEvent方法，事件會首先被派發(fā)到最頂級的DecorView也就是ViewGroup，再由ViewGroup遞歸傳遞到View的dispatchTouchEvent方法。對于View的dispatchTouchEvent方法，我們后面再做分析。
如果dispatchTransformedTouchEvent方法返回true，則表示子View消費掉該事件。那么就回到dispatchTouchEvent方法繼續(xù)執(zhí)行if語句里的代碼塊，將子View加入到mFirstTouchTarget鏈表的表頭，并且將該表頭賦值給newTouchTarget，同時 alreadyDispatchedToNewTouchTarget置為true，說明有子View消費掉了該down事件。
for循環(huán)執(zhí)行完畢后，如果newTouchTarget為null，且 mFirstTouchTarget不為null，即沒找到子View來消耗該事件，但為了保存Touch事件的鏈表不為空，則把newTouchTarget賦值為最早加進mFirstTouchTarget鏈表的target。
再看dispatchTouchEvent方法的第四個代碼塊：
// 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;
}
}
如果沒有找到消費Touch事件的子View，則直接把當(dāng)前的ViewGroup當(dāng)作普通的View看待，把事件傳遞給自己，即前面分析的dispatchTransformedTouchEvent方法中child為null的情況；如果之前的ACTION_DOWN事件被子View消費掉了，就會直接找到該子View對應(yīng)的Target，將ACTION_MOVE和ACTION_UP事件傳遞給它們。
這里需要注意的是，如果intercepted為true，也就是ACTION_MOVE和ACTION_UP事件被攔截了，則cancelChild為true，則會分發(fā)一次ACTION_CANCLE事件。
再看dispatchTouchEvent方法的第五個代碼塊：
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
...
if (onFilterTouchEventForSecurity(ev)) {
...
// 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;
}
如果當(dāng)前事件是ACTION_CANCLE或ACTION_UP，會調(diào)用resetTouchState方法清空Touch狀態(tài)。
至此，ViewGroup的dispatchTouchEvent方法分析完畢。
3、View的dispatchTouchEvent
在分析ViewGroup的dispatchTouchEvent方法時，里面多處調(diào)用了dispatchTransformedTouchEvent方法，最終將事件從ViewGroup傳遞到 View，那么事件在后續(xù)如何傳遞的，接下來繼續(xù)分析。
/**
* 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;
}
相比較ViewGroup的dispatchTouchEvent方法，View的dispatchTouchEvent方法要簡便得多。當(dāng)View沒有被其他窗口遮擋時，判斷mOnTouchListener是否為空，即判斷該View有沒有綁定OnTouchListener監(jiān)聽器。
從源碼里面可以找到，mOnTouchListener是通過setOnTouchListener方法來進行綁定的：
/**
* Register a callback to be invoked when a touch event is sent to this view.
* @param l the touch listener to attach to this view
*/
public void setOnTouchListener(OnTouchListener l) {
getListenerInfo().mOnTouchListener = l;
}
OnTouchListener監(jiān)聽器如下：
/**
* Interface definition for a callback to be invoked when a touch event is
* dispatched to this view. The callback will be invoked before the touch
* event is given to the view.
*/
public interface OnTouchListener {
/**
* Called when a touch event is dispatched to a view. This allows listeners to
* get a chance to respond before the target view.
*
* @param v The view the touch event has been dispatched to.
* @param event The MotionEvent object containing full information about
* the event.
* @return True if the listener has consumed the event, false otherwise.
*/
boolean onTouch(View v, MotionEvent event);
}
當(dāng)前View一旦執(zhí)行了setOnTouchListener方法，該View的mOnTouchListener就不為空，就會調(diào)用OnTouchListener監(jiān)聽器的OnTouch方法。從返回值可以看到，如果重寫的OnTouch方法返回true的話，那么result的值就為true，意味著該事件被消費掉了，就不會繼續(xù)執(zhí)行后面的onTouchEvent方法了；否則繼續(xù)執(zhí)行onTouchEvent方法。
4、View的onTouchEvent
onTouchEvent方法源碼如下：
/**
* Implement this method to handle touch screen motion events.
* <p>
* If this method is used to detect click actions, it is recommended that
* the actions be performed by implementing and calling
* {@link #performClick()}. This will ensure consistent system behavior,
* including:
* <ul>
* <li>obeying click sound preferences
* <li>dispatching OnClickListener calls
* <li>handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when
* accessibility features are enabled
* </ul>
*
* @param event The motion event.
* @return True if the event was handled, false otherwise.
*/
public boolean onTouchEvent(MotionEvent event) {
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
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);
}
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);
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;
}
該方法代碼比較多，但是思路非常清晰。可以從第一個if語句看到，即使View為 disable 狀態(tài)，其依然可以消耗事件。從后面的if語句可以看到，當(dāng) View 的 LongClick 或 Clickable 屬性，只要有一個為 true則能消耗事件，執(zhí)行onClick和onLongClick方法。
其中onClick是在ACTION_UP事件中執(zhí)行的，onLongClick是在ACTION_DOWN事件中執(zhí)行的，分別對應(yīng)performClick和checkForLongClick方法。
/**
* Call this view's OnClickListener, if it is defined. Performs all normal
* actions associated with clicking: reporting accessibility event, playing
* a sound, etc.
*
* @return True there was an assigned OnClickListener that was called, false
* otherwise is returned.
*/
public boolean performClick() {
final boolean result;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
return result;
}
上面代碼判斷mOnClickListener是否為空，即判斷該View有沒有綁定OnClickListener監(jiān)聽器。如果通過調(diào)用setOnClickListener方法綁定了OnClickListener監(jiān)聽器，則調(diào)用onClick方法。
/**
* Register a callback to be invoked when this view is clicked. If this view is not
* clickable, it becomes clickable.
*
* @param l The callback that will run
*
* @see #setClickable(boolean)
*/
public void setOnClickListener(@Nullable OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}
接著來看checkForLongClick方法的源碼：
private void checkForLongClick(int delayOffset, float x, float y) {
if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) {
mHasPerformedLongPress = false;
if (mPendingCheckForLongPress == null) {
mPendingCheckForLongPress = new CheckForLongPress();
}
mPendingCheckForLongPress.setAnchor(x, y);
mPendingCheckForLongPress.rememberWindowAttachCount();
postDelayed(mPendingCheckForLongPress,
ViewConfiguration.getLongPressTimeout() - delayOffset);
}
}
由于長按事件比較復(fù)雜，需要根據(jù)ACTION_DOWN事件開始計時，所以這里新建了一個CheckForLongPress對象，其實際為一個Runnable對象：
private final class CheckForLongPress implements Runnable {
private int mOriginalWindowAttachCount;
private float mX;
private float mY;
@Override
public void run() {
if (isPressed() && (mParent != null)
&& mOriginalWindowAttachCount == mWindowAttachCount) {
if (performLongClick(mX, mY)) {
mHasPerformedLongPress = true;
}
}
}
public void setAnchor(float x, float y) {
mX = x;
mY = y;
}
public void rememberWindowAttachCount() {
mOriginalWindowAttachCount = mWindowAttachCount;
}
}
run方法中調(diào)用了performLongClick 方法，繼續(xù)追蹤：
/**
* Calls this view's OnLongClickListener, if it is defined. Invokes the
* context menu if the OnLongClickListener did not consume the event,
* anchoring it to an (x,y) coordinate.
*
* @param x x coordinate of the anchoring touch event, or {@link Float#NaN}
* to disable anchoring
* @param y y coordinate of the anchoring touch event, or {@link Float#NaN}
* to disable anchoring
* @return {@code true} if one of the above receivers consumed the event,
* {@code false} otherwise
*/
public boolean performLongClick(float x, float y) {
mLongClickX = x;
mLongClickY = y;
final boolean handled = performLongClick();
mLongClickX = Float.NaN;
mLongClickY = Float.NaN;
return handled;
}
繼續(xù)調(diào)用了重載的performLongClick 方法：
/**
* Calls this view's OnLongClickListener, if it is defined. Invokes the
* context menu if the OnLongClickListener did not consume the event.
*
* @return {@code true} if one of the above receivers consumed the event,
* {@code false} otherwise
*/
public boolean performLongClick() {
return performLongClickInternal(mLongClickX, mLongClickY);
}
直接調(diào)用了performLongClickInternal方法：
/**
* Calls this view's OnLongClickListener, if it is defined. Invokes the
* context menu if the OnLongClickListener did not consume the event,
* optionally anchoring it to an (x,y) coordinate.
*
* @param x x coordinate of the anchoring touch event, or {@link Float#NaN}
* to disable anchoring
* @param y y coordinate of the anchoring touch event, or {@link Float#NaN}
* to disable anchoring
* @return {@code true} if one of the above receivers consumed the event,
* {@code false} otherwise
*/
private boolean performLongClickInternal(float x, float y) {
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_LONG_CLICKED);
boolean handled = false;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLongClickListener != null) {
handled = li.mOnLongClickListener.onLongClick(View.this);
}
if (!handled) {
final boolean isAnchored = !Float.isNaN(x) && !Float.isNaN(y);
handled = isAnchored ? showContextMenu(x, y) : showContextMenu();
}
if (handled) {
performHapticFeedback(HapticFeedbackConstants.LONG_PRESS);
}
return handled;
}
上面代碼判斷mOnLongClickListener是否為空，即判斷該View有沒有綁定OnLongClickListener監(jiān)聽器。如果通過調(diào)用setOnLongClickListener方法綁定了OnLongClickListener監(jiān)聽器，則調(diào)用onLongClick方法。
/**
* Register a callback to be invoked when this view is clicked and held. If this view is not
* long clickable, it becomes long clickable.
*
* @param l The callback that will run
*
* @see #setLongClickable(boolean)
*/
public void setOnLongClickListener(@Nullable OnLongClickListener l) {
if (!isLongClickable()) {
setLongClickable(true);
}
getListenerInfo().mOnLongClickListener = l;
}
從以上的代碼分析知道，如果在ACTION_DOWN事件中已經(jīng)執(zhí)行了onLongClick的話，則mHasPerformedLongPress變量會被置為true，這樣在ACTION_UP事件中，就會把onClick的回調(diào)remove掉，就不會再執(zhí)行onClick了。
至此，Touch事件的傳遞流程分析完畢。
總結(jié)
按照上面一步一步分析，流程確實比較復(fù)雜，只是便于理解具體如何傳遞的，最后再把其中的關(guān)鍵流程總結(jié)一下。主要有以下幾點：
事件從Activity.dispatchTouchEveent()開始傳遞，只要沒有攔截，就會從最上層(ViewGroup)開始一直往下傳遞，子View通過onTouchEvent()消費事件。如果事件從上往下一直傳遞到最底層的子View，但是該View并沒有消費該事件，那么該事件就會反序往上傳遞，即從該View傳遞給自己的ViewGroup，然后再傳給更上層的ViewGroup直至傳遞給Activity.onTouchEvent()。
事件從ViewGroup傳遞給子View時，其中ViewGroup可以通過onInterceptTouchEvent()方法對事件進行攔截，停止其往下傳遞，如果攔截（即返回true）后該事件會直接走到該ViewGroup中的onTouchEvent()方法，不會再往下傳遞給子View。如果從DOWN開始，之后的MOVE、UP都會直接在該ViewGroup.onTouchEvent()中進行處理。
如果子View之前在處理某個事件，但是后續(xù)被ViewGroup攔截，那么子View會接收到ACTION_CANCEL。如果View沒有消費ACTION_DOWN事件，之后其他的ACTION_MOVE和ACTION_UP等事件都不會傳遞過來。
OnTouchListener優(yōu)先于onTouchEvent()對事件進行消費，onLongClick優(yōu)先于oClick對事件進行消費。
這一塊的內(nèi)容詳細分析確實比較麻煩，但是整體疏通以后看起來大體還算比較簡單的。如果有疑問，歡迎留言一起相互探討共同進步。
原文鏈接：手把手教你讀懂源碼，View的Touch事件傳遞流程詳細剖析
View系列文章：
Android View從源碼的角度分析加載流程
Android View從源碼的角度分析繪制流程
Android View從源碼的角度分析事件的注冊和接收
