Android在監聽到屏幕上的手勢事件後，會根據視圖的層級結構從最外層的ViewGroup一直往下傳遞，事件可以在中途被某一層級的View截斷而不再向下傳，也就是消費了該事件。這個過程涉及到的方法主要是1、dispatchTouchEvent，2、onInterceptTouchEvent，3、onTouchEvent這三個方法，這三個方法什麼含義呢，下面是具體的解釋：
1、dispatchTouchEvent
dispatch是否分發事件，整個事件的驅動都在這個方法中，他會先調用自己的onInterceptTouchEvent，再遞歸調用child的dispatchTouchEvent，這樣就可以將事件一直傳下去，如果我們覆寫了這個方法，而且沒有調用super.dispatchTouchEvent()，也就是我們將事件調用的遞歸過程在這裏截斷了，child就不會收到該事件的傳遞，所以這裏可以對事件的進行第一次攔截，這裏返回true，表示該View已經消費了這個事件，事件傳遞結束；返回false，表示沒有消費，事件返回父控件onTouchEven進行處理。
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
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) {
//////////////////////////////////////////
// 這裏調用自身的onInterceptTouchEvent(); //
//////////////////////////////////////////
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;
}
// 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 (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 = buildOrderedChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
///////////////////////////////////////////
// 這裏開始遞歸子view的dispatchTouchEvent() //
///////////////////////////////////////////
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = customOrder
? getChildDrawingOrder(childrenCount, i) : i;
final View child = (preorderedList == null)
? children[childIndex] : preorderedList.get(childIndex);
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
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);
/////////////////////////////////////////////////////////////////////
// dispatchTransformedTouchEvent裏面調用了view的dispatchTouchEvent() //
/////////////////////////////////////////////////////////////////////
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;
}
}
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;
}2、onInterceptTouchEvent
intercept意思是攔截、截斷，所以該方法的作用就是判斷是否要將這個事件斷下來。返回true，事件截斷，進入onTouchEvent消費事件；返回false，不進行攔截，事件繼續向下傳遞，進入到子view的dispatchTouchEvent。
3、onTouchEvent
代表是否該view消費了該事件，也就是是否對該事件的發生進行了相應的處理，返回true一個事件就到這裏結束了，返回false說明事件沒有被消費，事件向上返回到父控件的onTouchEvent方法。
tips：
1、如果一個view沒有捕獲到ACTION_DOWN事件，或者捕獲到DOWN事件後在onTouchEvent裏返回了false，也就是沒有消費該事件，那麼將不會再收到後續事件。
2、View只有onTouchEvent方法，ViewGroup添加了dispatchTouchEvent和onInterceptTouchEvent方法，也就是說容器才需要判斷事件是否需要向下傳遞，view只需要處理事件就可以了，因爲view已經是視圖層裏的做小單位了
3、View也可以設置onTouchListener()，裏面有onTouch方法，如果一個View設置了onTouchListener，並且返回true，表明view消費了該事件，則不再執行onTouchEvent中的邏輯。
Reference
http://files.cnblogs.com/files/sunzn/PRE_andevcon_mastering-the-android-touch-system.pdf