本篇文章主要是以Activity上的觸摸事件爲例，事件從InputReader，InputDispatcher傳到ViewRootIpml開始談起。
首先是時序圖
其中最複雜的就是ViewGroup的dispatchTouchEvent方法將事件層層分發：
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();
}
// 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;
// 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;
}
}
}
// 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;
}下面具體分析：
1
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();
}首先一個完整的touch事件是以DOWN事件開始，以UP事件結束，中間可能貫穿多個MOVE事件，這個判斷的作用是，判斷一個新事件的開始，就將ViewGroup的成員變量mFirstTouchTarget的值置爲null，mFirstTouchTarget指向的是消耗了touch事件的子View，如果沒有子View消耗事件，該成員變量的值就是null的，這個值很重要，決定着後續MOVE和UP事件的傳遞流程
2
// 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;
}檢查當前ViewGroup是否調用onInterceptTouchEvent方法攔截事件，若攔截，則變量intercepted的值置爲true，否則就是false。
第一個判斷actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null的意思是：
1）如果是DOWN事件，那麼肯定走攔截的方法；
2）如果不是DOWN事件，則判斷mFirstTouchTarget是否爲null，即該ViewGroup的子View在之前的DOWN事件中是否消費了此次事件，如果消費了（即DOWN事件的onTouchEvent返回true，或者DOWN事件在dispatchTouchEvent分發的時候直接返回true），則此次非DOWN事件也會走到攔截的方法onInterceptTouchEvent中去，如果沒有消費，則直接將intercepted置爲true，不走該ViewGroup的攔截方法onInterceptTouchEvent方法，直接將intercepted置爲true。
3
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);//3.1
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;
}//3.2
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;
}
}
}3中首先是兩個判斷，主要就是判斷是DOWN事件，且沒有攔截（intercepted爲false）。
3.1中首先是判斷該ViewGroup有沒有child，如果沒有就沒有必要往下傳遞執行了，之後獲取下點擊的座標和所有的子View，一個for循環遍歷這個集合。注意這個循環是從ViewGroup的最上層的子View開始，依次往下遍歷。
3.2 中首先判斷子View能否接受point事件，並且這個point事件是否在這個view裏面，如果有一個不滿足，則continue，直接跳過本次循環；
然後調用dispatchTransformedTouchEvent方法，將事件在往子View分發，實現遞歸分發，知道最上層的view，如果在這個子View的某個child消費了事件（即DOWN事件的onTouchEvent返回true，或者DOWN事件在dispatchTouchEvent分發的時候直接返回true），則將newTouchTarget和mFirstTouchTarget都指向給子View（注意是一層一層的指向，即ViewGroup1指向ViewGroup2，ViewGroup2指向View1，而View1是真正消費事件的View），並且將alreadyDispatchedToNewTouchTarget置爲true。
4 dispatchTransformedTouchEvent方法中：
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);
}一般newPointerIdBits和oldPointerIdBits都是1，child.hasIdentityMatrix是true的，因此：
1）如果傳進來的child爲null，則調用本身ViewGroup的父類View的dispatchTouchEvent方法處理事件。此方法中最後會調用到ViewGroup的onTouchEvent方法，這個後面再說。
2）如果傳進來的child不爲null，則調用child的dispatchTouchEvent方法，將事件繼續往下傳遞。child如果是ViewGroup，則ViewGroup的分發dispatchTouchEvent是往子View傳遞，child如果是View，則View的dispatchTouchEvent是調用onTouchEvent去消費它。
5
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;
}
}經過層層循環調用dispatchTouchEvent，當事件一直傳遞到最上層的View的父佈局ViewGroup的dispatchTouchEvent時候，調用完3.2的代碼循環完以後，執行了每個touch點內的child的dispatchTouchEvent，onTouchEvent方法後，執行5處的代碼，
1）首先判斷mFirstTouchTarget是否爲null，如果爲null，調用dispatchTransformedTouchEvent方法，此時調用4處代碼，執行的是該ViewGroup的super.dispatchTouchEvent方法，內部會執行到該ViewGroup的onTouchEvent方法；
2）如果mFirstTouchTarget不爲null，即某個子View消費了這個事件，則進入else分支的while循環中，判斷alreadyDispatchedToNewTouchTarget && target == newTouchTarget是否成立，這個判斷是用於區分DOWN事件和其他事件的，如果是DOWN事件，則條件滿足，將handled置爲true，如果是非DOWN事件，則進入else分支，找到這個target對應的View，調用dispatchTransformedTouchEvent向這個View分發事件。
注意：如果是Down之後的MOVE或者UP事件，執行ViewGroup的dispatchTouchEvent時，執行完1、2以後，直接執行5處的代碼，如果之前的DOWN事件中判斷子View沒有消費掉事件，則直接執行此ViewGroup的onTouchEvent方法，事件到此ViewGroup就不往下傳遞，直接返回上層調用處了，若子View消費掉事件，進入else分支，將MOVE或者DOWN事件向目標View繼續向下傳遞（即調用子View的dispatchTouchEvent方法）。
總結：
首先一個完整的touch事件是以DOWN事件開始，以UP事件結束，中間可能貫穿多個MOVE事件。Down和後面的Move，Up事件是緊密聯繫的。
1 ViewGroup在執行dispatchTouchEvent的時候的onInterceptTouchEvent方法會不會執行到，主要是看：
1）如果是Down事件，肯定會執行到。
2）如果是Down後的Move或者Up事件，看之前的Down事件中，子View有沒有消費掉該事件，如果消費掉了該事件，則mFirstTouchTarget不爲null，纔會走onInterceptTouchEvent，如果在之前的Down事件中該ViewGroup沒有子View消費事件，則不會走到onInterceptTouchEvent，而是將intercepted直接置爲true。
從這裏也可以感覺出，Down事件可以看出是否用來消費事件，爲後面的Move和Up事件做準備的。
2）Move和Up事件不會走到3處的代碼，而是直接走到5處的代碼，判斷mFirstTouchTarget是否爲null，如果爲null，表明在之前的Down事件中，沒有子View消費事件，則進入dispatchTransformedTouchEvent方法，進入child==null的判斷，此時調用ViewGroup自身的super.dispatchTouchEvent方法，裏面會調用ViewGroup自身的onTouchEvent方法。
如果不爲null，表明在之前的Down事件中子View消費了事件，此時，走到else判斷，進入while循環中的else分支，調用dispatchTransformedTouchEvent將事件分發到消費了事件的子View處理。
2 對於註冊了onTouchListener , onClickListener , onLongClickListerer的View處理
1）setOnTouchListener()，此回調是在View的dispatchTouchEvent方法中，在onTouchEvent方法之前，會判斷有沒有註冊touch listener，如果有回調其中的onTouch方法，根據onTouch的返回值決定dispatchTouchEvent方法是否直接return。
2）setOnClickListener() 此回調是在View的onTouchEvent方法中case爲ACTION_UP的情況下調用onClick方法，改方法沒有返回值。
3）setOnLongClickListener()是在View的onTouchEvent方法中case爲ACTION_DOWN情況下，發送一個延遲消息實現，在ACTION_UP中會刪除這條消息。回調onLongClick方法，該方法有返回值，返回true後，在會掉完onLongClick後的ACTION_UP事件中不回調onClick，返回false之後，在ACTION_UP事件中依舊會回調onClick方法。
注意：設置setOnClickListener()和setOnLongClickListener()方法後，View在調用onTouchEvent方法時，就會局部變量clickable的值置爲true，就會走到Switch方法，根據Action類型不同操作，一旦走到Switch，最後都會return true，即onTouchEvent返回true。這裏就會影響到事件的分發。比如DOWN事件之後的MOVE UP事件都會傳遞到該View上來。