前面寫過一篇文章，說了下事件分發機制的方法和大致流程，本文嘗試從源碼的角度一層一層的看下分發機制。
源碼的查看：https://www.androidos.net.cn/sourcecode（可能是我下的源碼有問題，部分方法我是在線查看的）
Activity的事件分發機制
我們從activity的dispatchTouchEvent方法進行源碼分析：
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
點進去看到，onUserInteraction是個空方法，這裏是用來實現屏保功能的，當activity位於棧頂時，觸屏點擊home、menu、back會觸發。
public void onUserInteraction() {
}
然後我們來看第二個判斷
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
繼續往下看，
public abstract boolean superDispatchTouchEvent(MotionEvent event);
這玩意是個抽象方法，對應的Window也是個抽象類，我們能找到PhoneWindow，
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
然後我們在看下這個mDecor是啥，
找到這個類
public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks
他是繼承FrameLayout的，也就是mDecor.SuperDispatchTouchEvent即等同於Viegroup的分發機制。（即事件從activity傳遞到了viewgroup）
再看最後一個onTouch方法，
public boolean onTouchEvent(MotionEvent event) {
if (mWindow.shouldCloseOnTouch(this, event)) {
finish();
return true;
}
return false;
}
這裏默認返回false，就是不處理，向下層分發，那我們來看下這個判斷裏面。
public boolean shouldCloseOnTouch(Context context, MotionEvent event) {
final boolean isOutside =
event.getAction() == MotionEvent.ACTION_DOWN && isOutOfBounds(context, event)
|| event.getAction() == MotionEvent.ACTION_OUTSIDE;
if (mCloseOnTouchOutside && peekDecorView() != null && isOutside) {
return true;
}
return false;
}
這裏主要判斷是否在邊界外，在即消費事件，返回true，分發結束，反之返回false，activity層的分發也結束，扔給viewgroup繼續分發，直到被消費。
viewgroup的分發機制
我們從ViewGroup的dispatchTouchEvent方法進行源碼分析：
if (!canceled && !intercepted) {
// If the event is targeting accessibility 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);
代碼很長，挑一點看看（不完整，感興趣的自己查看），就是判斷事件沒有取消也沒有被攔截，然後給viewgroup內的子view進行遍歷，繼續看重點
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;
}
我們給這個dispatchTransformedTouchEvent點進去，
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;
}
這裏面進行了判斷，事件傳遞給了view（即child）進行分發，或者給他的上一層viewgroup進行分發，直到事件分發結束，即被消費。
回過頭來，我們看攔截的判斷
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;
}
這裏面的事件被攔截即 mFirsTouchTarget！=null不成立，即不攔截，mFirsTouchTarget！=null，disallowIntercept表示是否允許被攔截，是可以用代碼來控制的，經過判斷，允許被攔截再調用攔截的方法，
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;
}
這裏面進行多個判斷，默認返回false，即不攔截。攔截的要求大致上，這個事件是來自使用者的輸入、是down事件、這個是按鈕、點擊的事件只在viewgroup內。一旦攔截了，它就會向它的父類分發，也就是view的分發，但執行ViewGroup的onTouch() ->> onTouchEvent() ->> performClick（） ->> onClick()，即自己處理該事件，事件不會往下傳遞（具體請參考View事件的分發機制中的View.dispatchTouchEvent（））。（可以理解爲雖然是個viewgroup，但裏面並沒有子view，所以事件分發相當於view的事件分發），這個地方要注意。
View的事件分發機制
我們從View的dispatchTouchEvent方法進行源碼分析：（源碼可能下的有點問題，我們在線看下）
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;
}
}
它返回的是result，即決定是否繼續分發，返回true，即事件被消費，false即要調用onTouch方法。返回true有三種情況
1. view是可點擊的且handleScrollBarDragging
看下源碼（太長，看下什麼時候返回true）
* @return true if the event was handled as a scroll bar dragging, false otherwise.
*/
protected boolean handleScrollBarDragging(MotionEvent event) {
翻譯：如果作爲一個滾動條拖動事件處理返回true
2. view可點擊且onTouch事件不爲空
3. onTouch事件返回true
下面看下onTouchEvent方法
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
switch (action) {
case MotionEvent.ACTION_UP:
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
if ((viewFlags & TOOLTIP) == TOOLTIP) {
handleTooltipUp();
}
if (!clickable) {
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
}
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();
}
即view可點擊進入switch判斷具體DOWN、UP等事件，重點看下這個
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);
notifyEnterOrExitForAutoFillIfNeeded(true);
return result;
}
根據代碼我們知道只要我們通過setOnClickListener（）爲控件View註冊1個點擊事件，那麼就會給mOnClickListener變量賦值（即不爲空），則會往下回調onClick（） & performClick（）返回true。即調用onTouch事件要調用performClick事件，當這些執行完才能執行我們常見的onClick事件，至此，事件分發結束。
核心結論
事件逐層分發，判斷是否攔截，攔截就本層消費，否則向下分發，直至被消費。
寫在最後
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