在分析Android事件分發(fā)機制前，明確android的兩大基礎控件類型:View和ViewGroup。View即普通的控件，沒有子布局的，如Button、TextView. ViewGroup繼承自View，表示可以有子控件，如Linearlayout、Listview這些。今天我們先來了解View的事件分發(fā)機制。
先看下代碼，非常簡單，只有一個Button，分別給它注冊了OnClick和OnTouch的點擊事件。
btn.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { Log.i("Tag", "This is button onClick event"); } }); btn.setOnTouchListener(new View.OnTouchListener() { @Override public boolean onTouch(View v, MotionEvent event) { Log.i("Tag", "This is button onTouch action" + event.getAction()); return false; } });運行一下項目，結果如下：
I/Tag: This is button onTouch action0
I/Tag: This is button onTouch action2
I/Tag: This is button onTouch action2
I/Tag: This is button onTouch action1
I/Tag: This is button onClick event
可以看到，onTouch是有先于onClick執(zhí)行的，因此事件的傳遞順序是先onTouch，在到OnClick。具體為什么這樣，下面會通過源碼來說明。這時，我們可能注意到了，onTouch的方法是有返回值，這里是返回false，我們將它改為true再運行一次，結果如下：
I/Tag: This is button onTouch action0
I/Tag: This is button onTouch action2
I/Tag: This is button onTouch action2
I/Tag: This is button onTouch action2
I/Tag: This is button onTouch action1
對比兩次結果，我們發(fā)現onClick方法不再執(zhí)行，為什么會這樣，下面我將通過源碼給大家一步步理清這個思路。
查看源碼時，首先要知道所有View類型控件事件入口都是dispatchTouchEvent()，所以我們直接進入到View這個類里面的dispatchTouchEvent（）方法看一下。
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)) { //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; }從源碼第25行處可以看到，mOnTouchListener.onTouch()的方法首先被執(zhí)行，如果li != null && li.mOnTouchListener != null&& (mViewFlags & ENABLED_MASK) == ENABLED&& li.mOnTouchListener.onTouch(this, event)都為真的話，result賦值為true，否則就執(zhí)行onTouchEvent(event)方法。
從上面可以看到要符合條件有四個，
1、ListenerInfo li，它是view中的一個靜態(tài)類，里面定義view的事件的監(jiān)聽等等，所以有涉及到view的事件，ListenerInfo都會被實例化，因此li不為null
2、mOnTouchiListener是在setOnTouchListener方法里面賦值的，只要touch事件被注冊，mOnTouchiListener一定不會null
3、 (mViewFlags & ENABLED_MASK) == ENABLED，是判斷當前點擊的控件是否是enable的，button默認為enable，這個條件也恒定為true，
4、重點來了，li.mOnTouchListener.onTouch(this, event)就是回調控件onTouch方法，當這個條件也為true時，result=true,onTouchEvent（event）將不會被執(zhí)行。如果onTouch返回false，就會再執(zhí)行onTouchEvent(event)方法。
我們接著再進入到onTouchEvent方法查看源碼。
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); } 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; }從源碼的21行我們可以看出，該控件可點擊就會進入到switch判斷中，當我們觸發(fā)了手指離開的實際，則會進入到MotionEvent.ACTION_UP這個case當中。我們接著往下看，在源碼的50行，調用到了mPerformClick()方法，我們繼續(xù)進入到這個方法的源碼看看。
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; }現在我們可以看到，只要ListenerInfo和mOnClickListener不為null就會調用onClick這個方法，之前說過，只要有監(jiān)聽事件，ListenerInfo就不為null，帶mOnClickListener又是在哪里賦值呢？我們再繼續(xù)看下它的源碼。
public void setOnClickListener(@Nullable OnClickListener l) { if (!isClickable()) { setClickable(true); } getListenerInfo().mOnClickListener = l; }看到這里一切就清楚了，當我們調用setOnClickListener方法來給按鈕注冊一個點擊事件時，就會給mOnClickListener賦值。整個分發(fā)事件的順序是onTouch()-->onTouchEvent(event)-->performClick()-->OnClick()。
現在我們可以解決之前的問題。
1、onTouch方法是優(yōu)先于OnClick，所以是執(zhí)行了onTouch，再執(zhí)行onClick。
2、無論是dispatchTouchEvent還是onTouchEvent，如果返回true表示這個事件已經被消費、處理了，不再往下傳了。在dispathTouchEvent的源碼里可以看到，如果onTouchEvent返回了true，那么它也返回true。如果dispatchTouchEvent在執(zhí)行onTouch監(jiān)聽的時候，onTouch返回了true，那么它也返回true，這個事件提前被onTouch消費掉了。就不再執(zhí)行onTouchEvent了，更別說onClick監(jiān)聽了。
以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持武林網。
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