詳細分析Android中onTouch事件傳遞機制_Android

onTach介紹

ontach是Android系統中整個事件機制的基礎。Android中的其他事件，如onClick、onLongClick等都是以onTach為基礎的。

onTach包括從手指按下到離開手機螢幕的整個過程，在微觀形式上，具體表現為action_down、action_move和action_up等過程。

onTach兩種主要定義形式如下：

1.在自訂控制項中，常見的有重寫onTouchEvent(MotionEvent ev)方法。如在開發中經常可以看到重寫的onTouchEvent方法，

並且其中有針對不同的微觀表現（action_down、action_move和action_up等）做出的相應判斷，執行邏輯並可能返回不同的布爾值。

2.在代碼中，直接對現有控制項設定setOnTouchListener監聽器。並重寫監聽器的onTouch方法。onTouch回呼函數中有view和MotionEvent

onTouch事件傳遞機制

大家都知道一般我們使用的UI控制項都是繼承自共同的父類——View。所以View這個類應該掌管著onTouch事件的相關處理。那就讓我們去看看：在View中尋找Touch相關的方法，其中一個很容易地引起了我們的注意： dispatchTouchEvent(MotionEvent event) 。

根據方法名的意思應該是負責分發觸摸事件的，下面給出了源碼：

/** * 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)) { //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;}

源碼有點長，但我們不必每一行都看。首先注意到dispatchTouchEvent的傳回值是boolean類型的，注釋上的解釋：@return True if the event was handled by the view, false otherwise.也就是說如果該觸摸事件被這個View消費了就返回true，否則返回false。在方法中首先判斷了該event是否是否得到了焦點，如果沒有得到焦點直接返回false。然後讓我們把目光轉向if (li != null && li.mOnTouchListener != null&& (mViewFlags & ENABLED_MASK) == ENABLED&& li.mOnTouchListener.onTouch(this, event))這個片段，看到這裡有一個名為li的局部變數，屬於 ListenerInfo 類，經 mListenerInfo 賦值得到。ListenerInfo只是一個封裝類，裡面封裝了大量的監聽器。

再在 View 類中去尋找 mListenerInfo ，可以看到下面的代碼：

ListenerInfo getListenerInfo() { if (mListenerInfo != null) { return mListenerInfo; } mListenerInfo = new ListenerInfo(); return mListenerInfo;}

因此我們可以知道mListenerInfo是不為空白的，所以li也不是空，第一個判斷為true，然後看到li.mOnTouchListener，前面說過ListenerInfo是一個監聽器的封裝類，所以我們同樣去追蹤mOnTouchListener：

/** * 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;}

正是通過上面的方法來設定 mOnTouchListener 的，我想上面的方法大家肯定都很熟悉吧，正是我們平時經常用的 xxx.setOnTouchListener ，好了我們從中得知如果設定了OnTouchListener則第二個判斷也為true，第三個判斷為如果該View是否為enable，預設都是enable的，所以同樣為true。還剩最後一個：li.mOnTouchListener.onTouch(this, event) ，顯然是回調了第二個判斷中監聽器的onTouch()方法，如果onTouch()方法返回true,則上面四個判斷全部為true,dispatchTouchEvent()方法會返回true，並且不會執行if (!result && onTouchEvent(event))這個判斷；而在這個判斷中我們又看到了一個熟悉的方法：onTouchEvent() 。所以想要執行onTouchEvent，則在上面的四個判斷中必須至少有一個false。

那就假定我們在onTouch()方法中返回的是false，這樣就順利地執行了onTouchEvent，那就看看onTouchEvent的源碼吧：

/** * 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);  }  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;}

這段源碼比 dispatchTouchEvent 的還要長，不過同樣我們挑重點的看：
if (((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE)
看到這句話就大概知道了主要是判斷該view是否是可點擊的，如果可以點擊則接著執行，否則直接返回false。可以看到if裡面用switch來判斷是哪種觸摸事件，但在最後都是返回true的。

還有一點要注意：在 ACTION_UP 中會執行 performClick() 方法：

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;}

可以看到上面的li.mOnClickListener.onClick(this); ，沒錯，我們好像又有了新的發現。根據上面的經驗，這句代碼會去回調我們設定好的點擊事件監聽器。也就是我們平常用的xxx.setOnClickListener(listener);

/** * 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;}

我們可以看到上面方法設定正是mListenerInfo的點擊監聽器，驗證了上面的猜想。到了這裡onTouch事件的傳遞機制基本已經分析完成了,也算是告一段落了。

好了，這下我們可以解決開頭的問題了，順便我們再來小結一下：在dispatchTouchEvent中，如果設定了OnTouchListener並且View是enable的，那麼首先被執行的是OnTouchListener中的onTouch(View v, MotionEvent event) 。若onTouch返回true,則dispatchTouchEvent不再往下執行並且返回true；不然會執行onTouchEvent，在onTouchEvent中若View是可點擊的，則返回true，不然為false。還有在onTouchEvent中若View是可點擊以及當前觸摸事件為ACTION_UP，會執行performClick() ，回調OnClickListener的onClick方法。

下面是我畫的一張草圖：

還有一點值得注意的地方是：假如當前事件是ACTION_DOWN，只有dispatchTouchEvent返回true了之後該View才會接收到接下來的ACTION_MOVE,ACTION_UP事件，也就是說只有事件被消費了才能接收接下來的事件。

總結

以上就是關於Android中onTouch事件傳遞機制的詳細分析，希望對各位Android開發人員們的學習或者工作能有一定的協助，如果有疑問大家可以留言交流。