Android點擊事件派發機制源碼分析_Android

概述 

一直想寫篇關於Android事件派發機制的文章，卻一直沒寫，這兩天剛好是周末，有時間了，想想寫一篇吧，不然總是只停留在會用的層次上但是無法瞭解其內部機制。我用的是4.4源碼，開啟看看，挺複雜的，尤其是事件是怎麼從Activity派發出來的，太費解了。瞭解Windows訊息機制的人會發現，覺得Android的事件派發機制和Windows的訊息派發機制挺像的，其實這是一種典型的訊息“冒泡”機制，很多平台採用這個機制，訊息最先到達最底層View，然後它先進行判斷是不是它所需要的，否則就將訊息傳遞給它的子View，這樣一來，訊息就從水底的氣泡一樣向上浮了一點距離，以此類推，氣泡達到頂部和空氣接觸，破了（訊息被處理了），當然也有氣泡浮出到頂層了，還沒破（訊息無人處理），這個訊息將由系統來處理，對於Android來說，會由Activity來處理。 

Android點擊事件的派發機制 

1. 從Activity傳遞到底層View

 點擊事件用MotionEvent來表示，當一個點擊操作發生時，事件最先傳遞給當前Activity，由Activity的dispatchTouchEvent來進行事件派發，具體的工作是由Activity內部的Window來完成的，Window會將事件傳遞給decor view，decor view一般就是當前介面的底層容器（即setContentView所設定的View的父容器），通過Activity.getWindow.getDecorView（）可以獲得。另外，看下面代碼的的時候，主要看我注釋的地方，代碼很多很複雜，我無法一一說明，但是我注釋的地方都是關鍵點，是博主仔細讀代碼總結出來的。 

源碼解讀： 

事件是由哪裡傳遞給Activity的，這個我還不清楚，但是不要緊，我們從activity開始分析，已經足夠我們瞭解它的內部實現了。 

Code：Activity#dispatchTouchEvent

   /**   * Called to process touch screen events. You can override this to   * intercept all touch screen events before they are dispatched to the   * window. Be sure to call this implementation for touch screen events   * that should be handled normally.   *    * @param ev The touch screen event.   *    * @return boolean Return true if this event was consumed.   */  public boolean dispatchTouchEvent(MotionEvent ev) {    if (ev.getAction() == MotionEvent.ACTION_DOWN) {      //這個函數其實是個空函數，啥也沒幹，如果你沒重寫的話，不用關心      onUserInteraction();    }    //這裡事件開始交給Activity所附屬的Window進行派發，如果返回true，整個事件迴圈就結束了    //返回false意味著事件沒人處理，所有人的onTouchEvent都返回了false，那麼Activity就要來做最後的收場。    if (getWindow().superDispatchTouchEvent(ev)) {      return true;    }    //這裡，Activity來收場了，Activity的onTouchEvent被調用    return onTouchEvent(ev);  } 

Window是如何將事件傳遞給ViewGroup的

Code：Window#superDispatchTouchEvent 

  /**   * Used by custom windows, such as Dialog, to pass the touch screen event   * further down the view hierarchy. Application developers should   * not need to implement or call this.   *   */  public abstract boolean superDispatchTouchEvent(MotionEvent event);

這竟然是一個抽象函數，還註明了應用開發人員不要實現它或者調用它，這是什麼情況？再看看如下類的說明，大意是說：這個類可以控制頂級View的外觀和行為策略，而且還說這個類的唯一一個實現位於android.policy.PhoneWindow，當你要執行個體化這個Window類的時候，你並不知道它的細節，因為這個類會被重構，只有一個Factory 方法可以使用。好吧，還是很模糊啊，不太懂，不過我們可以看一下android.policy.PhoneWindow這個類，儘管執行個體化的時候此類會被重構，但是重構而已，功能是類似的。 

Abstract base class for a top-level window look and behavior policy. An instance of this class should be used as the top-level view added to the window manager. It provides standard UI policies such as a background, title area, default key processing, etc.The only existing implementation of this abstract class is android.policy.PhoneWindow, which you should instantiate when needing a Window. Eventually that class will be refactored and a factory method added for creating Window instances without knowing about a particular implementation.  

Code：PhoneWindow#superDispatchTouchEvent

@Override  public boolean superDispatchTouchEvent(MotionEvent event) {    return mDecor.superDispatchTouchEvent(event);  }這個邏輯很清晰了，PhoneWindow將事件傳遞給DecorView了，這個DecorView是啥呢，請看下面   private final class DecorView extends FrameLayout implements RootViewSurfaceTaker  // This is the top-level view of the window, containing the window decor.  private DecorView mDecor;  @Override  public final View getDecorView() {    if (mDecor == null) {      installDecor();    }    return mDecor;  }

順便說一下，平時Window用的最多的就是((ViewGroup)getWindow().getDecorView().findViewById(android.R.id.content)).getChildAt(0)即通過Activity來得到內部的View。這個mDecor顯然就是getWindow().getDecorView()返回的View，而我們通過setContentView設定的View是它的一個子View。目前事件傳遞到了DecorView 這裡，由於DecorView 繼承自FrameLayout且是我們的父View，所以最終事件會傳遞給我們的View，原因先不管了，換句話來說，事件肯定會傳遞到我們的View，不然我們的應用如何響應點擊事件呢。不過這不是我們的重點，重點是事件到了我們的View以後應該如何傳遞，這是對我們更有用的。從這裡開始，事件已經傳遞到我們的頂級View了，注意：頂級View實際上是最底層View，也叫根View。 

2.底層View對事件的分發過程 

點擊事件到底層View（一般是一個ViewGroup）以後，會調用ViewGroup的dispatchTouchEvent方法，然後的邏輯是這樣的：如果底層ViewGroup攔截事件即onInterceptTouchEvent返回true，則事件由ViewGroup處理，這個時候，如果ViewGroup的mOnTouchListener被設定，則會onTouch會被調用，否則，onTouchEvent會被調用，也就是說，如果都提供的話，onTouch會屏蔽掉onTouchEvent。在onTouchEvent中，如果設定了mOnClickListener，則onClick會被調用。如果頂層ViewGroup不攔截事件，則事件會傳遞給它的在點擊事件鏈上的子View，這個時候，子View的dispatchTouchEvent會被調用，到此為止，事件已經從最底層View傳遞給了上一層View，接下來的行為和其底層View一致，如此迴圈，完成整個事件派發。另外要說明的是，ViewGroup預設是不攔截點擊事件的，其onInterceptTouchEvent返回false。 

源碼解讀： 

Code：ViewGroup#dispatchTouchEvent 

  @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) {       //這裡判斷是否攔截點擊事件，如果攔截，則intercepted=true          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;       //這裡面一大堆是派發事件到子View，如果intercepted是true，則直接跳過      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 View[] children = mChildren;            final boolean customOrder = isChildrenDrawingOrderEnabled();            for (int i = childrenCount - 1; i >= 0; i--) {              final int childIndex = customOrder ?                  getChildDrawingOrder(childrenCount, i) : i;              final View child = children[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);              if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {                // Child wants to receive touch within its bounds.                mLastTouchDownTime = ev.getDownTime();                mLastTouchDownIndex = childIndex;                mLastTouchDownX = ev.getX();                mLastTouchDownY = ev.getY();                //注意下面兩句，如果有子View處理了點擊事件，則newTouchTarget會被賦值，                //同時alreadyDispatchedToNewTouchTarget也會為true，這兩個變數是直接影響下面的代碼邏輯的。                newTouchTarget = addTouchTarget(child, idBitsToAssign);                alreadyDispatchedToNewTouchTarget = true;                break;              }            }          }          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.     //這裡如果當前ViewGroup攔截了事件，或者其子View的onTouchEvent都返回了false，則事件會由ViewGroup處理      if (mFirstTouchTarget == null) {        // No touch targets so treat this as an ordinary view.       //這裡就是ViewGroup對點擊事件的處理        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;  }

 下面再看ViewGroup對點擊事件的處理

Code：ViewGroup#dispatchTransformedTouchEvent

   /**   * Transforms a motion event into the coordinate space of a particular child view,   * filters out irrelevant pointer ids, and overrides its action if necessary.   * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.   */  private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,      View child, int desiredPointerIdBits) {    final boolean handled;    // Canceling motions is a special case. We don't need to perform any transformations    // or filtering. The important part is the action, not the contents.    final int oldAction = event.getAction();    if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {      event.setAction(MotionEvent.ACTION_CANCEL);      if (child == null) {     //這裡就是ViewGroup對點擊事件的處理，其調用了View的dispatchTouchEvent方法        handled = super.dispatchTouchEvent(event);      } else {        handled = child.dispatchTouchEvent(event);      }      event.setAction(oldAction);      return handled;    }    // Calculate the number of pointers to deliver.    final int oldPointerIdBits = event.getPointerIdBits();    final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;    // If for some reason we ended up in an inconsistent state where it looks like we    // might produce a motion event with no pointers in it, then drop the event.    if (newPointerIdBits == 0) {      return false;    }    // If the number of pointers is the same and we don't need to perform any fancy    // irreversible transformations, then we can reuse the motion event for this    // dispatch as long as we are careful to revert any changes we make.    // Otherwise we need to make a copy.    final MotionEvent transformedEvent;    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);    }    // Perform any necessary transformations and dispatch.    if (child == null) {      handled = super.dispatchTouchEvent(transformedEvent);    } else {      final float offsetX = mScrollX - child.mLeft;      final float offsetY = mScrollY - child.mTop;      transformedEvent.offsetLocation(offsetX, offsetY);      if (! child.hasIdentityMatrix()) {        transformedEvent.transform(child.getInverseMatrix());      }      handled = child.dispatchTouchEvent(transformedEvent);    }    // Done.    transformedEvent.recycle();    return handled;  }

再看

Code：View#dispatchTouchEvent

  /**   * 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 (mInputEventConsistencyVerifier != null) {      mInputEventConsistencyVerifier.onTouchEvent(event, 0);    }    if (onFilterTouchEventForSecurity(event)) {      //noinspection SimplifiableIfStatement      ListenerInfo li = mListenerInfo;      if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED          && li.mOnTouchListener.onTouch(this, event)) {        return true;      }      if (onTouchEvent(event)) {        return true;      }    }    if (mInputEventConsistencyVerifier != null) {      mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);    }    return false;  }

這段代碼比較簡單，View對事件的處理是這樣的：如果設定了OnTouchListener就調用onTouch，否則就直接調用onTouchEvent，而onClick是在onTouchEvent內部通過performClick觸發的。簡單來說，事件如果被ViewGroup攔截或者子View的onTouchEvent都返回了false，則事件最終由ViewGroup處理。 

3.無人處理的點擊事件 

如果一個點擊事件，子View的onTouchEvent返回了false，則父View的onTouchEvent會被直接調用，以此類推。如果所有的View都不處理，則最終會由Activity來處理，這個時候，Activity的onTouchEvent會被調用。這個問題已經在1和2中做了說明。

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