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 Android教程網 >> Android技術 >> 關於Android編程 >> Android事件分發機制源碼分析

Android事件分發機制源碼分析

編輯:關於Android編程

Part1:事件來源以及傳遞順序

Activity分發事件源碼

public boolean dispatchTouchEvent(MotionEvent ev) {
    if (ev.getAction() == MotionEvent.ACTION_DOWN) {
        onUserInteraction();
    }
    if (getWindow().superDispatchTouchEvent(ev)) {
        return true;
    }
    return onTouchEvent(ev);
}

當觸屏、按下Home鍵、back鍵、menu鍵等都會觸發onUserInteraction(),可以重寫這個方法處理一些用戶交互。可以看到Activity將事件交給Window來處理。如果Window不能消費事件Activity調用onTouchEvent()自行處理事件。PhoneWindow是Window的唯一實現類,接下來分析PhoneWindow分發事件過程。

PhoneWindow分發事件源碼

public boolean superDispatchTouchEvent(MotionEvent event) {
    return mDecor.superDispatchTouchEvent(event);
}

mDecor是DecorView類的對象,DecorView繼承FrameLayout,PhoneWindow只是把事件交由頂級DecorView處理。由於DecorView繼承FrameLayout,FrameLayout繼承ViewGroup,所以,之後的事件分發過程與ViewGroup事件分發過程一樣。接下來Part2會介紹這部分。

小結

事件首先傳遞給Activity,Activity將事件傳遞給Window(PhoneWindow是其實現類),Window把事件交給頂級DecorView處理,如果Window沒有消費這個事件則Activity調用oTouchEvent()自行處理事件。
事件傳遞順序:

Created with Rapha?l 2.1.0ActivityWindowDecorView

Part2:ViewGroup事件分發過程

只針對主要流程以及相應代碼進行分析,不會貼出完整代碼。ViewGroup的事件分發過程主要在dispatchTouchEvent()方法中。

首先,對MotionEvent做個簡單介紹。事件序列開始於ACTION_DOWN,終於ACTION_UP。對於單指操作有ACTION_DOWN、ACTION_MOVE、ACTION_UP等事件序列組成。多指操作由ACTION_DOWN、ACTION_POINTER_DOWN、ACTION_MOVE、ACTION_POINTER_UP、ACTION_UP事件序列組成。pointer可以理解為觸摸點。多指對應的pointerId不變,pointerIndex在事件序列中是變化的。
更多參考:http://www.jianshu.com/p/0c863bbde8eb

事件序列的起始動作是ACTION_DOWN,在新事件序列到達時要做一些狀態清除操作。

// 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.
    /**由於一些特殊原因丟失ACTION_UP或者ACTION_CANCEL,
    *導致事件序列結束時mFirstTouchTarget(TouchTarget鏈表)未被清空,
    * 新事件序列到達時,要先清空mFirstTouchTarget
    */
    cancelAndClearTouchTargets(ev);
    //主要設置 mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT
    resetTouchState();
}

看看cancelAndClearTouchTargets()和resetTouchState()的具體實現。

/**
* Cancels and clears all touch targets.
*/
private void cancelAndClearTouchTargets( MotionEvent event )
{
    if ( mFirstTouchTarget != null )
    {
        boolean syntheticEvent = false;
        if ( event == null )
        {
            final long now = SystemClock.uptimeMillis();
            event = MotionEvent.obtain( now, now,
                            MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0 );
            event.setSource( InputDevice.SOURCE_TOUCHSCREEN );
            syntheticEvent = true;
        }

        for ( TouchTarget target = mFirstTouchTarget; target != null; target = target.next )
        {
            resetCancelNextUpFlag( target.child );
            dispatchTransformedTouchEvent( event, true, target.child, target.pointerIdBits );
        }
        clearTouchTargets();

        if ( syntheticEvent )
        {
            event.recycle();
        }
    }
}

dispatchTransformedTouchEvent()後面再分析,先來看clearTouchTargets()方法。

/**
 * Clears all touch targets.
 */
private void clearTouchTargets()
{
    TouchTarget target = mFirstTouchTarget;
    if ( target != null )
    {
        do
        {
            TouchTarget next = target.next;
            target.recycle();
            target = next;
        }
        while ( target != null );
        mFirstTouchTarget = null;
    }
}

顯而易見,clearTouchTargets()對mFirstTouchTarget指向的鏈表進行了清空操作。
接下來看看resetTouchState()方法的實現:

private void resetTouchState() {
        clearTouchTargets();
        resetCancelNextUpFlag(this);
        mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
        mNestedScrollAxes = SCROLL_AXIS_NONE;
    }

同樣對mFirstTouchTarget指向的鏈表進行了清空,更重要的是設置了~FLAG_DISALLOW_INTERCEPT標志位。引出一個結論,子View在ACTION_DOWN時調用ViewGroup的requestDisallowInterceptTouchEvent()方法是無效的。

接下來ViewGroup檢測是否要攔截事件:

/* Check for interception. */
final boolean intercepted;
/* ACTION_DOWN或者mFirstTouchTarget!=null時檢測是否要攔截事件 */
if ( actionMasked == MotionEvent.ACTION_DOWN
     || mFirstTouchTarget != null )
{
    final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
    if ( !disallowIntercept )
    {
        intercepted = onInterceptTouchEvent( ev );
        /* restore action in case it was changed */
        ev.setAction( action );  
    } 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;
}

通過上面代碼,可以看出,ViewGroup在ACTON_DOWN或mFirstTouchTarget!=null條件時都會檢測是否需要攔截事件;在mFirstTouchTarget!=null的情況下,可以通過設置FLAG_DISALLOW_INTERCEPT或~FLAG_DISALLOW_INTERCEPT標記位來決定ViewGroup是否允許攔截ACTION_DOWN之後的事件,在允許攔截的情況下是否攔截還取決於onInterceptTouchEvent()的返回值。對於滑動沖突,方案一:使用onInterceptTouchEvent()攔截事件;方案二:使用onInterceptTouchEvent()和requestDisallowInterceptTouchEvent()一起攔截事件。個人覺得方案一更為簡單實用。

接下來ViewGroup會遍歷Children,尋找能消費事件的Child。實現代碼如下:

/*
 * Check for cancelation.
 * PFLAG_CANCEL_NEXT_UP_EVENT標記位文檔解釋是Indicates whether the view is temporarily detached
 */
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;

/* View detached或者event類型為ACTION_CANCEL或者未被攔截 */
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 */

        /* 這裡處理多觸控情況,一個View如果有多指觸摸,用32位的int記錄不同Pointer */
        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   preorderedList  = buildOrderedChildList();
            final boolean       customOrder = preorderedList == null
                                  && isChildrenDrawingOrderEnabled();


            final View[] children = mChildren;
            for ( int i = childrenCount - 1; i >= 0; i-- )
            {
                final int childIndex = customOrder
                               ? getChildDrawingOrder( childrenCount, i ) : i;
                final View child = (preorderedList == null)
                           ? children[childIndex] : preorderedList.get( 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;
                }
                /* Child不可見並且無動畫直接跳過,或者Point不在child范圍內 */
                if ( !canViewReceivePointerEvents( child )
                     || !isTransformedTouchPointInView( x, y, child, null ) )
                {
                    ev.setTargetAccessibilityFocus( false );
                    continue;
                }

                /* mFirstTouchTarget鏈表已經存在消費該事件的Child,用於多點觸控 */
                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 );
                /* 如果Child能消費事件,Child加入到mFirstTouchTarget鏈表 */
                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;
        }
    }
}

在ACTION_DOW或者ACTION_POINTER_DOWN時ViewGroup遍歷Children,尋找能夠消費事件的Child。Child不在TouchTarget鏈表中,addTouchTarget(child, idBitsToAssign);Child已經存在TouchTarget鏈表中,多指觸摸同一View情況,newTouchTarget.pointerIdBits |= idBitsToAssign。ViewGroup對於多指觸控不同View的解決方案是使用鏈表,View對於多指觸控的方案是使用32位int來記錄每個Pointer。

找到能夠消費事件序列的Child後,ACTION_DOWN或ACTION_POINTER_DOWN之後的事件,在ViewGroup不攔截的情況下,直接交由Child處理;一旦被攔截,在dispatchTransformedTouchEventChild()方法中eventAction會置為ACTION_CANCEL,並且Child會從TouchTarget鏈表中清除,因此接收不到後續事件序列,都將交給ViewGroup處理。實現代碼如下:

/*
 * Dispatch to touch targets.
 * Child不能消費事件序列,交由ViewGroup處理
 */
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;
        /* 除去ACTION_DOWN或ACTION_POINTER_DOWN事件,因為在尋找過程中已經處理過 */
        if ( alreadyDispatchedToNewTouchTarget && target == newTouchTarget )
        {
            handled = true;
        } else {
            final boolean cancelChild = resetCancelNextUpFlag( target.child )
                            || intercepted;
            if ( dispatchTransformedTouchEvent( ev, cancelChild,
                                target.child, target.pointerIdBits ) )
            {
                handled = true;
            }
            /* PFLAG_CANCEL_NEXT_UP_EVENT重置或者ViewGroup攔截ACTION_DOWN或ACTION_POINTER_DOWN  之後的事件,清除相應TouchTarget */
            if ( cancelChild )
            {
                if ( predecessor == null )
                {
                    mFirstTouchTarget = next;
                } else {
                    predecessor.next = next;
                }
                target.recycle();
                target = next;
                continue;
            }
        }
        predecessor = target;
        target      = next;
    }
}

ViewGroup通過dispatchTransformedTouchEvent()方法將事件分發給Child。

/* Perform any necessary transformations and dispatch. */
if ( child == null )
{
    handled = super.dispatchTouchEvent( transformedEvent );
} else {
    /* 將event坐標轉換成Child坐標系內坐標 */
    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 );
}

未找到可消費事件的Child,ViewGroup自行處理事件序列;否則,將event坐標轉換成Child坐標系內坐標交由Child處理。

ACTION_UP觸發事件序列結束時清空TouchTarget,ACTION_PONITER_UP觸發時,清空相應pointer的target。

/* 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 );
}

小結

ViewGroup事件分發的流程圖整理如下:

Created with Rapha?l 2.1.0開始ACTION_DOWN清空mFirstTouchTarget鏈表,設置~FLAG_DISALLOW_INTERCEPT標記ACTION_DOWN或者mFirstTouchTarget!=nulldisallowInterceptintercepted=falseACTION_DOWN或者ACTION_POINTER_DOWN尋找能消費事件的ChildmFirstTouchTarget==nullViewGroup處理事件ACTION_UP或者ACTION_POINTER_UP作相應清除操作結束事件未被分發到ChildChild處理intercepted從TouchTarget鏈表清除Child對應TargetonInterceptTouchEvent()intercepted=trueyesnoyesnoyesnoyesnoyesnoyesnoyesnoyesnoyesno

ViewGroup的事件分發過程就分析完了,接下來分析View的事件分發過程,相對ViewGroup來說相對簡單。

Part3:View事件分發過程

注意:View只處理了單指觸控的情況,未實現多指觸控,如果有需要可以自己實現。針對View的事件分發只涉及單指情況。View的事件分發過程同樣在dispatchTouchEvent()方法中,主要對這個方法進行分析即可。

View的事件分發過程同樣在dispatchTouchEvent()方法中,主要對這個方法進行分析即可。

final int actionMasked = event.getActionMasked();
    if (actionMasked == MotionEvent.ACTION_DOWN) {
        // Defensive cleanup for new gesture
        stopNestedScroll();
}

上面代碼表示,ACTION_DOWN事件會使View停止滾動(如果View是能夠滾動的,比如ListView)。
接下來View就要開始處理事件了,代碼如下:

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;
    }
}

View狀態是ENABLE並且調用過setOnTouchListener()方法,事件是否能被OnTouchListener消費取決於onTouch()的返回值。未調用過setOnTouchListener()方法或者OnTouchListener未消費事件,由onTouchEvent()方法來處理事件,事件是否能被消費取決於onTouchEvent()的返回值。接下來看看onTouchEvent()具體實現。

if ( (viewFlags & ENABLED_MASK) == DISABLED )
{
    if ( event.getAction() == 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) );
}

其實,View是CLICKABLE或者LONG_CLICKABLE時返回結果都為true(具體可查看源碼),也就是View能夠消費事件。上面的情況是View是DISABLED狀態時,會在ACTION_UP或者(mPrivateFlags & PFLAG_PRESSED) != 0設置mPrivateFlags &= ~PFLAG_PRESSED。長按以及點擊事件執行前都會先對這個標記位進行判斷。View處於DISABLED狀態可以消費事件,但是單擊和長按事件不會執行。

if (mTouchDelegate != null) {
    if (mTouchDelegate.onTouchEvent(event)) {
        return true;
    }
}

這段代碼表示可以給View設置一個代理對象(別的View),使用代理對象的onTouchEvent()來處理事件。比如擴大View的接觸面積、幾個View同步處理事件都可以用到。

對於View的事件處理,主要分析對ACTION_DOWN和ACTION_UP進行分析。先來看對ACTION_DOWN事件的處理:

/*
 * 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 );
}

在滾動容器中的操作只是增加了個延時操作,本質還是和不在滾動容器中一樣的。來看看checkForLongClick()方法的實現:

private void checkForLongClick( int delayOffset )
{
    if ( (mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE )
    {
        mHasPerformedLongPress = false;

        if ( mPendingCheckForLongPress == null )
        {
            mPendingCheckForLongPress = new CheckForLongPress();
        }
        mPendingCheckForLongPress.rememberWindowAttachCount();
        postDelayed( mPendingCheckForLongPress,
                 ViewConfiguration.getLongPressTimeout() - delayOffset );
    }
}

postDelayed()向Handler的消息隊列插入一個待處理的Runable對象,並且設置延時,這也是為什麼需要長按一段時間,長按操作才會執行。長按操作的具體實現都在CheckForLongPress裡了。

private final class CheckForLongPress implements Runnable {
    private int mOriginalWindowAttachCount;

    @Override
    public void run()
    {
        if ( isPressed() && (mParent != null)
             && mOriginalWindowAttachCount == mWindowAttachCount )
        {
            if ( performLongClick() )
            {
                mHasPerformedLongPress = true;
            }
        }
    }


    public void rememberWindowAttachCount()
    {
        mOriginalWindowAttachCount = mWindowAttachCount;
    }
}

看到重點了,performLongClick()會執行setOnLongClickListener()方法設置的OnLongClickListener的onLongClick()方法。

最後來看看View對ACTION_UP事件的處理:

if ( !mHasPerformedLongPress )
{
    /* 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();
        }
    }
}

如果mHasPerformedLongPress為false (可能OnLongClickListener為空或者onLongCkcik()方法返回false),移除隊列中的CheckForLongPress對象,然後如果OnClickListener不為空執行onClick()方法。

注意:給一個Button設置OnLongClickListener和OnClickListener,onLongClick()方法返回false。這種情況長按和點擊都會執行,驗證方法不能使用System.out.print()來進行輸出驗證,因為System.out是一個有緩存的輸出流,print()並不會立即輸出,使用println()才會立即輸出。

小結

View的狀態是CLICKABLE或者LONG_CLICKABLE都能夠消費事件,如果是DISABLED狀態則不會觸發長按和點擊事件。單擊事件優先級最低,因為最後才會處理單擊事件。

至此,Android事件分發機制分析完畢。

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