Android事件分發機制-------View

當觸摸一個View時，首先會調用View的dispatchTouchEvent(MotionEvent event)方法，關乎著事件的分發，所以首先看看這個方法

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;
    }

看關鍵性的代碼，包含了四個判斷

 

li：是ListenerInfo，點進去可以看到是各種Listener，OnFocusChangeListener，OnScrollChangeListener，OnClickListener等等。

li.mOnTouchListener ：很明顯是View的TouchListener，設置了setOnTouchListener()就不為空

(mViewFlags & ENABLED_MASK) == ENABLED：View是否是Enable的

li.mOnTouchListener.onTouch(this, event)：onTouchListener的返回值，是否消耗了touch事件

也就是說這四個條件都滿足，dispatchTouchEvent方法就返回true，否則進入下面的判斷

接著看下一步

 

 if (!result && onTouchEvent(event)) {
                result = true;
            }

當上面的四個條件有一個不滿足時，就會執行onTouchEvent方法，onTouchEvent返回true結果就返回true。那麼下面就看看onTouchEvent(event)的源碼

 

 

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;
    }

額，這段代碼有點長，看看下面幾點。
7行判斷view是否是disable的，如果是，官方文檔解釋，這個view如果是CLICKABLE、LONG_CLICKABLE、CONTEXT_CLICKABLE，將會消耗掉touch事件，返回true，但是不會去響應點擊事件。

 

接著看下面的代碼，下面如果不是Disable的並且可點擊會進入switch的語句，在Up事件中會執行一個performClick()方法，點進入看

 

 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;
    }

這裡可以看到mOnClickListener 不為空就會執行onClick方法，mOnClickListener 的賦值就是我們常用的setOnclickListener，並且返回true消耗掉了touch事件。也就是說onTouch事件要先於onClick事件，並且dispatchTouchEvent方法裡面的四個條件不能全部滿足才會執行（主要是onTouch不能返回true）onClick

看MotionEvent.ACTION_DOWN

 

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;

這裡會檢測該view是否在一個滾動的容器裡面，如果在會有一個小的延遲再反饋的按下的狀態，以防的滾動

 

然後會去檢測是否是長按的事件
 

onTouchEvent()的返回結果

從源碼可以看出，只要View是Enable的並且滿足CLICKABLE、LONG_CLICKABLE、CONTEXT_CLICKABLE其中的一個條件都會返回true，消耗掉，否則返回false

 

 

 

下面用demo測試

自定義一個MyView繼承於View，重寫裡面的dispatchTouchEvent，onTouchEvent方法

1.在界面畫出該View，並且觸摸view，發現只打印下面

 

 @Override
    public boolean dispatchTouchEvent(MotionEvent event) {
        Log.i(TAG, "dispatchTouchEvent");
        return super.dispatchTouchEvent(event);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()){
            case MotionEvent.ACTION_DOWN:
                Log.i(TAG, "onTouchEvent  ACTION_DOWN");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.i(TAG, "onTouchEvent  ACTION_MOVE");
                    break;
            case MotionEvent.ACTION_UP:
                Log.i(TAG, "onTouchEvent  ACTION_UP");
                break;
        }
        return super.onTouchEvent(event);
    }

 

無論滑動還是離開都不會觸發其他事件。根據上面的源碼解釋mOnTouchListener為空（並沒有設置onTouchListener的監聽）所以會進入onTouchEvent方法，而該view又是disable的，所以onTouchEvent返回false，自然dispatchTouchEvent返回也是false。因為必須dispatchTouchEvent返回true才會接收到接下來的事件，所以才只接收到down的事件，不信我們把dispatchTouchEvent的返回值設置為true試試

2.dispatchTouchEvent的返回值設置為true

 

 @Override
    public boolean dispatchTouchEvent(MotionEvent event) {
        Log.i(TAG, "dispatchTouchEvent");
        super.dispatchTouchEvent(event);
        return true;
//        return super.dispatchTouchEvent(event);
    }

 

打印日志如下

可以看到有接收到後續的事件

 

3.設置onTouchListener

主界面設置view的touchListener監聽

 

view = (MyView) findViewById(R.id.view);
        view.setOnTouchListener(new View.OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                return true;
            }
        });

MyView

 

 

@Override
    public boolean dispatchTouchEvent(MotionEvent event) {
        Log.i(TAG, "dispatchTouchEvent");
        return super.dispatchTouchEvent(event);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                Log.i(TAG, "onTouchEvent  ACTION_DOWN");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.i(TAG, "onTouchEvent  ACTION_MOVE");
                break;
            case MotionEvent.ACTION_UP:
                Log.i(TAG, "onTouchEvent  ACTION_UP");
                break;
        }
        return super.onTouchEvent(event);
    }

打印日志如下

 

發現並不會執行onTouchEvent，原理很簡單因為onTouch返回了true，使onTouchEvent不會執行，直接給dispatchTouchEvent返回了true，所以也可以接收到後續事件

如果把onTouch方法返回false，打印如下

返回false自然會執行onTouchEvent方法，由於是disable並且不可點擊，的所以給dispatchTouchEvent返回了false，自然不會有後續事件

 

還有很多其他的情況都可以根據源碼來分析出來執行結果。