View事件分發機制，看完這些你一定能懂

當一個點擊事件發生時，事件的傳遞順序是從Activity -> PhoneWindow -> DecorView（當前界面的容器，setContentView最終得到的佈局就是添加到DecorView），然後事件就傳遞給setContentView這個佈局設置的View樹上。事件傳遞的入口是從Activity dispatchTouchEvent 開始處理。

    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        //將事件傳遞到window上，然後再到DecorView
        if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }
        return onTouchEvent(ev);
    }

這裏就不去追蹤上面的代碼的執行過程了，可以查看下PhoneWindow的superDispatchTouchEvent 方法。
接下來我們瞭解下Activity ViewGroup 和 View 這三者之間的事件分發處理的邏輯。接下來就引出了衆所周知的三個核心方法

1、 public boolean dispatchTouchEvent(MotionEvent ev) {}
2、 public boolean onInterceptTouchEvent(MotionEvent ev) {}
3、 public boolean onTouchEvent(MotionEvent event) {}

Activity 和View 只有dispatchTouchEvent 和 onTouchEvent 方法而ViewGroup有這個三個方法。
接下來我們通過日誌和操縱dispatchTouchEvent 、onInterceptTouchEvent、onTouchEvent 來觀察下事件到底是如何傳遞的。

public class EventDeliverActivity extends Activity {

    private static final String TAG = "EventDeliverActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_event_deliver);

        /*
        //情況四 案例
        findViewById(R.id.view_1).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.d(TAG, "onClick: view_1");
            }
        });

        findViewById(R.id.view_1).setOnLongClickListener(new View.OnLongClickListener() {
            @Override
            public boolean onLongClick(View v) {
                Log.d(TAG, "onLongClick: view_1");
                return false;
            }
        });

        findViewById(R.id.view_1).setOnTouchListener(new View.OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                Log.d(TAG, "onTouch: view_1");
                return false;
            }
        });
        */
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        int action = ev.getAction();
        switch (action){
            case MotionEvent.ACTION_DOWN:
                Log.d(TAG, " dispatchTouchEvent: ACTION_DOWN");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.d(TAG, " dispatchTouchEvent: ACTION_MOVE");
                break;
            case MotionEvent.ACTION_UP:
                Log.d(TAG, " dispatchTouchEvent: ACTION_UP");
                break;
        }
        return super.dispatchTouchEvent(ev);
    }


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

public class ViewGroup_1 extends FrameLayout {

    private static final String TAG = "ViewGroup-1";

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        int action = ev.getAction();
        switch (action){
            case MotionEvent.ACTION_DOWN:
                Log.d(TAG, " dispatchTouchEvent: ACTION_DOWN");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.d(TAG, " dispatchTouchEvent: ACTION_MOVE");
                break;
            case MotionEvent.ACTION_UP:
                Log.d(TAG, " dispatchTouchEvent: ACTION_UP");
                break;
        }
        return super.dispatchTouchEvent(ev);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        int action = ev.getAction();
        switch (action){
            case MotionEvent.ACTION_DOWN:
                Log.d(TAG, " onInterceptTouchEvent: ACTION_DOWN");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.d(TAG, " onInterceptTouchEvent: ACTION_MOVE");
                break;
            case MotionEvent.ACTION_UP:
                Log.d(TAG, " onInterceptTouchEvent: ACTION_UP");
                break;
        }
        return super.onInterceptTouchEvent(ev);
    }

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

public class View_1 extends TextView {

    private static final String TAG = "View_1";

    @Override
    public boolean dispatchTouchEvent(MotionEvent event) {
        int action = event.getAction();
        switch (action){
            case MotionEvent.ACTION_DOWN:
                Log.d(TAG, " dispatchTouchEvent: ACTION_DOWN");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.d(TAG, " dispatchTouchEvent: ACTION_MOVE");
                break;
            case MotionEvent.ACTION_UP:
                Log.d(TAG, " dispatchTouchEvent: ACTION_UP");
                break;
        }
        return super.dispatchTouchEvent(event);
        //return true;
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int action = event.getAction();
        switch (action){
            case MotionEvent.ACTION_DOWN:
                Log.d(TAG, " onTouchEvent: ACTION_DOWN");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.d(TAG, " onTouchEvent: ACTION_MOVE");
                break;
            case MotionEvent.ACTION_UP:
                Log.d(TAG, " onTouchEvent: ACTION_UP");
                break;
        }
        return super.onTouchEvent(event);
        //return true;
    }
}

爲了節約篇幅刪除了一些構造方法。下面我們通過控制三個核心方法，是否分發、是否中斷、是否處理來觀察事件的執行過程。

情況一 Activity -> ViewGroup_1 -> View_1 所有的事件方法都不做任何處理

事件傳遞順序是：

 DOWN事件:
         EventDeliverActivity:  dispatchTouchEvent: ACTION_DOWN
         ViewGroup-1:  dispatchTouchEvent: ACTION_DOWN
         ViewGroup-1:  onInterceptTouchEvent: ACTION_DOWN
         View_1:  dispatchTouchEvent: ACTION_DOWN
         View_1:  onTouchEvent: ACTION_DOWN
         ViewGroup-1:  onTouchEvent: ACTION_DOWN
         EventDeliverActivity:  onTouchEvent: ACTION_DOWN
MOVE事件：
         EventDeliverActivity:  dispatchTouchEvent: ACTION_MOVE
         EventDeliverActivity:  onTouchEvent: ACTION_MOVE
         EventDeliverActivity:  dispatchTouchEvent: ACTION_MOVE
         EventDeliverActivity:  onTouchEvent: ACTION_MOVE
UP事件：
         EventDeliverActivity:  dispatchTouchEvent: ACTION_UP
         EventDeliverActivity:  onTouchEvent: ACTION_UP

綜上得出結論： 事件傳遞的過程是 先dispatchTouchEvent -> onInterceptTouchEvent -> onTouchEvent並且dispatchTouchEvent 和 onInterceptTouchEvent在同一級調用就是說Activity 調用完了dispatchTouchEvent，會調用DecorView的 dispatchTouchEvent 和onInterceptTouchEvent方法，接着調用ViewGroup-1的dispatchTouchEvent 和onInterceptTouchEvent方法 ，然後調用View_1的dispatchTouchEvent方法，接着在從onTouchEvent依次往上開始回調父View的onTouchEvent。

情況二 Activity -> ViewGroup_1 -> View_1 在View_1的dispatchTouchEvent 或者 onTouchEvent 返回true

對View_1 的 dispatchTouchEvent return true

一、對View_1 的 dispatchTouchEvent return true

DOWN事件:
         EventDeliverActivity:  dispatchTouchEvent: ACTION_DOWN
         ViewGroup-1:  dispatchTouchEvent: ACTION_DOWN
         ViewGroup-1:  onInterceptTouchEvent: ACTION_DOWN
         View_1:  dispatchTouchEvent: ACTION_DOWN
MOVE事件：
         EventDeliverActivity:  dispatchTouchEvent: ACTION_MOVE
         ViewGroup-1:  dispatchTouchEvent: ACTION_MOVE
         ViewGroup-1:  onInterceptTouchEvent: ACTION_MOVE
         View_1:  dispatchTouchEvent: ACTION_MOVE
UP事件：
         EventDeliverActivity:  dispatchTouchEvent: ACTION_UP
         ViewGroup-1:  dispatchTouchEvent: ACTION_UP
         ViewGroup-1:  onInterceptTouchEvent: ACTION_UP
         View_1:  dispatchTouchEvent: ACTION_UP
         
二、對View_1 的 onTouchEvent return true

DOWN事件:
         EventDeliverActivity:  dispatchTouchEvent: ACTION_DOWN
         ViewGroup-1:  dispatchTouchEvent: ACTION_DOWN
         ViewGroup-1:  onInterceptTouchEvent: ACTION_DOWN
         View_1:  dispatchTouchEvent: ACTION_DOWN
         View_1:  onTouchEvent: ACTION_DOWN
MOVE事件：
         EventDeliverActivity:  dispatchTouchEvent: ACTION_MOVE
         ViewGroup-1:  dispatchTouchEvent: ACTION_MOVE
         ViewGroup-1:  onInterceptTouchEvent: ACTION_MOVE
         View_1:  dispatchTouchEvent: ACTION_MOVE
         View_1:  onTouchEvent: ACTION_MOVE
UP事件：
         EventDeliverActivity:  dispatchTouchEvent: ACTION_UP
         ViewGroup-1:  dispatchTouchEvent: ACTION_UP
         ViewGroup-1:  onInterceptTouchEvent: ACTION_UP
         View_1:  dispatchTouchEvent: ACTION_UP
         View_1:  onTouchEvent: ACTION_UP

綜上得出結論: dispatchTouchEvent 或者 onTouchEvent 返回true 將會直接導致事件被終止，如何終止？ 如DOWN事件它會不繼續往下傳遞，而是走MOVE和UP事件。調用順序是Activity 的dispatchTouchEvent DOWN事件 然後會調用DecorView的 dispatchTouchEvent 和onInterceptTouchEvent方法接着會調用ViewGroup_1的 dispatchTouchEvent 和onInterceptTouchEvent方法的DOWN事件，然後調用View_1的dispatchTouchEvent DOWN事件， 然後DOWN事件就終止了，繼續MOVE和UP時間。對於ViewGroup_1 也是同樣的道理這裏就不舉例了。

情況三 Activity -> ViewGroup_1 -> View_1 在ViewGroup_1的onInterceptTouchEvent 返回true

DOWN事件:
         EventDeliverActivity:  dispatchTouchEvent: ACTION_DOWN
         ViewGroup-1:  dispatchTouchEvent: ACTION_DOWN
         ViewGroup-1:  onInterceptTouchEvent: ACTION_DOWN
         ViewGroup-1:  onTouchEvent: ACTION_DOWN
         EventDeliverActivity:  onTouchEvent: ACTION_DOWN
MOVE事件：
         EventDeliverActivity:  dispatchTouchEvent: ACTION_MOVE
         EventDeliverActivity:  onTouchEvent: ACTION_MOVE
UP事件：
         EventDeliverActivity:  dispatchTouchEvent: ACTION_UP
         EventDeliverActivity:  onTouchEvent: ACTION_UP

綜上得出結論，如果在ViewGroup_1的onInterceptTouchEvent 返回true 它會導致事件不在向下傳，而是直接給它自己的onTouchEvent處理，然後根據onTouchEvent返回情況，在調用自己的父View的onTouchEvent。 簡單說就是改變了事件的傳遞方向。 好比一個球從十樓開始自由落體，dispatchTouchEvent 向下走，onInterceptTouchEvent是決定是否要反彈，onTouchEvent是向上走。 現在在中間樓層的onInterceptTouchEvent 返回true， 那麼球就從中間樓層開始反彈了，而不是一直掉到一樓纔開始反彈向上走。

情況四 看看View_1 onTouch onClick 以及 onLongClick方法的調用時機

DOWN事件:
         EventDeliverActivity:  dispatchTouchEvent: ACTION_DOWN
         ViewGroup-1:  dispatchTouchEvent: ACTION_DOWN
         ViewGroup-1:  onInterceptTouchEvent: ACTION_DOWN
         View_1:  dispatchTouchEvent: ACTION_DOWN
         EventDeliverActivity: onTouch: view_1
         View_1:  onTouchEvent: ACTION_DOWN
MOVE事件：
         EventDeliverActivity:  dispatchTouchEvent: ACTION_MOVE
         ViewGroup-1:  dispatchTouchEvent: ACTION_MOVE
         ViewGroup-1:  onInterceptTouchEvent: ACTION_MOVE
         View_1:  dispatchTouchEvent: ACTION_MOVE
         EventDeliverActivity: onTouch: view_1
         View_1:  onTouchEvent: ACTION_MOVE
UP事件：
         EventDeliverActivity:  dispatchTouchEvent: ACTION_UP
         ViewGroup-1:  dispatchTouchEvent: ACTION_UP
         ViewGroup-1:  onInterceptTouchEvent: ACTION_UP
         View_1:  dispatchTouchEvent: ACTION_UP
         EventDeliverActivity: onTouch: view_1
         View_1:  onTouchEvent: ACTION_UP
         EventDeliverActivity: onClick: view_1

綜上上面的打印日誌分析得出， view_1的onClick事件發生在UP事件以後響應，並且在View_1的onTouchEvent 中響應。view_1 的 onTouch 方法發生在 dispatchTouchEvent 和 onTouchEvent 之間。

結合源碼查看驗證，分別在View.java 的 public boolean dispatchTouchEvent(MotionEvent event) {}方法中，可以看到先是調用 li.mOnTouchListener.onTouch(this, event)方法， 如果返回true的話，就會得到result=true if (!result && onTouchEvent(event)) {} 就可以得出 onTouchEvent 不會被執行。如果返回false onTouchEvent方法被執行，接着在ACTION_UP事件處理時，performClickInternal() -> performClick()調用
調用onClick方法在onTouchEvent ACTION_DOWN 事件中 checkForLongClick(0, x, y); 觸發longClick事件。

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

        if (onFilterTouchEventForSecurity(event)) {
            if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
                result = true;
            }
            //noinspection SimplifiableIfStatement
            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnTouchListener != null
                    && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {
                    //onTouch方法的返回值講直接決定了onTouchEvent是否被調用，如果返回true 代表事件被消費處理
               
                result = true;
            }
			// onTouchEvent 處理onClick和onLongClick事件
            if (!result && onTouchEvent(event)) {
                result = true;
            }
        }
        return result;
    }

    public boolean onTouchEvent(MotionEvent event) {
        final float x = event.getX();
        final float y = event.getY();
        final int viewFlags = mViewFlags;
        final int action = event.getAction();

        final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
                || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
                || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;

        if ((viewFlags & ENABLED_MASK) == DISABLED) {
            if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
                setPressed(false);
            }
            mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
            // A disabled view that is clickable still consumes the touch
            // events, it just doesn't respond to them.
            return clickable;
        }
        if (mTouchDelegate != null) {
            if (mTouchDelegate.onTouchEvent(event)) {
                return true;
            }
        }

        if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
            switch (action) {
                case MotionEvent.ACTION_UP:
                    mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
                    if ((viewFlags & TOOLTIP) == TOOLTIP) {
                        handleTooltipUp();
                    }
                    if (!clickable) {
                        removeTapCallback();
                        removeLongPressCallback();
                        mInContextButtonPress = false;
                        mHasPerformedLongPress = false;
                        mIgnoreNextUpEvent = false;
                        break;
                    }
                    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)) {
                                	//處理點擊事件
                                    performClickInternal();
                                }
                            }
                        }

                        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:
                    if (event.getSource() == InputDevice.SOURCE_TOUCHSCREEN) {
                        mPrivateFlags3 |= PFLAG3_FINGER_DOWN;
                    }
                    mHasPerformedLongPress = false;

                    if (!clickable) {
                    	//檢查是否是長按事件
                        checkForLongClick(0, x, y);
                        break;
                    }

                    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, x, y);
                    }
                    break;

                case MotionEvent.ACTION_CANCEL:
                    if (clickable) {
                        setPressed(false);
                    }
                    removeTapCallback();
                    removeLongPressCallback();
                    mInContextButtonPress = false;
                    mHasPerformedLongPress = false;
                    mIgnoreNextUpEvent = false;
                    mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
                    break;

                case MotionEvent.ACTION_MOVE:
                    if (clickable) {
                        drawableHotspotChanged(x, y);
                    }

                    // Be lenient about moving outside of buttons
                    if (!pointInView(x, y, mTouchSlop)) {
                        // Outside button
                        // Remove any future long press/tap checks
                        removeTapCallback();
                        removeLongPressCallback();
                        if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
                            setPressed(false);
                        }
                        mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
                    }
                    break;
            }

            return true;
        }

        return false;
    }

最後通過一張圖表示上面的幾次實驗的結果。