View和ViewGroup事件分發理解

一：View事件分發理解

1.View事件執行的順序爲View.onDispatcher-->View.setOnTouchEvent-->View.onTouchEvent，如果setOnTouchEvent中onTouch方法返回true表示該事件被消費，則表示

不執行onTouchEvent，onTouch執行的條件是註冊了OnTouch事件，並且View爲可點擊狀態，具體代碼如下

 public boolean dispatchTouchEvent(MotionEvent event) {
        if (!onFilterTouchEventForSecurity(event)) {
            return false;
        }

        if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED &&
                mOnTouchListener.onTouch(this, event)) {
            return true;
        }
        return onTouchEvent(event);
    }

2.onTouch方法會優先於OnClickLisener執行，因爲OnClick是在onTouchEvent中調用當dispatchTouchEvent在進行事件分發的時候，只有前一個action返回true，纔會觸發下一個action

public boolean onTouchEvent(MotionEvent event) {
    final int viewFlags = mViewFlags;
    if ((viewFlags & ENABLED_MASK) == DISABLED) {
        // 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));
    }
    if (mTouchDelegate != null) {
        if (mTouchDelegate.onTouchEvent(event)) {
            return true;
        }
    }
    if (((viewFlags & CLICKABLE) == CLICKABLE ||
            (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_UP:
                boolean prepressed = (mPrivateFlags & PREPRESSED) != 0;
                if ((mPrivateFlags & 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 (!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();
                            }
                        }
                    }
                    if (mUnsetPressedState == null) {
                        mUnsetPressedState = new UnsetPressedState();
                    }
                    if (prepressed) {
                        mPrivateFlags |= PRESSED;
                        refreshDrawableState();
                        postDelayed(mUnsetPressedState,
                                ViewConfiguration.getPressedStateDuration());
                    } else if (!post(mUnsetPressedState)) {
                        // If the post failed, unpress right now
                        mUnsetPressedState.run();
                    }
                    removeTapCallback();
                }
                break;
            case MotionEvent.ACTION_DOWN:
                if (mPendingCheckForTap == null) {
                    mPendingCheckForTap = new CheckForTap();
                }
                mPrivateFlags |= PREPRESSED;
                mHasPerformedLongPress = false;
                postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
                break;
            case MotionEvent.ACTION_CANCEL:
                mPrivateFlags &= ~PRESSED;
                refreshDrawableState();
                removeTapCallback();
                break;
            case MotionEvent.ACTION_MOVE:
                final int x = (int) event.getX();
                final int y = (int) event.getY();
                // Be lenient about moving outside of buttons
                int slop = mTouchSlop;
                if ((x < 0 - slop) || (x >= getWidth() + slop) ||
                        (y < 0 - slop) || (y >= getHeight() + slop)) {
                    // Outside button
                    removeTapCallback();
                    if ((mPrivateFlags & PRESSED) != 0) {
                        // Remove any future long press/tap checks
                        removeLongPressCallback();
                        // Need to switch from pressed to not pressed
                        mPrivateFlags &= ~PRESSED;
                        refreshDrawableState();
                    }
                }
                break;
        }
        return true;
    }
    return false;
}

2.setOnLongClickListener和setOnClickLisenter是否只能執行一個？

如果設置setOnLongClickLisener中返回爲false，兩個都會執行，如果返回true，會屏蔽setOnClickLisener方法

二  ViewGroup事件分發理解

1.ViewGroup繼承View覆蓋了其dispatchTouchEvent方法，當在Activity中點擊一個button，首先會調用該Activity中的dispatchTouchEvent，會拿到setContentVIew中

根節點調用其dispatchTouchEvent方法。

2.在ACTION_DOWN裏，首選判斷事件是否被攔截，如果沒有被攔截，則尋找子View，一旦子View.dispatchTouchEvent 則將這個子View賦值給Target，代表這個子View會消費該事件，如果沒有找到這個子View，會調用Super.dispatchTouchEvent 也就是自身來處理該事件，如果自身也不能處理就會調用Activity中onTouchEvent，最終會調用target.dispatchTouchEvent(ev).

3.如果攔截了事件，那麼會調用target的ACTION_CANCLE方法，並將target=null，返回true，事件不會向下傳遞

public boolean dispatchTouchEvent(MotionEvent ev) {
    if (ev.getAction() == MotionEvent.ACTION_DOWN) {
        onUserInteraction();
    }
    //這個最終傳遞到setContentView對應的View中
    if (getWindow().superDispatchTouchEvent(ev)) {
        return true;
    }
    //如果ContentView沒有對時間進行處理，統一由Activity的onTouchEvent()來處理
    return onTouchEvent(ev);
}

public boolean dispatchTouchEvent(MotionEvent ev) {
    final int action = ev.getAction();
    final float xf = ev.getX();
    final float yf = ev.getY();
    final float scrolledXFloat = xf + mScrollX;
    final float scrolledYFloat = yf + mScrollY;
    final Rect frame = mTempRect;
    boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
    if (action == MotionEvent.ACTION_DOWN) {
        if (mMotionTarget != null) {
            mMotionTarget = null;
        }
        if (disallowIntercept || !onInterceptTouchEvent(ev)) {
            ev.setAction(MotionEvent.ACTION_DOWN);
            final int scrolledXInt = (int) scrolledXFloat;
            final int scrolledYInt = (int) scrolledYFloat;
            final View[] children = mChildren;
            final int count = mChildrenCount;
            for (int i = count - 1; i >= 0; i--) {
                final View child = children[i];
                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE
                        || child.getAnimation() != null) {
                    child.getHitRect(frame);
                    if (frame.contains(scrolledXInt, scrolledYInt)) {
                        final float xc = scrolledXFloat - child.mLeft;
                        final float yc = scrolledYFloat - child.mTop;
                        ev.setLocation(xc, yc);
                        child.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
                        if (child.dispatchTouchEvent(ev))  {
                            mMotionTarget = child;
                            return true;
                        }
                    }
                }
            }
        }
    }
    boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||
            (action == MotionEvent.ACTION_CANCEL);
    if (isUpOrCancel) {
        mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
    }
    final View target = mMotionTarget;
    if (target == null) {
        ev.setLocation(xf, yf);
        if ((mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {
            ev.setAction(MotionEvent.ACTION_CANCEL);
            mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
        }
        return super.dispatchTouchEvent(ev);
    }
    if (!disallowIntercept && onInterceptTouchEvent(ev)) {
        final float xc = scrolledXFloat - (float) target.mLeft;
        final float yc = scrolledYFloat - (float) target.mTop;
        mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
        ev.setAction(MotionEvent.ACTION_CANCEL);
        ev.setLocation(xc, yc);
        if (!target.dispatchTouchEvent(ev)) {
        }
        mMotionTarget = null;
        return true;
    }
    if (isUpOrCancel) {
        mMotionTarget = null;
    }
    final float xc = scrolledXFloat - (float) target.mLeft;
    final float yc = scrolledYFloat - (float) target.mTop;
    ev.setLocation(xc, yc);
    if ((target.mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {
        ev.setAction(MotionEvent.ACTION_CANCEL);
        target.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
        mMotionTarget = null;
    }
    return target.dispatchTouchEvent(ev);
}

常見問題分析解決：

1. touch監聽器沒被調用到？ 
   看View.dispatchTouchEvent(),ViewGroup.dispatchTouchEvent() 
   a)如果是View非使能，直接用setEnabled(true) 
   b)如果是事件被這個View的viewparent攔截了。可以修改這個viewparent的onInterceptTouchTouchEvent(),或者在這個View中調用getParent().requestDisallowInterceptTouchEvent()

2. 雙層滑動模塊嵌套後發生滑動不了的現象? 
   看ViewGroup.dispatchTouchEvent() 
   如果是事件被這個View的viewparent攔截了。可以修改這個viewparent的onInterceptTouchTouchEvent(),或者在這個View中調用getParent().requestDisallowInterceptTouchEvent()

3. 設置了onClickListener後，點擊View沒有反應? 
   看View.onTouchEvent() 
   a)如果是View非使能，直接用setEnabled(true) 
   b)可能覆蓋了onTouchEvent(),需要在覆蓋的方法調用super.onTouchEvent()或者手動調用performClick()

4. 點擊兩下View才調用onClickListener的bug? 
   看View.onTouchEvent() 
   這個其實是安卓的設計，當某個View調用了setFocusableInTouchMode(true)後，第一次點擊會引起這個View的focus，第二次點擊纔會調用onClickListener，只需要設置setFocusableInTouchMode(false)即可。

5.ListView中Item帶Button，Button click事件和OnItemClick事件的衝突

       1.這種可以在itemview中進行事件攔截，攔截後不會調用button dispatcherTouchEvent方法，可以在adapter中通過requestDisallowInterceptTouchEvent（true）進行攔截

   2.覆蓋Button的onTouchEvent方法，返回false