android4.4之GestureDetector（手勢識別）

應用如果需要復雜的手勢匹配，這時候可以使用GestureDetector來實現。

實現步驟：

1、實現OnGestureListener類，也可繼承SimpleOnGestureListener類然後復寫相應函數；

2、創建一個GestureDetector類對象，然後new一個第1步中自定義的監聽類對象作為參數穿進去；

3、在接收到MotionEvent事件時，調用OnGestureListener.onTouchEvent(onTouchEvent)函數。

對手勢匹配做的一些自定義處理在手勢監聽類中實現。

下面對以上三步做一些簡單分析：先看GestureDetector構造函數

    public GestureDetector(Context context, OnGestureListener listener, Handler handler) {
        if (handler != null) {
            mHandler = new GestureHandler(handler);
        } else {
            mHandler = new GestureHandler();
        }
        mListener = listener;
        if (listener instanceof OnDoubleTapListener) {
            setOnDoubleTapListener((OnDoubleTapListener) listener);
        }
        init(context);
    }

從構造函數看出new一個GestureDetector對象需要一個OnGestureListener對象，該對象保存在mListener中，如果手勢監聽是繼承SimpleOnGestureListener類，那麼還會調用setOnDoubleTapListener()。init函數做一些初始化函數，先不看。

接著看第3步OnGestureListener.onTouchEvent(onTouchEvent)函數處理。此函數稍微有點長，分段閱讀

    public boolean onTouchEvent(MotionEvent ev) {
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(ev, 0);
        }

        final int action = ev.getAction();

        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev);

        final boolean pointerUp =
                (action & MotionEvent.ACTION_MASK) == MotionEvent.ACTION_POINTER_UP;
        final int skipIndex = pointerUp ? ev.getActionIndex() : -1;

        // Determine focal point
        float sumX = 0, sumY = 0;
        final int count = ev.getPointerCount();
        for (int i = 0; i < count; i++) {
            if (skipIndex == i) continue;
            sumX += ev.getX(i);
            sumY += ev.getY(i);
        }
        final int div = pointerUp ? count - 1 : count;
        final float focusX = sumX / div;
        final float focusY = sumY / div;

        boolean handled = false;

InputEventConsistencyVerifier是調試用的，mVelocityTracker是一個速度追蹤類對象，緊接著的這段代碼邏輯是求一個MotionEvent的中心點，這邊調試時發現getPointerCount只有一個點而已，不管是點擊或滑動。接著往下看

        switch (action & MotionEvent.ACTION_MASK) {
        case MotionEvent.ACTION_POINTER_DOWN:
            mDownFocusX = mLastFocusX = focusX;
            mDownFocusY = mLastFocusY = focusY;
            // Cancel long press and taps
            cancelTaps();
            break;

        case MotionEvent.ACTION_POINTER_UP:
            mDownFocusX = mLastFocusX = focusX;
            mDownFocusY = mLastFocusY = focusY;

            // Check the dot product of current velocities.
            // If the pointer that left was opposing another velocity vector, clear.
            mVelocityTracker.computeCurrentVelocity(1000, mMaximumFlingVelocity);
            final int upIndex = ev.getActionIndex();
            final int id1 = ev.getPointerId(upIndex);
            final float x1 = mVelocityTracker.getXVelocity(id1);
            final float y1 = mVelocityTracker.getYVelocity(id1);
            for (int i = 0; i < count; i++) {
                if (i == upIndex) continue;

                final int id2 = ev.getPointerId(i);
                final float x = x1 * mVelocityTracker.getXVelocity(id2);
                final float y = y1 * mVelocityTracker.getYVelocity(id2);

                final float dot = x + y;
                if (dot < 0) {
                    mVelocityTracker.clear();
                    break;
                }
            }
            break;

        case MotionEvent.ACTION_DOWN:
            if (mDoubleTapListener != null) {
                boolean hadTapMessage = mHandler.hasMessages(TAP);
                if (hadTapMessage) mHandler.removeMessages(TAP);
                if ((mCurrentDownEvent != null) && (mPreviousUpEvent != null) && hadTapMessage &&
                        isConsideredDoubleTap(mCurrentDownEvent, mPreviousUpEvent, ev)) {
                    // This is a second tap
                    mIsDoubleTapping = true;
                    // Give a callback with the first tap of the double-tap
                    handled |= mDoubleTapListener.onDoubleTap(mCurrentDownEvent);
                    // Give a callback with down event of the double-tap
                    handled |= mDoubleTapListener.onDoubleTapEvent(ev);
                } else {
                    // This is a first tap
                    mHandler.sendEmptyMessageDelayed(TAP, DOUBLE_TAP_TIMEOUT);
                }
            }

            mDownFocusX = mLastFocusX = focusX;
            mDownFocusY = mLastFocusY = focusY;
            if (mCurrentDownEvent != null) {
                mCurrentDownEvent.recycle();
            }
            mCurrentDownEvent = MotionEvent.obtain(ev);
            mAlwaysInTapRegion = true;
            mAlwaysInBiggerTapRegion = true;
            mStillDown = true;
            mInLongPress = false;
            mDeferConfirmSingleTap = false;
            
            if (mIsLongpressEnabled) {
                mHandler.removeMessages(LONG_PRESS);
                mHandler.sendEmptyMessageAtTime(LONG_PRESS, mCurrentDownEvent.getDownTime()
                        + TAP_TIMEOUT + LONGPRESS_TIMEOUT);
            }
            mHandler.sendEmptyMessageAtTime(SHOW_PRESS, mCurrentDownEvent.getDownTime() + TAP_TIMEOUT);
            handled |= mListener.onDown(ev);
            break;

對於ACTION_DOWN事件，調用mListener.onDown(ev);處理，mListener是指向第1步中定義的手勢監聽類對象。對於MotionEvent.ACTION_MOVE事件，調用mListener.onScroll(mCurrentDownEvent, ev, scrollX, scrollY);函數，mCurrentDownEvent是當前滑動手勢中的按下點，ev是當前觸摸點，scrollX、scrollY是當前觸摸點跟上一個觸摸點的x/y軸偏移量。對於ACTION_UP事件，調用mListener.onSingleTapUp(ev)或者mListener.onFling(mCurrentDownEvent, ev, velocityX, velocityY);函數。

未完待續。