Android捏缩放。捏时对焦点/图像移位

Question

我一直致力于使用非常光滑的捏拉缩放创建自定义视图。到目前为止，我已经进行了缩放和平移工作，但是当我在屏幕的不同区域捏缩放时，图像移动到屏幕的相对位置。

例如，如果图像在左侧并且我在左侧捏缩放，则图像将向右移动。

我确定我在某个地方犯了一个简单的错误，但我不能为我的生活找出问题所在。我在过去的几天里尝试过大量的改变，花了几个小时搜索SO无济于事。

这是我到目前为止的代码：

private class PinchView extends View {

    private ScaleGestureDetector mScaleDetector;
    private float mScaleFactor = 1.f;
    private static final int INVALID_POINTER_ID = -1;

    // The ‘active pointer’ is the one currently moving our object.
    private int mActivePointerId = INVALID_POINTER_ID;

    private Drawable mIcon;
    private float mPosX;
    private float mPosY;

    private float mLastTouchX;
    private float mLastTouchY;


    private float focusX;
    private float focusY;



    public PinchView(Context context) {
        super(context);
        mIcon = context.getResources().getDrawable(R.drawable.ic_launcher);
        mIcon.setBounds(0, 0, mIcon.getIntrinsicWidth(), mIcon.getIntrinsicHeight());

        // Create our ScaleGestureDetector
        mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        // Let the ScaleGestureDetector inspect all events.
        mScaleDetector.onTouchEvent(ev);

        final int action = ev.getAction();
        switch (action & MotionEvent.ACTION_MASK) {
        case MotionEvent.ACTION_DOWN: {
            final float x = ev.getX()/mScaleFactor;
            final float y = ev.getY()/mScaleFactor;

            mLastTouchX = x;
            mLastTouchY = y;
            mActivePointerId = ev.getPointerId(0);
            break;
        }

        case MotionEvent.ACTION_MOVE: {
            final int pointerIndex = ev.findPointerIndex(mActivePointerId);
            final float x = ev.getX(pointerIndex)/mScaleFactor;
            final float y = ev.getY(pointerIndex)/mScaleFactor;

            // Only move if the ScaleGestureDetector isn't processing a gesture.
            if (!mScaleDetector.isInProgress()) {
                final float dx = x - mLastTouchX;
                final float dy = y - mLastTouchY;

                mPosX += dx;
                mPosY += dy;

                invalidate();
            }

            mLastTouchX = x;
            mLastTouchY = y;

            break;
        }

        case MotionEvent.ACTION_UP: {
            mActivePointerId = INVALID_POINTER_ID;
            break;
        }

        case MotionEvent.ACTION_CANCEL: {
            mActivePointerId = INVALID_POINTER_ID;
            break;
        }

        case MotionEvent.ACTION_POINTER_UP: {
            final int pointerIndex = (ev.getAction() & MotionEvent.ACTION_POINTER_INDEX_MASK) 
                    >> MotionEvent.ACTION_POINTER_INDEX_SHIFT;
            final int pointerId = ev.getPointerId(pointerIndex);
            if (pointerId == mActivePointerId) {
                // This was our active pointer going up. Choose a new
                // active pointer and adjust accordingly.
                final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
                mLastTouchX = ev.getX(newPointerIndex)/mScaleFactor;
                mLastTouchY = ev.getY(newPointerIndex)/mScaleFactor;
                mActivePointerId = ev.getPointerId(newPointerIndex);
            }



            break;
        }
        }

        return true;
    }

    @Override
    public void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        canvas.save();

     //   canvas.scale(mScaleFactor, mScaleFactor);
        canvas.scale(mScaleFactor, mScaleFactor, focusX, focusY);
        canvas.translate(mPosX, mPosY);
        mIcon.draw(canvas);
        canvas.restore();
    }

    private float lastFocusX = -1;
    private float lastFocusY = -1;

    private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {



        @Override
        public boolean onScaleBegin(ScaleGestureDetector detector) {

        //  float x = detector.getFocusX();
        //  float y = detector.getFocusY();

            lastFocusX = -1;
            lastFocusY = -1;


            return true;
        }

        @Override
        public boolean onScale(ScaleGestureDetector detector) {
            mScaleFactor *= detector.getScaleFactor();

            focusX = detector.getFocusX();
            focusY = detector.getFocusY();

            if(lastFocusX == -1) lastFocusX = focusX;
            if(lastFocusY == -1) lastFocusY = focusY;


            mPosX += (focusX - lastFocusX);
            mPosY += (focusY - lastFocusY);




            // Don't let the object get too small or too large.
            mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f));

          lastFocusX = focusX;
          lastFocusY = focusY;

            invalidate();
            return true;
        }



    }
}