如何在Android中实现画布的“双指拖动或缩放”？

Question

我想在两个手指上实现画布的拖动和缩放。 我试过以下方法。

第1步： onTouchEvent我已经检查过getPointerCount == 2然后拖动画布类似于单指拖动。 代码：

if(ev.getPointerCount() == 2) {

                final int action = ev.getAction();
                switch (action & MotionEvent.ACTION_MASK) {
                    case MotionEvent.ACTION_POINTER_DOWN: {

                        final float x = ev.getX(0);
                        final float y = ev.getY(0);

                        mLastTouchX = x;
                        mLastTouchY = y;    

                        break;
                    }

                    case MotionEvent.ACTION_MOVE: {

                        final float x = ev.getX(0);
                        final float y = ev.getY(0);
                        // 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;

                        //invalidate();
                        break;
                    }

                    case MotionEvent.ACTION_UP: {
                        Log.i(TAG,"Double Touch : ACTION_UP SingleTouch:"+isSingleTouch );
                        mActivePointerId = INVALID_POINTER_ID;
                        break;
                    }

                    case MotionEvent.ACTION_CANCEL: {
                        Log.i(TAG,"Double Touch : ACTION_CANCEL SingleTouch:"+isSingleTouch );
                        mActivePointerId = INVALID_POINTER_ID;
                        break;
                    }

                    case MotionEvent.ACTION_POINTER_UP: {
                        Log.i(TAG,"Double Touch : ACTION_POINTER_UP SingleTouch:"+isSingleTouch );
                        // Extract the index of the pointer that left the touch sensor
                        final int pointerIndex = (action & 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);
                            mLastTouchY = ev.getY(newPointerIndex);
                            mActivePointerId = ev.getPointerId(newPointerIndex);
                        }
                        break;
                    }
                }

第2步：添加Scale Gesture Detector

代码：

public boolean onTouchEvent(MotionEvent ev) {

            mScaleDetector.onTouchEvent(ev);
                   /*Some code */          
}