android多点触控统一的原理(使用 event.getAction()&MotionEvent.ACTION_MASK的原因)

看到下面代码中用了AND位运算是为了什么呢？

首先来看看这些常量的值

例如：ACTION_MASK & ACTION_POINTER_2_DOWN 即0x000000ff & 0×00000105=0x0000005

可以看到，and运算的结果总是小于等于0x000000ff，那就是说and之后，无论你多少根手指加进来，都是会ACTION_POINTER_DOWN或者ACTION_POINTER_UP

imgitem.setOnTouchListener(new OnTouchListener() {

			            PointF start = new PointF();
			            PointF mid = new PointF();
			            float oldDist = 1f;
			            Matrix matrix = new Matrix();
			            Matrix savedMatrix = new Matrix();
			            static final int NONE = 0;
			            static final int DRAG = 1;
			            static final int ZOOM = 2;
			            int mode = NONE;

			            long startTime;

			            public boolean onTouch(View v, MotionEvent event) {
			                ImageView view = (ImageView) v;
			                view.setScaleType(ScaleType.MATRIX);

			                switch (event.getAction() & MotionEvent.ACTION_MASK) {

			                case MotionEvent.ACTION_DOWN:
			                    matrix.set(view.getImageMatrix());
			                    savedMatrix.set(matrix);
			                    start.set(event.getX(), event.getY());
			                    mode = DRAG;
			                    // isclick = true;
			                    startTime = System.currentTimeMillis();
			                    break;

			                case MotionEvent.ACTION_POINTER_DOWN:
			                    // isclick = false;
			                    oldDist = spacing(event);
			                    if (oldDist > 10f) {
			                        savedMatrix.set(matrix);
			                        midPoint(mid, event);
			                        mode = ZOOM;
			                    }
			                    break;

			                case MotionEvent.ACTION_UP:
			                    /*if (mode == DRAG && System.currentTimeMillis() - startTime < 150) {
			                        finish();
			                        overridePendingTransition(android.R.anim.fade_in, android.R.anim.fade_out);
			                    }*/
			                    break;
			                case MotionEvent.ACTION_POINTER_UP:
			                    mode = NONE;
			                    break;
			                case MotionEvent.ACTION_MOVE:
			                    // isclick = false;
			                    if (mode == DRAG) {
			                        matrix.set(savedMatrix);
			                        matrix.postTranslate(event.getX() - start.x, event.getY() - start.y);
			                    } else if (mode == ZOOM) {
			                        float newDist = spacing(event);
			                        if (newDist > 10f) {
			                            matrix.set(savedMatrix);
			                            float scale = newDist / oldDist;
			                            matrix.postScale(scale, scale, mid.x, mid.y);
			                        }
			                    }
			                    break;
			                }

			                view.setImageMatrix(matrix);

			                return true;
			            }

			            private float spacing(MotionEvent event) {
			                float x = event.getX(0) - event.getX(1);
			                float y = event.getY(0) - event.getY(1);
			                return FloatMath.sqrt(x * x + y * y);
			            }

			            private void midPoint(PointF point, MotionEvent event) {
			                float x = event.getX(0) + event.getX(1);
			                float y = event.getY(0) + event.getY(1);
			                point.set(x / 2, y / 2);
			            }

			        });