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？

public boolean onTouchEvent(MotionEvent event) {
int action = event.getAction();
switch (action & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_DOWN:
showMsg("ACTION_DOWN" + action);
break;
case MotionEvent.ACTION_UP:
showMsg("ACTION_UP" + action);
break;
case MotionEvent.ACTION_POINTER_UP:
showMsg("ACTION_POINTER_UP" + action);
break;
case MotionEvent.ACTION_POINTER_DOWN:
showMsg("ACTION_POINTER_DOWN" + action);
break;
}
return super.onTouchEvent(event);
}

首先来看看这些常量的值

ACTION_MASK     0x000000ff
ACTION_DOWN     0x00000000         ACTION_UP      0x00000001
       ACTION_MOVE      0x00000002
ACTION_POINTER_DOWN       0x00000005            ACTION_POINTER_UP
       0x00000006
ACTION_POINTER_1_DOWN     0x00000005            ACTION_POINTER_1_UP
     0x00000006
ACTION_POINTER_2_DOWN     0x00000105            ACTION_POINTER_2_UP
     0x00000106

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);
}

});