在ACTION_MOVE期间，准确地确定手指在Android中的移动距离

Question

In my onTouchEvent method's ACTION_MOVE I'm attempting to find out how many pixels the users finger has moved during a touch gesture - this amount will be updated every frame and an object on the screen will be moved by the same amount.

This is what I'm doing so far:

case MotionEvent.ACTION_MOVE: {

            movedByY = oldYPosition - event.getY();

            oldYPosition = event.getY();

}

I'm only really concerned with movement in the Y direction and the above seems fairly simple.

Once I've got the amount, I move my object by the same amount - something like this.

myObject.currentYPos -= movedByY;

This kind of works, however, it isn't precise enough. When moving downwards, the finger gets slightly ahead of the object (object moves too slow) and when moving upwards, the object gets slightly ahead of the finger (object moves too fast). The slower you move your finger, the bigger the discrepency.

Note I can't simply set the object's coordinates to match the finger because that's not what I'm trying to achieve. If the user places a finger down on the screen and moves it, the object should mimic it's movements extactly but not 'jump to' where it is.

Let's say the object is at 100,100 and the user places their finger down at 600 x 586 and moves 100 pixels upwards, then the object should follow this path and end up at 100 x 0 - and as we're updating this every frame, at any given frame the amount the object as moved, should match the amount the finger has moved exactly .

I can't figure out why it's moving at a different speed to the finger. Any clues would be appreciated.

Answer 1

I managed to get around the problem by working out the movedByY value in my GLRendering thread (in my update() method).

So, in ACTION_MOVE - I did this:

case MotionEvent.ACTION_MOVE: {

        onTouchYCoord = event.getY();

}

And then in my logic update method, used that value to get the correct amount like so:

logicYCoord = (int) onTouchYCoord ;

movedByY = logicYCoord -oldYPosition;

oldYPosition = logicYCoord;

Then I update the sprite's position:

myObject.currentYPos += movedByY;

This works perfectly and the object stays with the finger. Clearly, the problem was the fact that I was using a value from the UI Thread which was could have changed at any time. This way, I'm guaranteed the value I'm using in my calculations stays consistent by taking a snapshot of it.