How would I implement a swipe-based circular control like this?

Question

I am working on an Android application, and I have a TextView where I display a price (for example 50$).

I would like to have a circular control similar to this picture:

Answer 1

I've just written the following code and only tested it theoretically.

private final double stepSizeAngle = Math.PI / 10f; //Angle diff to increase/decrease dial by 1$
private final double dialStartValue = 50.0;

//Center of your dial
private float dialCenterX = 500;
private float dialCenterY = 500;

private float fingerStartDiffX;
private float fingerStartDiffY;

private double currentDialValueExact = dialStartValue;


public boolean onTouchEvent(MotionEvent event) {
    int eventaction = event.getAction();

    switch (eventaction) {
        case MotionEvent.ACTION_DOWN: 
            //Vector between startpoint and center
            fingerStartDiffX = event.getX() - dialCenterX;
            fingerStartDiffY = event.getY() - dialCenterY;
            break;

        case MotionEvent.ACTION_MOVE:
            //Vector between current point and center
            float xDiff = event.getX() - dialCenterX;
            float yDiff = event.getY() - dialCenterY;

            //Range from -PI to +PI
            double alpha = Math.atan2(fingerStartDiffY, yDiff) - Math.atan2(fingerStartDiffX, xDiff);

            //calculate exact difference between last move and current move.
            //This will take positive and negative direction into account.
            double dialIncrease = alpha / stepSizeAngle;        
            currentDialValueExact += dialIncrease;

            //Round down if we're above the start value and up if we are below
            setDialValue((int)(currentDialValueExact > dialStartValue ? Math.floor(currentDialValueExact) : Math.ceil(currentDialValueExact));

            //set fingerStartDiff to the current position to allow multiple rounds on the dial
            fingerStartDiffX = xDiff;
            fingerStartDiffY = yDiff;
            break;
    }

    // tell the system that we handled the event and no further processing is required
    return true; 
}

private void setDialValue(int value) {
    //assign value
}

If you would like to change the direction, simply do alpha = -alpha.