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Moving minutes to Android watches

I am trying to write a simple proof of concept of an application that allows the user to rotate the minute hand of a clock. I find it hard to find the right logic for OnTouchEvent.

So far, I have the following code:

    public boolean onTouchEvent(MotionEvent e) {

       float x = e.getX();
       float y = e.getY();

    switch (e.getAction()) {
    case MotionEvent.ACTION_MOVE:
        //find an approximate angle between them.

        float dx = x-cx;
        float dy = y-cy;
        double a=Math.atan2(dy,dx);

        this.degree = Math.toDegrees(a);
        this.invalidate();
    }
    return true;
    }


protected void onDraw(Canvas canvas) {
    super .onDraw(canvas);

    boolean changed = mChanged;
    if (changed) {
        mChanged = false;
    }

    int availableWidth = getRight() - getLeft();
    int availableHeight = getBottom() - getTop();

    int x = availableWidth / 2;
    int y = availableHeight / 2;

    cx = x;
    cy = y;

    final Drawable dial = mDial;

    int w = dial.getIntrinsicWidth() + 100;
    int h = dial.getIntrinsicHeight() + 100;

    boolean scaled = false;

    if (availableWidth < w || availableHeight < h) {
        scaled = true;

        float scale = Math.min((float) availableWidth / (float) w, (float) availableHeight / (float) h);

        canvas.save();
        canvas.scale(scale, scale, x, y);
    }

    if (changed)
    {
        dial.setBounds(x - (w / 2), y - (h / 2), x + (w / 2), y + (h / 2));
    }

    dial.draw(canvas);

    canvas.save();
    float hour = mHour / 12.0f * 360.0f;

    canvas.rotate(hour, x, y);

    final Drawable hourHand = mHourHand;

    if (changed) {

        w = hourHand.getIntrinsicWidth() + 30;
        h = hourHand.getIntrinsicHeight() + 30;

        hourHand.setBounds(x - (w / 2), y - (h / 2), x + (w / 2), y  + (h / 2));
    }

    hourHand.draw(canvas);
    canvas.restore();

    canvas.save();
    float minute = mMinutes / 60.0f * 360.0f;

    if (bearing == 0)
    {
        canvas.rotate(minute, x, y);
    }
    else
    {
        canvas.rotate((float)bearing, x, y);
    }

    final Drawable minuteHand = mMinuteHand;

    if (changed) {

        w = minuteHand.getIntrinsicWidth() + 30;
        h = minuteHand.getIntrinsicHeight() + 30;

        minuteHand.setBounds(x - w, y - h, x + w, y  + h);
    }

    minuteHand.draw(canvas);
    canvas.restore();

    if (scaled) {
        canvas.restore();
    }
}

Then, based on this, my OnDraw method rotates the minute hand to the specified "this.degree" (just calls canvas.rotate). I guess my math is here. I tried to follow the example here: Calculate the rotation angle in a pie chart , but it still does not rotate the minute hand correctly. Any help would be greatly appreciated.

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2 answers

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//first we save the initial matrix, so we can easily get
//back to this state after we're done rotating
canvas.save();
//I *think* you need to negate the center offsets here,
//because you are conceptually moving the canvas, rather
//than moving the center directly
canvas.translate(-cx, -cy);
//<perform the rotation and draw the clock hand>
//...

//and now restore the matrix back to the initial state   
canvas.restore();
+2

... .... onTouch()

public float getRotationAngle(float x, float y) {
        float dx = x - cx;
        float dy = y - cy;
        double a = Math.atan2(dy, dx);

        double degree = Math.toDegrees(a)+90;
        if(angle<0){
            degree=degree+360;
        }
        return (float) degree;
}

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