Android, How can I rotate an arrow (image) around a fixed point?

Question

I have an arrow image that I want to rotate from 0 to 180 degree (like the needle in a meter.) One point of the arrow is fixed in middle and at bottom of the screen and head of arrow should move. Length of arrow is fix (it is image). Also I have two buttons and I want arrow to turn left when button left is touched and turn right when right button is touched.

What is the logic of this process?

Answer 1

This is actually pretty simple if you are using a canvas to do your drawing(as you should in your case).

Given that you know the coordinates for the point around which the image should rotate, you can do it like this:

private void doDraw(Canvas canvas) {
canvas.save();
float px = ...;
float py = ...;
canvas.rotate(degrees, px, py);
arrow.draw(canvas);
canvas.restore();
}

degrees will be a integer value that you increment/decrement when the user clicks the L or R buttons. canvas.rotate takes care of the rest!