Issue With Canvas.rotate() In Android .how Does It Exactly Work?

Please find the code of my onDraw method below(.I'm trying to rotate the canvas(//Rotate call -b) by 25 degrees after drawing the arc .But i find that the arc is still drawn from 0

Solution 1:

The Canvas maintains a Matrix that is responsible for all transformations on it. Even for rotation. As you can see in the documentation, the rotate method says:

Preconcat the current matrix with the specified rotation.

All transformations are done on the CanvasMatrix,hence, on the Canvas. The arc you draw isn't rotated. You first rotate the Canvas and then draw on it.

Hence, in your code, call -a works, not call -b.

EDIT: For issues like postrotate and prerotate check the Matrix class(postRotate and preRotate methods).

Few Examples: this and this.

And few things you might want to read : this and this.