iphone sdk CGAffineTransform getting the angle of rotation of an object

Question

how do i calculate the angle of rotation for any given object (ie a uiimageview)?

Answer 1

Technically you can't, because the transform can include a skew operation which turns the image into a parallelogram and the rotation angle isn't defined anymore.

Anyway, since the rotation matrix generates

 cos(x)  sin(x)   0
-sin(x)  cos(x)   0
   0        0     1

You can recover the angle with

return atan2(transform.b, transform.a);