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