Transforms - iOS Core Animation - page 74

Listing 5.1 Rotating a Layer by 45 Degrees Using affineTransform
@interface ViewController ()
@property ( nonatomic , weak ) IBOutlet UIView *layerView;
@end
@implementation ViewController
- ( void )viewDidLoad
{
[ super viewDidLoad ];
//rotate the layer 45 degrees
CGAffineTransform transform = CGAffineTransformMakeRotation( M_PI_4 );
self . layerView . layer .affineTransform = transform;
}
@end
Note that the value we've used for the angle is a constant called M_PI_4 , not the number
45 as you might have expected. The transform functions on iOS use radians rather than
degrees for all angular units. Radians are usually specified using multiples of the
mathematical constant π (pi). π radians equates to 180 degrees, so π divided by 4 is
equivalent to 45 degrees.
The C math library (which is automatically included in every iOS project) conveniently
provides constants for common multiples of π, and M_PI_4 is the constant representing π
divided by 4. If you struggle to think in terms of radians, you can use these macros to
convert to and from degrees:
#define RADIANS_TO_DEGREES (x) ((x)/ M_PI * 180.0 )
#define DEGREES_TO_RADIANS (x) ((x)/ 180.0 * M_PI )
Combining Transforms
Core Graphics also provides a second set of functions that can be used to apply a further
transform on top of an existing one. This is useful if you want to create a single transform
matrix that both scales and rotates a layer, for example. These functions are as follows:
CGAffineTransformRotate( CGAffineTransform t, CGFloat angle)
CGAffineTransformScale( CGAffineTransform t, CGFloat sx, CGFloat sy)
CGAffineTransformTranslate( CGAffineTransform t, CGFloat tx, CGFloat ty)