Transforms - iOS Core Animation - page 73

Creating a CGAffineTransform
A full explanation of matrix mathematics is beyond the scope of this topic, and if you are
not already familiar with matrices, the idea of a transform matrix can seem quite daunting.
Fortunately, Core Graphics provides a number of built-in functions for building up arbitrary
transforms out of simple ones without requiring the developer to do any math. The
following functions each create a new CGAffineTransform matrix from scratch:
CGAffineTransformMakeRotation( CGFloat angle)
CGAffineTransformMakeScale( CGFloat sx, CGFloat sy)
CGAffineTransformMakeTranslation( CGFloat tx, CGFloat ty)
The rotation and scale transforms are fairly self-explanatory—they rotate and scale a vector
respectively. A translation transform just adds the specified x and y values to the vector—
so if the vector represents a point, it moves the point.
Let's demonstrate the effect of these functions with a simple project. We'll start with an
ordinary view and apply a 45-degree rotation transform (see Figure 5.3).
Figure 5.3 A view rotated 45 degrees using an affine transform
A UIView can be transformed by setting its transform property, but as with all layout
properties, the UIView transform is really just a wrapper around an equivalent
CALayer feature.
CALayer also has a transform property, but its type is CATransform3D , not
CGAffineTransform . We come back to that later in the chapter, but for now it's not
what we're looking for. The CALayer equivalent to the UIView transform property is
called affineTransform . Listing 5.1 shows the code for rotating a layer by 45 degrees
in the clockwise direction using the affineTransform property.