Programming iOS 4

Graphics Context Transforms

Just as a UIView can have a transform, so can a graphics context. However, applying a transform to a graphics context has no effect on the drawing that’s already in it; it affects only the drawing that takes place after it is applied, altering the way the coordinates you provide are mapped onto the graphics context’s area. A graphics context’s transform is called its CTM, for “current transformation matrix.”

It is quite usual to take full advantage of a graphics context’s CTM to save yourself from performing even simple calculations. You can multiply the current transform by any CGAffineTransform using CGContextConcatCTM; there are also convenience functions for applying a translate, scale, or rotate transform to the current transform.

The base transform for a graphics context is already set for you when you obtain the context; this is how the system is able to map context drawing coordinates onto screen coordinates. Whatever transforms you apply are applied to the current transform, so the base transform remains in effect and drawing continues to work. You can always return to the base transform after applying your own transforms by wrapping your code in calls to CGContextSaveGState and CGContextRestoreGState.

For example, we have hitherto been drawing our upward-pointing arrow with code that knows how to place that arrow at only one location: the top left of its rectangle is hard-coded at (80,0). This is silly. It makes the code hard to understand, as ...