Programming iOS 6, 3rd Edition

Graphics Context Settings

When you draw in a graphics context, the drawing obeys the context’s current settings. Thus, the procedure is always to configure the context’s settings first, and then draw. For example, to draw a red line followed by a blue line, you would first set the context’s line color to red, and then draw the first line; then you’d set the context’s line color to blue, and then draw the second line. To the eye, it appears that the redness and blueness are properties of the individual lines, but in fact, at the time you draw each line, line color is a feature of the entire graphics context. This is true regardless of whether you use UIKit methods or Core Graphics functions.

A graphics context thus has, at every moment, a state, which is the sum total of all its settings; the way a piece of drawing looks is the result of what the graphics context’s state was at the moment that piece of drawing was performed. To help you manipulate entire states, the graphics context provides a stack for holding states. Every time you call CGContextSaveGState, the context pushes the entire current state onto the stack; every time you call CGContextRestoreGState, the context retrieves the state from the top of the stack (the state that was most recently pushed) and sets itself to that state.

Thus, a common pattern is: call CGContextSaveGState; manipulate the context’s settings, thus changing its state; draw; call CGContextRestoreGState to restore the state and the settings to what they ...