Pushing/Popping Graphics Context - ios5

Question

I've been going through the Standford University IOS lessons and I am stuck on understanding what UIGraphicsPushContext and UIGraphicsPopContext does (lesson 4). From the notes, it is explained that these methods can be used to avoid affecting the current graphics states from other utility methods. Here is the example provided in the notes:
- (void)drawGreenCircle:(CGContextRef)ctxt {
UIGraphicsPushContext(ctxt);
[[UIColor greenColor] setFill];
// draw my circle
UIGraphicsPopContext();
}
- (void)drawRect:(CGRect)aRect {
CGContextRef context = UIGraphicsGetCurrentContext();
[[UIColor redColor] setFill];
// do some stuff
[self drawGreenCircle:context];
// do more stuff and expect fill color to be red
}
However, when I try to test this, I don't seem to get this result. Below is a simple test case I've made that gets the current context in drawRect, sets the color, calls a utility method that sets the color to green, returns to drawRect and draws a line. I would expect based on the notes on the Standford site that the line would be red since I pushed/popped the context in my drawGreenCircle. (I realize I am not actually drawing any circle in drawGreenCircle) But I get a green line in drawRect. It seems that my drawGreenCircle method did change the color and did not revert it back to red. I would like to know what I am missing about this concept.
- (void)drawGreenCircle:(CGContextRef)ctxt {
//Doesn't actually draw circle --Just testing if the color reverts
//to red in drawRect after this returns
UIGraphicsPushContext(ctxt);
[[UIColor greenColor] setStroke];
UIGraphicsPopContext();
}
- (void)drawRect:(CGRect)aRect {
CGContextRef context = UIGraphicsGetCurrentContext();
[[UIColor redColor] setStroke];
// do some stuff
[self drawGreenCircle:context];
CGContextBeginPath(context);
CGContextMoveToPoint(context, 100, 100);
CGContextAddLineToPoint(context, 500, 100);
CGContextClosePath(context);
CGContextDrawPath(context, kCGPathFillStroke);
}

Answer 1

You need CGContextSaveGState and CGContextRestoreGState instead. The following code works:
- (void)drawRect:(CGRect)rect {
CGContextRef context = UIGraphicsGetCurrentContext();
[[UIColor greenColor] setFill];
[[UIColor blackColor] setStroke];
[self drawBlueCircle:context];
CGContextMoveToPoint(context, 55, 5);
CGContextAddLineToPoint(context, 105, 100);
CGContextAddLineToPoint(context, 5, 100);
CGContextClosePath(context);
CGContextDrawPath(context, kCGPathFillStroke);
}
- (void)drawBlueCircle:(CGContextRef)context {
CGContextSaveGState(context);
[[UIColor blueColor] setFill];
CGContextAddEllipseInRect(context, CGRectMake(0, 0, 100, 100));
CGContextDrawPath(context, kCGPathFillStroke);
CGContextRestoreGState(context);
}
Source