On iOS, why UIBezierPath drawing doesn't require a context? - iphone

Question

On iOS, we can draw a line in drawRect using
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextBeginPath (context);
CGContextMoveToPoint(context, 0, 0);
CGContextAddLineToPoint(context, 100, 100);
CGContextStrokePath(context);
but we can also draw a rectangle if we remove the above code, and just use:
UIBezierPath *path = [UIBezierPath bezierPathWithRect:CGRectMake(0, 0, 100, 100)];
[path stroke];
Two related questions:
1) Why doesn't UIBezierPath need to get or use the current context?
2) What if I have two context: one for screen, and one is a bitmap context, then how to tell which context to draw to for UIBezierPath? I thought it might be UIGraphicsSetCurrentContext but it doesn't exist.

Answer 1

UIBezierPath does use a context. It uses the current UIKit graphics context. This is exactly the same thing that you're already getting with UIGraphicsGetCurrentContext().
If you want UIBezierPath to use a different context you can use UIGraphicsPushContext(), but you have to remember to use UIGraphicsPopContext() when you're done.

Answer 2

I thought it might be useful to mention that CGContextFillRect is ~8.5x faster than using a UIBezierPath from what I can tell (in case performance is a factor and assuming you don't need to use a UIBezierPath for more complex drawing).
I added some timing to Apple's HazardMap example (http://developer.apple.com/library/ios/#samplecode/HazardMap/Introduction/Intro.html) and the time in ms per rect is ~0.00064 ms/rect for the CGContextFillRect approach versus ~0.00543 ms/rect for the UIBezierPath approach, presumably b/c the latter requires more message passing overhead.
i.e. I'm comparing using
CGContextFillRect(ctx, boundaryCGRect);
versus using
UIBezierPath* path = [UIBezierPath bezierPathWithRect:boundaryCGRect];
[path fill];
in the inner loop in HazardMapView (plus the above-mentioned changes to push / pop the context that is passed to HazardMapView drawMapRect:zoomScale:inContext:).
ETA

Answer 3

On iOS, we can draw a line in drawRect using
I've highlighted the important part of this statement. Inside of drawRect:, a context has already been set up for you by UIKit, and any object-based drawing instructions go directly into that context. UIBezierPath is indeed using that context, it just doesn't need to be passed in explicitly.
In Cocoa Touch, there must always be a drawing context (in this case, the context will eventually be painted onto the screen). If you were not inside drawRect:, you'd have to create a context yourself.
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextBeginPath (context);
CGContextMoveToPoint(context, 0, 0);
Notice that the first function call is GetCurrentContext(). When you're using CoreGraphics' functional drawing interface, you need to pass a context into each function, but you're not creating one here, you're just retrieving the one that already exists.
Graphics contexts are in a stack. If you want to draw into a context you've created, you push it onto the stack using UIGraphicsPushContext() (as Kevin already mentioned), then pop back to the previous one.