Why does -[UIColor setFill] work without referring to a drawing context?

Question

It escapes me why this code, inside drawRect:, works:

UIBezierPath *buildingFloor = [[UIBezierPath alloc] init];
// draw the shape with addLineToPoint
[[UIColor colorWithRed:1 green:0 blue:0 alpha:1.0] setFill]; // I'm sending setFill to UIColor object?
[buildingFloor fill]; // Fills it with the current fill color

My point is, the UIColor object gets a message setFill and then somehow the stack understands that this UIColor will now be the fill color, isn't this just weird and wrong? At the very least I would expect setting fill by calling some CGContext method... But now instead of representing a color, UIColor goes on and does something to change the context of my drawing.

Could someone explain what is happening behind the scenes, because I'm totally lost here?

I did check out these references before posting:

http://developer.apple.com/library/ios/#documentation/UIKit/Reference/UIColor_Class/Reference/Reference.html http://developer.apple.com/library/ios/#documentation/uikit/reference/UIBezierPath_class/Reference/Reference.html

Answer 1

The implementation of UIColor setFill is written to get the current graphics context and then to set the color on that current context. Essentially it does this for you:

UIColor *color = ... // some color
CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextSetFillColorWithColor(ctx, color.CGColor);

Answer 2

My point is, UIColor object gets a message setFill and then somehow the stack understands that this UIColor will now be the fill color, isn't this just weird and wrong? At the very least I would expect setting fill by calling some CGContext method... But now instead of representing a color, UIColor goes on and does something to change the context of my drawing.

It is because all this is taking place within the current CGContext. That is why your code works only if there is a current CGContext (as there is, for example, in drawRect: or in a UIGraphicsBeginImageContextWithOptions block).

It will probably help you a lot, at this stage of your iOS study, to read the Drawing chapter of my book: http://www.apeth.com/iOSBook/ch15.html#_graphics_contexts