Question

This code appears to do nothing. Build successful, no errors. No rectangle drawn on screen.

- (void)viewDidLoad
{

    [super viewDidLoad];

    UIColor *reliantMagenta             = [UIColor colorWithRed:208.0f / 255.0f green:27.0f / 255.0f blue:124.0f / 255.0f alpha:1];

    CALayer *reliantCanvasLayer         = [CALayer layer];

    reliantCanvasLayer.frame            = CGRectMake(0, 0, 640, 960);

    [[[self view] layer] addSublayer:reliantCanvasLayer];

    CGContextRef ctx                    = UIGraphicsGetCurrentContext();
    CGRect leftRect                     = CGRectMake(0, 0, 200, 300);

    CGContextSaveGState(ctx);
    CGContextSetFillColorWithColor(ctx, reliantMagenta.CGColor);

    CGContextFillRect(ctx, leftRect);
    CGContextRestoreGState(ctx);

}

I am just learning Quartz and really thrashing with it. If you want to explain the relationship between UIViews, CALayers, CGLayers, and context that would be a big help too, but not required, just having trouble understanding what is going on.

Answer 1

If you’re starting out with Quartz, then you should start with the Quartz 2D Programming Guide, which walks through all of this. Your key mistake here is that there is no context available in viewDidLoad. Drawing of this kind is generally done in drawRect:. Your call to UIGraphicsGetCurrentContext() returns NULL at this point.

After reading through the Programming Guide, you may have more questions, but that’s where you should start to learn about custom drawing.