Fill UIImage like a gauge using CG/Quartz

Question

Lets say i have a an image (a semi-circle gauge) with colors starting from green to yellow to red. How can i programmatically clip and fill the image for a given percentage dynamically using CoreGraphics/Quartz ?

Answer 1

You can use a CALayer with a custom mask.

The mask will be a CAShapeLayer with a path that defines the given percentage of the guage.

CALayer *guageLayer = //your CALayer, could be the backing view

CAShapeLayer *guageMask = [[CAShapeLayer alloc] init];
guageMask.path = [self _pathForCurrentGuage]; //bezier path based on your current percentage

guageLayer.mask = guageMask;