Draw part of a circle

Question

For an iPhone application I want to draw a circle, that is only for an x percentage filled.

Something like this:

I have no problems calculating the radius, the degrees or the radians, that is no problem. Also drawing the circle is already done. But how do I get the iPhone SDK to draw the part that is filled.

I can draw a rectangle that size, but not part of a circle.

I just want to draw that on a a normal context.

Hope someone can give me any pointers here.

Answer 1

A lot of people have showed you how this can be done in Core Graphics but it can also be done with Core Animation which gives the big addition of easily being able to animate the percentage of the pie shape.

The following code will create both the ring and the partly filled layers (even though you said that you already can draw the ring) since its nice to have both the ring and the pie shape to be drawn using the same method.

If you animate the strokeStart or strokeEnd properties of the pieShape layer you will have the percentage animate. As with all Core Animation code you will need to add QuartzCore.framework to your project and include <QuartzCore/QuartzCore.h> in your code.

// Create a white ring that fills the entire frame and is 2 points wide.
// Its frame is inset 1 point to fit for the 2 point stroke width
CGFloat radius = MIN(self.frame.size.width,self.frame.size.height)/2;
CGFloat inset  = 1;
CAShapeLayer *ring = [CAShapeLayer layer];
ring.path = [UIBezierPath bezierPathWithRoundedRect:CGRectInset(self.bounds, inset, inset) 
                                       cornerRadius:radius-inset].CGPath;

ring.fillColor   = [UIColor clearColor].CGColor;
ring.strokeColor = [UIColor whiteColor].CGColor;
ring.lineWidth   = 2;

// Create a white pie-chart-like shape inside the white ring (above).
// The outside of the shape should be inside the ring, therefore the
// frame needs to be inset radius/2 (for its outside to be on 
// the outside of the ring) + 2 (to be 2 points in).
CAShapeLayer *pieShape = [CAShapeLayer layer];
inset = radius/2 + 2; // The inset is updated here
pieShape.path = [UIBezierPath bezierPathWithRoundedRect:CGRectInset(self.bounds, inset, inset)
                                        cornerRadius:radius-inset].CGPath;
pieShape.fillColor   = [UIColor clearColor].CGColor;
pieShape.strokeColor = [UIColor whiteColor].CGColor;
pieShape.lineWidth   = (radius-inset)*2;   

// Add sublayers
// NOTE: the following code is used in a UIView subclass (thus self is a view)
// If you instead chose to use this code in a view controller you should instead
// use self.view.layer to access the view of your view controller.
[self.layer addSublayer:ring];
[self.layer addSublayer:pieShape];