iPhone Core Animation - Drawing a Circle

Question

I wish to create an animation. The best way I can explain this is if you can imagine drawing a circle. It starts at the top or 12 o clock and draws clockwise all the way around until it becomes a complete circle over the space of 10 or so seconds.

The closet I have come to this is to draw a point rotating around a center point using Core Animation (using the sample code here). But I am at a loss on how to draw the circle? Any suggestions are very welcome.

Many Thanks :)

Answer 1

What you really should do is to animate the stroke of a CAShapeLayer where the path is a circle. This will be accelerated using Core Animation and is less messy then to draw part of a circle in -drawRect:.

The code below will create a circle shape layer in the center of the screen and animate the stroke of it clockwise so that it looks as if it is being drawn. You can of course use any shape you'd like. (You can read this article on Ole Begemanns blog to learn more about how to animate the stroke of a shape layer.)

Note: that the stoke properties are not the same as the border properties on the layer. To change the width of the stroke you should use "lineWidth" instead of "borderWitdh" etc.

// Set up the shape of the circle
int radius = 100;
CAShapeLayer *circle = [CAShapeLayer layer];
// Make a circular shape
circle.path = [UIBezierPath bezierPathWithRoundedRect:CGRectMake(0, 0, 2.0*radius, 2.0*radius) 
                                         cornerRadius:radius].CGPath;
// Center the shape in self.view
circle.position = CGPointMake(CGRectGetMidX(self.view.frame)-radius, 
                              CGRectGetMidY(self.view.frame)-radius);

// Configure the apperence of the circle
circle.fillColor = [UIColor clearColor].CGColor;
circle.strokeColor = [UIColor blackColor].CGColor;
circle.lineWidth = 5;

// Add to parent layer
[self.view.layer addSublayer:circle];

// Configure animation
CABasicAnimation *drawAnimation = [CABasicAnimation animationWithKeyPath:@"strokeEnd"];
drawAnimation.duration            = 10.0; // "animate over 10 seconds or so.."
drawAnimation.repeatCount         = 1.0;  // Animate only once..

// Animate from no part of the stroke being drawn to the entire stroke being drawn
drawAnimation.fromValue = [NSNumber numberWithFloat:0.0f];
drawAnimation.toValue   = [NSNumber numberWithFloat:1.0f];

// Experiment with timing to get the appearence to look the way you want
drawAnimation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseIn];

// Add the animation to the circle
[circle addAnimation:drawAnimation forKey:@"drawCircleAnimation"];