我正在尝试制作一个动画并且顺时针填充黑色的圆圈，圆形动画的持续时间预定义为20秒，我想要做的是前10秒将填充黑色和圆圈的其余部分将用红色填充，并且在总共20秒后它将变成2个彩色的圆形圆圈，但我找不到它的任何程序，这里是我绘制圆形动画的代码：
-(void)makeCircle{
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;
//centre the shape in self.view
circle.position = CGPointMake(CGRectGetMidX(self.view.frame)-radius, CGRectGetMidY(self.view.frame)-radius);
//configure the appearence 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 = 20.0;//animate over 10 seconds or so...
drawAnimation.repeatCount=1.0;//animate only once;
drawAnimation.removedOnCompletion=NO;
drawAnimation.fromValue = [NSNumber numberWithFloat:0.0f];
drawAnimation.toValue = [NSNumber numberWithFloat:1.0f];
drawAnimation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseIn];
[circle addAnimation:drawAnimation forKey:@"drawCircleAnimation"];
}
但我需要知道我该怎么做？