CABasicAnimation - Setting start stroke position

问题:

I'm animating the drawing of a basic circle. This works fine, except the animation begins drawing at the 3 o'clock position. Does anyone know how I can make it start at 12 o'clock?

self.circle = [CAShapeLayer layer];
self.circle.fillColor = nil;
self.circle.lineWidth = 7;
self.circle.strokeColor = [UIColor blackColor].CGColor;
self.circle.bounds = CGRectMake(0, 0, 200, 200);
self.circle.path = [UIBezierPath bezierPathWithOvalInRect:self.circle.bounds].CGPath;
[self.view.layer addSublayer:self.circle];

CABasicAnimation *drawAnimation = [CABasicAnimation animationWithKeyPath:@"strokeEnd"];
drawAnimation.duration            = 5.0;
drawAnimation.repeatCount         = 1.0;
drawAnimation.removedOnCompletion = NO;
drawAnimation.fromValue = [NSNumber numberWithFloat:0.0f];
drawAnimation.toValue   = [NSNumber numberWithFloat:1.0f];
drawAnimation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseIn];
[self.circle addAnimation:drawAnimation forKey:@"drawCircleAnimation"];

回答1:

You can use bezierPathWithArcCenter instead of bezierPathWithOvalInRect, because that allows to specify a start and end angle:

CGFloat radius = self.circle.bounds.size.width/2; // Assuming that width == height
self.circle.path = [UIBezierPath bezierPathWithArcCenter:CGPointMake(radius, radius)
                                                  radius:radius
                                              startAngle:(-M_PI/2)
                                                endAngle:(3*M_PI/2)
                                               clockwise:YES].CGPath;

See the bezierPathWithArcCenter documentation for the meaning of the angles.