iOS 画圆

    _demoView = [[UIView alloc] initWithFrame:CGRectMake(100, 100, 100, 100)];
    [self.view addSubview:_demoView];

    CAShapeLayer *shapeLayer = [CAShapeLayer layer];
    shapeLayer.frame = _demoView.bounds;
    UIBezierPath *path = [UIBezierPath bezierPathWithOvalInRect:_demoView.bounds];
    shapeLayer.path = path.CGPath;
//    shapeLayer.strokeEnd = 1.0f;
//    shapeLayer.strokeStart = 0.75f;
    shapeLayer.fillColor = [UIColor clearColor].CGColor;
    shapeLayer.lineWidth = 2.0f;
    shapeLayer.strokeColor = [UIColor redColor].CGColor;
    [_demoView.layer addSublayer:shapeLayer];
    CABasicAnimation *pathAnima = [CABasicAnimation animationWithKeyPath:@"strokeEnd"];
    pathAnima.duration = 3.0f;
    pathAnima.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseOut];
    pathAnima.fromValue = [NSNumber numberWithFloat:0.0f];
    pathAnima.toValue = [NSNumber numberWithFloat:1.0f];
    pathAnima.fillMode = kCAFillModeForwards;
    pathAnima.removedOnCompletion = NO;
    [shapeLayer addAnimation:pathAnima forKey:@"strokeEndAnimation"];