圆环loading动画

- (void)addRingLayer {
    CAShapeLayer *ringLayer = [CAShapeLayer layer];
    ringLayer.frame = CGRectMake(100, 100, 100, 100);
    
    UIBezierPath *path = [UIBezierPath bezierPath];
    //不能是CGPointMake(150, 150），因为path在ringLayer的内部
    [path addArcWithCenter:CGPointMake(50, 50) radius:50 startAngle:-0.5 * M_PI endAngle:1.5 * M_PI clockwise:YES];
    ringLayer.path = path.CGPath;
    ringLayer.lineWidth = 2;
    ringLayer.strokeColor = UIColor.redColor.CGColor;
//    ringLayer.fillColor = UIColor.greenColor.CGColor;
    ringLayer.fillColor = nil;
    ringLayer.backgroundColor = nil;
    
    CABasicAnimation *rotationAnimation = [CABasicAnimation animationWithKeyPath:@"transform.rotation"];
    rotationAnimation.byValue = @(2.0 * M_PI);
    rotationAnimation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionLinear];
    
    CABasicAnimation *strokeStartAnimation = [CABasicAnimation animationWithKeyPath:@"strokeStart"];
    strokeStartAnimation.fromValue = @(0);
    strokeStartAnimation.toValue = @(1);
    strokeStartAnimation.duration = 1.2;
    strokeStartAnimation.beginTime = 0.5;
    strokeStartAnimation.timingFunction = [CAMediaTimingFunction functionWithControlPoints:0.4 :0.0 :0.2 :1.0];
    
    CABasicAnimation *strokeEndAnimation = [CABasicAnimation animationWithKeyPath:@"strokeEnd"];
    strokeEndAnimation.fromValue = @(0);
    strokeEndAnimation.toValue = @(1);
    strokeEndAnimation.duration = 0.7;
    strokeEndAnimation.timingFunction = [CAMediaTimingFunction functionWithControlPoints:0.4 :0.0 :0.2 :1.0];
    
    CAAnimationGroup *group = [CAAnimationGroup animation];
    group.animations = @[rotationAnimation, strokeEndAnimation, strokeStartAnimation];
    //不能是strokeStartAnimation.duration + strokeEndAnimation.duration，否则会看到一个完整的环停留一下。
    group.duration = strokeStartAnimation.duration + strokeStartAnimation.beginTime;
    group.removedOnCompletion = NO;
    group.repeatCount = HUGE_VAL;
    group.fillMode = kCAFillModeForwards;
    
    [ringLayer addAnimation:group forKey:@"animation"];
    
    [self.view.layer addSublayer:ringLayer];
    
//    UIView *greenView = [[UIView alloc] initWithFrame:CGRectMake(100, 100, 100, 100)];
//    greenView.backgroundColor = UIColor.greenColor;
//    [self.view addSubview:greenView];
}