SrokeStart 与 StrokeEnd 动画

效果：

实现代码：

@interface ViewController ()

@property (nonatomic, strong) NSTimer      *timer;      // 定时器
@property (nonatomic, strong) CAShapeLayer *shapeLayer; // 形状layer

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    // 设置背景色
    self.view.backgroundColor = [UIColor colorWithRed:0.878 green:0.878 blue:0.878 alpha:1];

    // 创建椭圆形贝塞尔曲线
    UIBezierPath *oval        = [UIBezierPath bezierPathWithOvalInRect:CGRectMake(0, 0, 100, 100)];
    
    // 创建CAShapeLayer
    _shapeLayer               = [CAShapeLayer layer];
    _shapeLayer.frame         = CGRectMake(0, 0, 100, 100);
    _shapeLayer.position      = self.view.center;
    
    // 修改CAShapeLayer的线条相关值
    _shapeLayer.fillColor     = [UIColor clearColor].CGColor;
    _shapeLayer.strokeColor   = [UIColor redColor].CGColor;
    _shapeLayer.lineWidth     = 2.f;
    _shapeLayer.strokeStart   = 0.f;
    _shapeLayer.strokeEnd     = 0.f;

    // 建立贝塞尔曲线与CAShapeLayer之间的关联
    _shapeLayer.path          = oval.CGPath;
    
    // 添加并显示
    [self.view.layer addSublayer:_shapeLayer];
    
    // 创建定时器
    _timer = [NSTimer scheduledTimerWithTimeInterval:1.f
                                              target:self
                                            selector:@selector(animationEventTypeTwo)
                                            userInfo:nil
                                             repeats:YES];
}

/**
 *  动画效果1
 */
- (void)animationEventTypeOne {
    // 执行隐式动画
    _shapeLayer.strokeEnd = arc4random() % 100 / 100.f;
}

/**
 *  动画效果2
 */
- (void)animationEventTypeTwo {
    CGFloat valueOne = arc4random() % 100 / 100.f;
    CGFloat valueTwo = arc4random() % 100 / 100.f;
    
    // 执行隐式动画
    _shapeLayer.strokeStart = valueOne < valueTwo ? valueOne : valueTwo;
    _shapeLayer.strokeEnd   = valueOne > valueTwo ? valueOne : valueTwo;
}