可以對圖層按path進行指定裁剪

//#import "ViewController.h"
//
//@interface ViewController ()
//
//@end
//
//@implementation ViewController
//
//- (void)viewDidLoad {
//
//    [super viewDidLoad];
//
//    // 創建一個藍色的Layer
//    CALayer *foregroundLayer        = [CALayer layer];
//    foregroundLayer.bounds          = CGRectMake(0, 0, 100, 100);
 
//    foregroundLayer.backgroundColor = [UIColor redColor].CGColor;
//
//    // 創建一個路徑
//    UIBezierPath *apath = [UIBezierPath bezierPathWithOvalInRect:CGRectMake(20, 20, 60, 60)];
//
//    // 創建maskLayer
//    CAShapeLayer *maskLayer = [CAShapeLayer layer];
//    maskLayer.path = apath.CGPath;
//    maskLayer.fillColor = [UIColor greenColor].CGColor;
 
//    maskLayer.fillRule = kCAFillRuleEvenOdd;
//
//    // 設置位置
//    foregroundLayer.position = self.view.center;
//    // 設置mask
//    foregroundLayer.mask = maskLayer;
//
//    [self.view.layer addSublayer:foregroundLayer];
//
//}
//
//@end


#import "ViewController.h"

static CGFloat num;

@interface ViewController ()

@property (nonatomic, strong) CAShapeLayer 
 
*circle;
@property (nonatomic, strong) CADisplayLink *link;

@end

@implementation ViewController

@synthesize circle;

- (void)viewDidLoad {
    
    [super viewDidLoad];
    
//    //創建一個CAShape
//    CALayer *bgLayer = [CALayer layer];
//
//    //設置大小顏色和位置
//    bgLayer.bounds          = CGRectMake(0, 0, 200, 200);
//    bgLayer.backgroundColor = [UIColor redColor].CGColor;
//    bgLayer.position        = self.view.center;
    
    CAGradientLayer *bgLayer = [CAGradientLayer layer];
    bgLayer.bounds = CGRectMake(0, 0, 200, 200);
    bgLayer.position        = self.view.center;

    bgLayer.colors = [NSArray arrayWithObjects:
                       (id)[UIColor colorWithRed:0 green:143/255.0 blue:234/255.0 alpha:1.0].CGColor,
                       (id)[UIColor colorWithRed:0 green:173/255.0 blue:234/255.0 alpha:1.0].CGColor,
                       (id)[UIColor whiteColor].CGColor, nil];
    
    
    //創建一個CAShapeLayer作為MaskLayer
    circle = [CAShapeLayer layer];
    
    //設置路徑
    circle.path      = [UIBezierPath bezierPathWithArcCenter:CGPointMake(100, 100)
                                                      radius:20
                                                  startAngle:0
                                                    endAngle:2 * M_PI
                                                   clockwise:YES].CGPath;
//    circle.lineWidth = 5;
//    circle.fillColor = [UIColor greenColor].CGColor;
//    circle.fillRule  = kCAFillRuleEvenOdd;
    
    //設置maskLayer
    bgLayer.mask = circle;
    
    [self.view.layer addSublayer:bgLayer];
    
    //添加計時器 這個只是一個附加動畫
    //self.link = [CADisplayLink displayLinkWithTarget:self selector:@selector(action)];
    //[self.link addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
}

- (void)action {
    
    num ++;
    
    circle.path      = [UIBezierPath bezierPathWithArcCenter:CGPointMake(100, 100)
                                                      radius:20 + num
                                                  startAngle:0
                                                    endAngle:2 * M_PI
                                                   clockwise:YES].CGPath;
    
    if (num > 1000) {
        [self.link invalidate];
    }
}

@end

CALayer的mask屬性