通过CAGradientLayer类实现色度差动画

#import "ViewController.h"

@interface ViewController ()
{
    CAGradientLayer *_gradientLayer;
}
@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    UIImageView *_imageView = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@""]];
    _imageView.center       = self.view.center;
    [self.view addSubview:_imageView];

    _gradientLayer            = [CAGradientLayer layer];
    _gradientLayer.frame      = _imageView.bounds;

    /**设置渐变方向*/
    _gradientLayer.startPoint = CGPointMake(, );
    _gradientLayer.endPoint   = CGPointMake(, );

    /**设置颜色*/
    _gradientLayer.colors     = @[ (__bridge id)[UIColor clearColor].CGColor,
                                   (__bridge id)[UIColor redColor].CGColor
                                 ];

    /**设置颜色分割点*/
    _gradientLayer.locations  = @[@(0.5),@(1.0f)];

    [_imageView.layer addSublayer:_gradientLayer];

    [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerEvent) userInfo:nil repeats:YES];

}
- (void)timerEvent
{
   _gradientLayer.colors      = @[ (__bridge id)[UIColor clearColor].CGColor,
                                   (__bridge id)[UIColor colorWithRed:arc4random()%/255.0
                                                                green:arc4random()%/255.0
                                                                 blue:arc4random()%/255.0
                                                                alpha:1.0].CGColor
                                   ];

}