#import "AppDelegate.h"

@interface AppDelegate ()
<
 CAAnimationDelegate
>

@property (nonatomic, strong) CALayer     *mask;
@property (nonatomic, strong) UIImageView *imageView;
@end

@implementation AppDelegate


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];

    self.imageView       = [[UIImageView alloc]initWithFrame:self.window.frame];
    self.imageView.image = [UIImage imageNamed:@"twitterScreen"];
    [self.window addSubview:self.imageView];
    
    //初始化mask
    self.mask          = [CALayer layer];
    self.mask.contents = (__bridge id _Nullable)([UIImage imageNamed:@"twitterBird"].CGImage);
    self.mask.position = self.window.center;
    self.mask.bounds   = CGRectMake(0, 0, 100, 80);
    self.imageView.layer.mask = self.mask;
    
    [self creawteBaseAnimateMask];
    
    self.window.rootViewController = [[UIViewController alloc]init];
    self.window.backgroundColor    = [UIColor colorWithRed:70/255.f green:154/255.f blue:233/255.f alpha:1];
    [self.window makeKeyAndVisible];
    
    [UIApplication sharedApplication].statusBarHidden = YES;
    return YES;
}


/**
 添加动画
 */
- (void)creawteBaseAnimateMask {
    CAKeyframeAnimation *key = [CAKeyframeAnimation animationWithKeyPath:@"bounds"];
    key.duration  = 1;
    key.delegate  = self;
    key.beginTime = CACurrentMediaTime() + 1;
    
    NSValue *initalBounds = [NSValue valueWithCGRect:self.mask.bounds];
    NSValue *secondBounds = [NSValue valueWithCGRect:CGRectMake(0, 0, 80, 64)];
    NSValue *finalBounds  = [NSValue valueWithCGRect:CGRectMake(0, 0, 2000, 2000)];
    
    key.values   = @[initalBounds,secondBounds,finalBounds];
    key.keyTimes = @[@0, @0.3, @1];
    key.timingFunctions = @[
                            [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut],
                            [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseOut]
                            ];
    [self.mask addAnimation:key forKey:@"bounds"];
}

- (void)animationDidStop:(CAAnimation *)anim finished:(BOOL)flag {
    self.imageView.layer.mask = nil;
}
@end

效果基本和twitter一样，就不上图了