[iOS]编写单例的正确方式

class TheOneAndOnlyKraken {
static let sharedInstance = TheOneAndOnlyKraken()
private init() {} //This prevents others from using the default '()' initializer for this class.
}

@interface Kraken : NSObject
@end

@implementation Kraken

+ (instancetype)sharedInstance {
static Kraken *sharedInstance = nil;
static dispatch_once_t onceToken;

dispatch_once(&onceToken, ^{
sharedInstance = [[Kraken alloc] init];
});
return sharedInstance;
}

@end