iOS设计模式（单例模式）

单例模式（Singleton）
保证一个类仅有一个实例，并提供一个该实例的全局访问点。

定义包含三层含义：一是某个类只能有一个实例；二是它必须自行创建这个实例；三是它必须向整个系统提供这个实例。

OC

@interface ZZSingleton : NSObject
+ (instancetype)shareSingleton;
@end


@implementation ZZSingleton

static ZZSingleton *shareSingleton = nil;

+ (instancetype)shareSingleton {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        shareSingleton = [[super allocWithZone:NULL] init];
    });
    return shareSingleton;
}

+ (instancetype)allocWithZone:(struct _NSZone *)zone {
    return [ZZSingleton shareSingleton];
}

- (id)copyWithZone:(struct _NSZone *)zone {
    return [ZZSingleton shareSingleton];
}

@end


#######
   ZZSingleton *one = [ZZSingleton shareSingleton];
    NSLog(@"one = %@", one);
    
    ZZSingleton *two = [ZZSingleton shareSingleton];
    NSLog(@"two = %@", two);
    
    ZZSingleton *three = [[ZZSingleton alloc] init];
    NSLog(@"three = %@", three);
    
    ZZSingleton *four = [[ZZSingleton alloc] init];
    NSLog(@"four = %@", [four copy]);
    
    ZZSingleton *five = [ZZSingleton new];
    NSLog(@"five = %@", five);