单例的init死循环

https://my.oschina.net/nerozhao/blog/466731

第一种，在init里面抛出异常，定义私有的init

- (instancetype)init {
    [self doesNotRecognizeSelector:_cmd];
    return nil;
}

- (instancetype)initPrivate {
    self = [super init];
    if (self) {
    }
    return self;
}

+ (instancetype)sharedInstance {
    static MySingleton *sharedInstance;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[self alloc] initPrivate];
    });
    return sharedInstance;
}

第二种，在init里面直接返回单例

- (instancetype)init {
    return [[self class] sharedInstance];
}

- (instancetype)initPrivate {
    self = [super init];
    if (self) {
    }
    return self;
}

+ (instancetype)sharedInstance {
    static MySingleton2 *sharedInstance;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[self alloc] initPrivate];
    });
    return sharedInstance;
}

个人比较倾向于第二种