在Objective-C中使用init方法创建单例的安全方法 [英] Safe way to create singleton with init method in Objective-C

问题描述

我想采用GCD方法将共享实例用于下一步，因此我创建了以下代码：

I would like to take the GCD approach of using shared instances to the next step so I created the following code:

@implementation MyClass

static id sharedInstance;

#pragma mark Initialization

+ (instancetype)sharedInstance {
    static dispatch_once_t once;
    dispatch_once(&once, ^{
        sharedInstance = [[self alloc] init];
    });
    return sharedInstance;
}

- (instancetype)init {
    if (sharedInstance) {
        return sharedInstance;
    }
    @synchronized(self) {
        self = [super init];
        if (self) {
            sharedInstance = self;
        }
        return self;
    }
}

@end

I假设 sharedInstance 方法似乎没问题，但我不确定init方法。创建这个的原因是我不希望人们使用我的SDK，使用init方法，如果他们这样做......使它成为防弹。

I assume the sharedInstance method seems to be ok but I am unsure about the init method. The reason for creating this is that I don't want people using my SDK, to use the init method, and if they do ... make it bullet proof.

推荐答案

而不是透明地将对 init 的调用重定向到单例实现，这可能会导致SDK用户的行为非常混乱，我建议不要允许完全调用init：

Instead of transparently redirecting calls to init to the singleton implementation which can cause very confusing behaviour for the users of your SDK, I suggest not allowing to call init at all:

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

- (instancetype)init {
    @throw [NSException exceptionWithName:NSInternalInconsistencyException reason:@"..." userInfo:nil];
}

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

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