在Objective C中使用GCD的dispatch_once创建单例

Question

如果您可以定位iOS 4.0或更高版本

使用GCD,它是在Objective C(线程安全)中创建单例的最佳方法吗？

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

Answer 1

这是创建类实例的完全可接受且线程安全的方法.它在技术上可能不是"单例"(因为它们只能有1个这样的对象),但只要你只使用该[Foo sharedFoo]方法来访问对象,这就足够了.

Answer 2

instancetype

instancetype只是众多语言扩展中的一种Objective-C,每个新版本都添加了更多.

知道它,喜欢它.

并将其作为一个例子,说明如何关注低级细节可以让您深入了解改变Objective-C的强大新方法.

请参考此处:instancetype

+ (instancetype)sharedInstance
{
    static dispatch_once_t once;
    static id sharedInstance;

    dispatch_once(&once, ^
    {
        sharedInstance = [self new];
    });    
    return sharedInstance;
}

+ (Class*)sharedInstance
{
    static dispatch_once_t once;
    static Class *sharedInstance;

    dispatch_once(&once, ^
    {
        sharedInstance = [self new];
    });    
    return sharedInstance;
}

Answer 3

MySingleton.h

@interface MySingleton : NSObject

+(instancetype)sharedInstance;

+(instancetype)alloc __attribute__((unavailable("alloc not available, call sharedInstance instead")));
-(instancetype)init __attribute__((unavailable("init not available, call sharedInstance instead")));
+(instancetype)new __attribute__((unavailable("new not available, call sharedInstance instead")));
-(instancetype)copy __attribute__((unavailable("copy not available, call sharedInstance instead")));

@end

MySingleton.m

@implementation MySingleton

+(instancetype)sharedInstance {
    static dispatch_once_t pred;
    static id shared = nil;
    dispatch_once(&pred, ^{
        shared = [[super alloc] initUniqueInstance];
    });
    return shared;
}

-(instancetype)initUniqueInstance {
    return [super init];
}

@end

Answer 4

您可以避免分配类覆盖alloc方法.

@implementation MyClass

static BOOL useinside = NO;
static id _sharedObject = nil;


+(id) alloc {
    if (!useinside) {
        @throw [NSException exceptionWithName:@"Singleton Vialotaion" reason:@"You are violating the singleton class usage. Please call +sharedInstance method" userInfo:nil];
    }
    else {
        return [super alloc];
    }
}

+(id)sharedInstance
{
    static dispatch_once_t p = 0;
    dispatch_once(&p, ^{
        useinside = YES;
        _sharedObject = [[MyClass alloc] init];
        useinside = NO;
    });   
    // returns the same object each time
    return _sharedObject;
}

Answer 5

戴夫是对的,这很好.您可能需要查看Apple关于创建单例的文档,以获取有关实现其他一些方法的提示,以确保在类选择不使用sharedFoo方法时只能创建一个方法.