单例--iOS

GCD单例：

+ (id)sharedInstance {
    static ModulesManager * sharedManager = nil;
    static dispatch_once_t once;
    dispatch_once(&once, ^{
        sharedManager = [[self alloc] init];
    });

    return sharedManager;
}

为了防止外界通过alloc方法申请到另一块内存，只生成单个实例，需要覆盖父类的allocWithZone方法 需要注意线程安全

+(id)allocWithZone:(struct _NSZone *)zone{
    @synchronized(self){
        if (sharedSingleton == nil) {
            sharedSingleton = [super allocWithZone:zone];
        }
    }
    return sharedSingleton;
}

+(id)copyWithZone:(struct _NSZone *)zone{
    return sharedSingleton;
}