为什么Apple建议使用dispatch_once在ARC下实现单例模式？ [英] Why does Apple recommend to use dispatch_once for implementing the singleton pattern under ARC?

问题描述

在ARC下的单例的共享实例访问器中使用dispatch_once的确切原因是什么？

What's the exact reason for using dispatch_once in the shared instance accessor of a singleton under ARC?

+ (MyClass *)sharedInstance
{
    //  Static local predicate must be initialized to 0
    static MyClass *sharedInstance = nil;
    static dispatch_once_t onceToken = 0;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[MyClass alloc] init];
        // Do any other initialisation stuff here
    });
    return sharedInstance;
}

在后台异步实例化单例不是一个坏主意吗？我的意思是如果我请求共享实例并立即依赖它会发生什么，但dispatch_once要到圣诞节才能创建我的对象？它不会马上回来吗？至少这似乎是Grand Central Dispatch的重点。

Isn't it a bad idea to instantiate the singleton asynchronously in the background? I mean what happens if I request that shared instance and rely on it immediately, but dispatch_once takes until Christmas to create my object? It doesn't return immediately right? At least that seems to be the whole point of Grand Central Dispatch.

那他们为什么这样做？

推荐答案

dispatch_once（）绝对是同步的。并非所有GCD方法都异步执行（例如， dispatch_sync（）是同步的）。使用 dispatch_once（）取代以下习语：

dispatch_once() is absolutely synchronous. Not all GCD methods do things asynchronously (case in point, dispatch_sync() is synchronous). The use of dispatch_once() replaces the following idiom:

+ (MyClass *)sharedInstance {
    static MyClass *sharedInstance;
    @synchronized(self) {
        if (sharedInstance == nil) {
            sharedInstance = [[MyClass alloc] init];
        }
    }
    return sharedInstance;
}

dispatch_once（）这是因为它更快。它在语义上也更清晰，因为它还可以保护您免受多个线程的影响，这些线程执行您的sharedInstance的alloc init - 如果它们都在相同的时间尝试。它不允许创建两个实例。 dispatch_once（）的整个想法是只执行一次，这正是我们正在做的事情。

The benefit of dispatch_once() over this is that it's faster. It's also semantically cleaner, because it also protects you from multiple threads doing alloc init of your sharedInstance--if they all try at the same exact time. It won't allow two instances to be created. The entire idea of dispatch_once() is "perform something once and only once", which is precisely what we're doing.

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