objective-c - Apple 的 Sample MVCNetworking 中的 Singleton Class NetworkManager 是否正确？

Question

这里是示例代码的链接 http://developer.apple.com/library/ios/#samplecode/MVCNetworking/Introduction/Intro.html

下面是文件 NetworkManager.m 中的代码片段

+ (NetworkManager *)sharedManager
// See comment in header.
{
    static NetworkManager * sNetworkManager;

    // This can be called on any thread, so we synchronise.  We only do this in 
    // the sNetworkManager case because, once sNetworkManager goes non-nil, it can 
    // never go nil again.

    if (sNetworkManager == nil) {
        @synchronized (self) {
            sNetworkManager = [[NetworkManager alloc] init];
            assert(sNetworkManager != nil);
        }
    }
    return sNetworkManager;
}

显然这里存在线程安全问题。当有多个线程时，可能会创建两个 NetworkManager 实例。所以 Apple 犯了一个错误，对吧？

Answer 1

是的，你是对的。它会在并发环境中出现问题。更好的方法是在 alloc 之前使用双重检查:

+ (NetworkManager *)sharedManager
{
    static NetworkManager * sNetworkManager;
    if (sNetworkManager == nil) {
        @synchronized (self) {
            if (sNetworkManager == nil) {
                sNetworkManager = [[NetworkManager alloc] init];
                assert(sNetworkManager != nil);
            }
        }
    }
    return sNetworkManager;
}

而且有很多方法可以使用 Ojbective-C 编写单例，查看这篇文章:What should my Objective-C singleton look like?

更新

BobCromwell 是对的。 double check lock苹果不推荐，苹果Threading Programming Guide中的文档:

A double-checked lock is an attempt to reduce the overhead of taking a lock by testing the locking criteria prior to taking the lock. Because double-checked locks are potentially unsafe, the system does not provide explicit support for them and their use is discouraged.`