阻止`performSelectorInBackground：`运行两次？ [英] Prevent `performSelectorInBackground:` from running twice?

问题描述

所以我使用 performSelectorInBackground：@selector（loadStuff）在后台加载东西。这需要一些时间。

So I'm using performSelectorInBackground:@selector(loadStuff) to load things in the background. It takes some time.

用户可能想重新加载项目，而上述方法是在后台运行。如果I performSelectorInBackground：@selector（loadStuff），而另一个方法已经运行，我会遇到各种错误。

The user may want to re-load the items, while the method above is running in the background. If I performSelectorInBackground:@selector(loadStuff) again while the other method is already running, I get all kinds of errors.

有没有一种简单的方法来处理这种情况？

Is there an easy way to handle this situation?

我想停止已经运行的后台方法，然后启动新的。

I'd like to stop the already-running-in-the-background method, then start the new one. (Or if there is a better way of accomplishing the end goal, that would be fine as well).

推荐答案

如果你想要一个更好的方法来完成最终目标，要重新开始，可以取消连接并创建一个新连接。因为你要在后台线程上运行这个方法，你需要确保只有一个线程可以一次访问相关的实例变量：

If you want to start over, you can cancel the connection and create a new one. Since you're going to run this method on a background thread you need to make sure that only one thread can access the related instance variables at a time:

- (void)loadStuff
{
    @synchronized(self) {
        if (currentConnection != nil)
            [currentConnection cancel];
            [currentConnection release];
        }
        currentConnection = [[NSURLConnection alloc] initWithRequest:request 
                                                            delegate:self 
                                                    startImmediately:YES];
    }
}

另一种方法是使用标志指示忙碌状态。例如，如果你想只是返回如果长期运行的方法已经在另一个线程上进行，你可以这样做：

Another way to do it is to use a flag to indicate a busy state. For example, if you wanted to just return if the long-running method were already in process on another thread, you could do this:

- (void)loadStuff
{
    @synchronized(self) {
        if (loadingStuff == YES)
            return;
        }
        loadingStuff = YES;
    }

    NSURLRequest *request = ...
    NSURLResponse *response;
    NSError *error;
    [NSURLConnection sendSynchronousRequest:request returningReseponse:&response error:&error];

    @synchronized(self) {
        loadingStuff = NO;
    }
}

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