dispatch_semaphore_wait不等待信号量 [英] dispatch_semaphore_wait does not wait on semaphore

问题描述

我开发了以下方法，该方法检查应用程序与服务器通信的能力.该方法执行一个简单的查询，并且知道如果得到结果，则应连接该应用程序(基本ping机制).

I have developed the following method, which checks the app's ability to communicate with the server. The method performs a simple query and knows that if it gets a result, the app should be connected (basic ping mechanism).

- (BOOL)isAppConnected
{
    __block BOOL isConnected = NO;

    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    [[SFRestAPI sharedInstance] performSOQLQuery:@"SELECT id FROM Account LIMIT 1"
                                           failBlock:^(NSError *e) {
                                               isConnected = NO;
                                               NSLog(@"NOT CONNECTED %@", e);
                                               NSLog(@"fail block ON THE MAIN THREAD? %hhd", [NSThread isMainThread]);

                                               dispatch_semaphore_signal(semaphore);

                                           } completeBlock:^(NSDictionary *dict) {
                                               isConnected = YES;
                                               NSLog(@"%@", dict);
                                               NSLog(@"complete block ON THE MAIN THREAD? %hhd", [NSThread isMainThread]);

                                               dispatch_semaphore_signal(semaphore);
                                           }];

    // if the wait times-out we will receive a non-zero result and can assume no connection to SF
    //When using: DISPATCH_TIME_FOREVER the app hangs forever!!
    int waitResult = dispatch_semaphore_wait(semaphore, 30 * NSEC_PER_SEC); 
    NSLog(@"waitResult: %d", waitResult);

    return isConnected;
}

我正在使用 Apple文档

我的目标是等待响应或短暂的超时，以弄清我们是否真的有有效的连接.

My goal is to wait on the response or a short timeout to figure out if we really have a valid connection.

使用上面的代码，"dispatch_semaphore_wait"实际上从不等待，即执行不会在该行停止，而是立即继续(总是将49作为结果返回给dispatch_semaphore_wait调用).除非我使用DISPATCH_TIME_FOREVER，否则在这种情况下应用程序将永远挂起...

With the code above, 'dispatch_semaphore_wait' never actually waits, i.e. execution does not stop at that line but it continues immediately (always returning 49 as the result to the dispatch_semaphore_wait call). That is unless I use DISPATCH_TIME_FOREVER in which case the app hangs forever...

此刻，我正在从主线程调用此方法.我知道这是个坏主意，但我想在重构之前按预期工作.

At the moment I am calling this method from the main thread. I am aware that this is a bad idea, but I wanted to see this working as expected before refactoring.

什么可能导致此行为?谢谢.

What could be causing this behaviour? Thanks.

推荐答案

dispatch_semaphore_wait的参数不是延迟，而是信号量应唤醒的时间.1月1日午夜30秒后，您的手机将醒来.1970年(或2001年，不确定).使用dispatch_time函数.

The parameter for dispatch_semaphore_wait is not a delay, but the time when the semaphore should wake up. Yours will wake up 30 seconds after Midnight, Jan 1st. 1970 (or 2001, not sure). Use the dispatch_time function.

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