Objective C - 如何等待异步调用完成

Question

我是 iOS 的开发新手。我已经搜索并尝试了几种方法，但程序不想等待异步调用完成。

调试时，函数CheckForHost先returns-1作为retVal。这会导致调用该函数的方法继续。一秒钟后，程序返回 CheckForHost 函数，将正确的值设置为 retVal。我也试过 NSCondition 但也不走运...

有人可以告诉我我做错了什么或者应该做些什么吗？非常感谢您的帮助！

代码如下：

-(int)CheckForHost
{
    InternetActive = -1;
    HostActive = -1;
   dispatch_queue_t myQueue = dispatch_queue_create("my queue", NULL);
    __block int retVal = -1;
    dispatch_async(myQueue, ^{
        [self HostInit];
        dispatch_async(dispatch_get_main_queue(), ^{
            [internetReachable stopNotifier];
            [hostReachable stopNotifier];
            [[NSNotificationCenter defaultCenter] removeObserver:self];
            if (InternetActive == 0) {
                retVal = 0;
            } else if (HostActive == 0) {
                retVal = 1;
            } else
                retVal = 2;
        });
    });
    return retVal;
}

-(void)HostInit
{
    // check for internet connection
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(checkNetworkStatus:) name:kReachabilityChangedNotification object:nil];
    internetReachable = [Reachability reachabilityForInternetConnection];
    [internetReachable startNotifier];
    // check if a pathway to a random host exists
    hostReachable = [Reachability reachabilityWithHostName:@"www.apple.com"];
    [hostReachable startNotifier];
    // now patiently wait for the notification
}

-(void)checkNetworkStatus:(NSNotification *)notice
{
    // called after network status changes
    NetworkStatus internetStatus = [internetReachable currentReachabilityStatus];
    switch (internetStatus)
    {
        case NotReachable:
        {
            NSLog(@"The internet is down.");
            InternetActive = 0;
            break;
        }
        case ReachableViaWiFi:
        {
            NSLog(@"The internet is working via WIFI.");
            InternetActive = 1;
            break;
        }
        case ReachableViaWWAN:
        {
            NSLog(@"The internet is working via WWAN.");
            InternetActive = 1;
            break;
        }
    }
    NetworkStatus hostStatus = [hostReachable currentReachabilityStatus];
    switch (hostStatus)
    {
        case NotReachable:
        {
            NSLog(@"A gateway to the host server is down.");
            HostActive = 0;
            break;
        }
        case ReachableViaWiFi:
        {
            NSLog(@"A gateway to the host server is working via WIFI.");
            HostActive = 1;
            break;
        }
        case ReachableViaWWAN:
        {
            NSLog(@"A gateway to the host server is working via WWAN.");
            HostActive = 1;
            break;
        }
    }
}

Answer 1

嗯，如果要等待执行完成，就不要异步执行了。不等待就是异步的意思

因此，如果您想等待，请同步执行块或使用 semaphore。（我没有说同步执行是个好主意，我只是说，该怎么做。）

Answer 2

我会使用积木，它们既简单又可靠

这是一个带有块的函数示例：

- (void)checkForHostWithBlock:(void (^)(int myReturnedInt))completion{

 //Our function does stuff here, i'll copy paste some of your code

InternetActive = -1;
    HostActive = -1;
   dispatch_queue_t myQueue = dispatch_queue_create("my queue", NULL);
    __block int retVal = -1;
    dispatch_async(myQueue, ^{
        [self HostInit];
        dispatch_async(dispatch_get_main_queue(), ^{
            [internetReachable stopNotifier];
            [hostReachable stopNotifier];
            [[NSNotificationCenter defaultCenter] removeObserver:self];
            if (InternetActive == 0) {
                retVal = 0;
            } else if (HostActive == 0) {
                retVal = 1;
            } else
                retVal = 2;
            if (completion){
                completion(retVal);
            }
        });
    });
}

给你。

您可以像调用任何其他方法一样调用该方法，并且可以使用自动完成功能来确保正确地写下所有内容。

[self checkForHostWithBlock:^(int myReturnInt){
   //Executed code when the block is called in checkForHost:
   NSLog(@"Here is my int : %i  !!", myReturnInt);
}];

请注意，块参数可以为 nil，但您需要确保在您的方法中检查它。

Objective C - 如何等待异步调用完成

Objective C - How to wait for the asynchronous call to finish

asynchronous

objective-c