DISPATCH_TIME_FOREVER如何在ios应用中工作？

Question

您好，我在应用程序中使用DISPATCH_TIME_FOREVER进行API调用。当网络速度较慢时，它似乎阻塞了主线程。我对此DISPATCH_TIME_FOREVER不太了解。谁能指导我。这是我正在使用的代码。

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

    NSMutableDictionary *postDic=[[NSMutableDictionary alloc]init];

    [postDic setObject:@"getProfileDetails" forKey:@"cmd"];
    [postDic setObject:[[NSUserDefaults standardUserDefaults]objectForKey:@"accountnumber"] forKey:@"accountNumber"];
    [postDic setObject:[[NSUserDefaults standardUserDefaults]objectForKey:@"deviceID"] forKey:@"deviceId"];


    NSData *postData;
    dispatch_group_t group=dispatch_group_create();


    if(postDic!=nil)
        postData=[NSJSONSerialization dataWithJSONObject:postDic options:0 error:nil];
    NSString *postLength=[NSString stringWithFormat:@"%lu", (unsigned long)[postData length]];

    // remote ip
    NSString *remoteAddress=[NSString stringWithFormat:@"%@",[[NSUserDefaults standardUserDefaults]objectForKey:@"remoteIP"]];


    NSURL *url=[NSURL URLWithString:remoteAddress];
    NSMutableURLRequest *request =[[NSMutableURLRequest alloc]init];
    [request setURL:url];
    [request setHTTPMethod:@"Post"];
    [request setValue:postLength forHTTPHeaderField:@"Content-Length"];


    [request setValue:[NSString stringWithFormat:@"%@ %@",[[NSUserDefaults standardUserDefaults] valueForKey:@"tokenType"],[[NSUserDefaults standardUserDefaults] valueForKey:@"accessToken"]] forHTTPHeaderField:@"Authorization"];


    [request setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Content-Type"];
    [request setTimeoutInterval:10.0];
    [request setHTTPBody:postData];

    dispatch_group_enter(group);

    NSURLSessionConfiguration *profileConfiguration=[NSURLSessionConfiguration defaultSessionConfiguration];
    profileConfiguration.timeoutIntervalForResource=30.0;
    profileConfiguration.timeoutIntervalForRequest=30.0;
    NSURLSession *profileSession=[NSURLSession sessionWithConfiguration:profileConfiguration];
    NSURLSessionDataTask *profileTask=[profileSession dataTaskWithRequest:request
                                                          completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) 
        {
            NSLog(@"profile Error is %@",error);

            NSDictionary* lineUpStationsResponse;


            if(data!=nil)
                profileResponse=[NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableLeaves error:&error];


            // If result is null then send empty dictionary back to viewcontroller to display that login customer not subscribed to any service

            if(error!=nil)
            {
                completionBlock(postDic,nil,error);
            }

            if([NSJSONSerialization isValidJSONObject:lineUpStationsResponse])
            {
                completionBlock(postDic,profileResponse,error);
            }
            else
            {
                NSDictionary *resultDic=[[NSDictionary alloc]init];
                completionBlock(postDic,resultDic,error);
            }

           // NSLog(@"profile is %@",profileResponse);

            dispatch_group_leave(group);

        }];

    [profileTask resume];

    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

});