ios - AFHTTPSessionManager 忽略 HTTPMaximumConnectionsPerHost

Question

我正在尝试使用 AFNetworking2.0 和新的 NSURLSession 类来获取一些数据。特别是我对 NSURLSessionConfiguration 的新 HTTPMaximumConnectionsPerHost 属性很感兴趣，但不幸的是它似乎没有像我预期的那样工作。

@interface ViewController ()
@property (nonatomic, strong) AFHTTPSessionManager *httpSessionManager;
@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    NSURLSessionConfiguration *sessionConfig = [[NSURLSessionConfiguration defaultSessionConfiguration] copy];
    sessionConfig.HTTPMaximumConnectionsPerHost = 5;

    self.httpSessionManager = [[AFHTTPSessionManager alloc] initWithSessionConfiguration:sessionConfig];
    self.httpSessionManager.responseSerializer = [AFImageResponseSerializer serializer];

    for (int i = 0; i < 100; ++i) {
        //http://i.dailymail.co.uk/i/pix/2012/07/03/article-2168112-13E70C13000005DC-867_964x946.jpg // 400k picture, the one from deviantart is 2k
        [self.httpSessionManager GET:@"http://a.deviantart.net/avatars/t/o/toaster-kitten.jpg" parameters:nil success:^(NSURLSessionDataTask *task, id responseObject) {
            NSLog(@"has finished task %p, context id %lu", task, task.taskIdentifier);
        } failure:^(NSURLSessionDataTask *task, NSError *error) {
            NSLog(@"has failed task %p, context id %lu", task, task.taskIdentifier);
        }];
    }
}

这是我的下载示例实现。

我还调整了 AFHTTPSessionManager 以生成日志:

- (NSURLSessionDataTask *)GET:(NSString *)URLString
                   parameters:(NSDictionary *)parameters
                      success:(void (^)(NSURLSessionDataTask *task, id responseObject))success
                      failure:(void (^)(NSURLSessionDataTask *task, NSError *error))failure
{
    NSMutableURLRequest *request = [self.requestSerializer requestWithMethod:@"GET" URLString:[[NSURL URLWithString:URLString relativeToURL:self.baseURL] absoluteString] parameters:parameters error:nil];

    __block NSURLSessionDataTask *task = [self dataTaskWithRequest:request completionHandler:^(NSURLResponse * __unused response, id responseObject, NSError *error) {
        if (error) {
            if (failure) {
                failure(task, error);
            }
        } else {
            if (success) {
                success(task, responseObject);
            }
        }
    }];

    [task addObserver:self forKeyPath:@"countOfBytesReceived" options:NSKeyValueObservingOptionOld context:(__bridge void*)task];
    NSLog(@"created task %p id %u", task, task.taskIdentifier);

    [task resume];

    return task;
}


- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{
    if([[change objectForKey:@"old"] integerValue] == 0 && [keyPath isEqualToString:@"countOfBytesReceived"]){
        NSLog(@"dtask %p id %u: started", object, ((NSURLSessionDataTask*)object).taskIdentifier);
        @try {
            if ((__bridge void *)object == context) {
                [object removeObserver:self forKeyPath:@"countOfBytesReceived" context:(__bridge void*)object];
            }
        }
        @catch (NSException *exception) {
        }
    }
}

使用此代码，我会收到下一条日志:

2014-04-02 17:10:38.950 test_block[2704:60b] created task 0x1701a4fa0 id 0
2014-04-02 17:10:38.972 test_block[2704:60b] created task 0x1781a5be0 id 1
2014-04-02 17:10:38.974 test_block[2704:60b] created task 0x1701a5160 id 2
2014-04-02 17:10:38.976 test_block[2704:60b] created task 0x1701a5320 id 3
2014-04-02 17:10:38.977 test_block[2704:60b] created task 0x1701a54e0 id 4
2014-04-02 17:10:38.979 test_block[2704:60b] created task 0x1701a56a0 id 5
2014-04-02 17:10:38.980 test_block[2704:60b] created task 0x1701a5940 id 6
2014-04-02 17:10:38.982 test_block[2704:1803] dtask 0x1701a4fa0 id 0: started
2014-04-02 17:10:38.984 test_block[2704:6403] dtask 0x1781a5be0 id 1: started
2014-04-02 17:10:38.986 test_block[2704:4b03] dtask 0x1701a56a0 id 5: started
2014-04-02 17:10:38.987 test_block[2704:5b03] dtask 0x1701a5160 id 2: started
2014-04-02 17:10:38.988 test_block[2704:6703] dtask 0x1701a5320 id 3: started
2014-04-02 17:10:38.990 test_block[2704:60b] created task 0x1781a5da0 id 7
2014-04-02 17:10:38.990 test_block[2704:1803] dtask 0x1701a54e0 id 4: started
2014-04-02 17:10:38.992 test_block[2704:6903] dtask 0x1701a5940 id 6: started
2014-04-02 17:10:38.993 test_block[2704:60b] created task 0x1701a4ec0 id 8
2014-04-02 17:10:38.993 test_block[2704:1803] dtask 0x1781a5da0 id 7: started
2014-04-02 17:10:38.994 test_block[2704:60b] created task 0x1781a5cc0 id 9
2014-04-02 17:10:38.996 test_block[2704:6403] dtask 0x1701a4ec0 id 8: started
2014-04-02 17:10:38.997 test_block[2704:60b] created task 0x1701a5240 id 10
2014-04-02 17:10:38.998 test_block[2704:5103] dtask 0x1781a5cc0 id 9: started
2014-04-02 17:10:38.999 test_block[2704:60b] created task 0x1781a6200 id 11
2014-04-02 17:10:39.391 test_block[2704:4603] dtask 0x1701a5240 id 10: started
2014-04-02 17:10:39.399 test_block[2704:60b] created task 0x1781a62e0 id 12
2014-04-02 17:10:39.406 test_block[2704:3507] dtask 0x1781a6200 id 11: started
2014-04-02 17:10:39.409 test_block[2704:60b] created task 0x1701a63c0 id 13
2014-04-02 17:10:39.411 test_block[2704:60b] created task 0x1701a6580 id 14
2014-04-02 17:10:39.414 test_block[2704:1803] dtask 0x1781a62e0 id 12: started
2014-04-02 17:10:39.418 test_block[2704:3507] dtask 0x1701a63c0 id 13: started
2014-04-02 17:10:39.421 test_block[2704:60b] created task 0x1781a6ac0 id 15
2014-04-02 17:10:39.423 test_block[2704:5b03] dtask 0x1701a6580 id 14: started
2014-04-02 17:10:39.425 test_block[2704:60b] created task 0x1701a5cc0 id 16
2014-04-02 17:10:39.427 test_block[2704:5b03] dtask 0x1781a6ac0 id 15: started
2014-04-02 17:10:39.428 test_block[2704:60b] created task 0x1701a5f60 id 17
2014-04-02 17:10:39.430 test_block[2704:5103] dtask 0x1701a5cc0 id 16: started
2014-04-02 17:10:39.431 test_block[2704:60b] created task 0x1781a64a0 id 18
2014-04-02 17:10:39.433 test_block[2704:3507] dtask 0x1701a5f60 id 17: started
2014-04-02 17:10:39.434 test_block[2704:60b] created task 0x1701a64a0 id 19
2014-04-02 17:10:39.435 test_block[2704:3507] dtask 0x1781a64a0 id 18: started
2014-04-02 17:10:39.437 test_block[2704:60b] created task 0x1701a6820 id 20
2014-04-02 17:10:39.438 test_block[2704:3507] dtask 0x1701a64a0 id 19: started
2014-04-02 17:10:39.440 test_block[2704:60b] created task 0x1781a6580 id 21
2014-04-02 17:10:39.441 test_block[2704:3507] dtask 0x1701a6820 id 20: started
2014-04-02 17:10:39.442 test_block[2704:60b] created task 0x1701a6120 id 22
2014-04-02 17:10:39.444 test_block[2704:3507] dtask 0x1781a6580 id 21: started
2014-04-02 17:10:39.445 test_block[2704:60b] created task 0x1701a7540 id 23
2014-04-02 17:10:39.446 test_block[2704:3507] dtask 0x1701a6120 id 22: started
2014-04-02 17:10:39.447 test_block[2704:60b] created task 0x1781a6c80 id 24
2014-04-02 17:10:39.448 test_block[2704:60b] created task 0x1701a5e80 id 25
2014-04-02 17:10:39.450 test_block[2704:60b] created task 0x1701a70e0 id 26
2014-04-02 17:10:39.450 test_block[2704:5103] dtask 0x1701a7540 id 23: started
2014-04-02 17:10:39.451 test_block[2704:5b03] dtask 0x1781a6c80 id 24: started
2014-04-02 17:10:39.453 test_block[2704:5103] dtask 0x1701a5e80 id 25: started
2014-04-02 17:10:39.454 test_block[2704:60b] created task 0x1701a7a80 id 27
...
2014-04-02 17:10:39.694 test_block[2704:60b] has finished task 0x1781aaaa0, context id 79
2014-04-02 17:10:39.695 test_block[2704:60b] has finished task 0x1701ac320, context id 80
2014-04-02 17:10:39.695 test_block[2704:60b] has finished task 0x1701abfa0, context id 78
2014-04-02 17:10:39.696 test_block[2704:60b] has finished task 0x1701ac4e0, context id 81
2014-04-02 17:10:39.697 test_block[2704:60b] has finished task 0x1701ac6a0, context id 82
2014-04-02 17:10:39.698 test_block[2704:60b] has finished task 0x1701ac860, context id 83
2014-04-02 17:10:39.699 test_block[2704:60b] has finished task 0x1781aac60, context id 84
2014-04-02 17:10:39.699 test_block[2704:60b] has finished task 0x1701aca20, context id 85
2014-04-02 17:10:39.700 test_block[2704:60b] has finished task 0x1781ab1a0, context id 86
2014-04-02 17:10:39.700 test_block[2704:60b] has finished task 0x1781ab360, context id 87
2014-04-02 17:10:39.701 test_block[2704:60b] has finished task 0x1701acbe0, context id 88
2014-04-02 17:10:39.702 test_block[2704:60b] has finished task 0x1701acda0, context id 89
2014-04-02 17:10:39.702 test_block[2704:60b] has finished task 0x1781ab6e0, context id 92
2014-04-02 17:10:39.703 test_block[2704:60b] has finished task 0x1781ab520, context id 90
2014-04-02 17:10:39.705 test_block[2704:60b] has finished task 0x1701ace80, context id 91
2014-04-02 17:10:39.713 test_block[2704:60b] has finished task 0x1781ab8a0, context id 94
2014-04-02 17:10:39.714 test_block[2704:60b] has finished task 0x1781aba60, context id 95
2014-04-02 17:10:39.714 test_block[2704:60b] has finished task 0x1701ad040, context id 93
2014-04-02 17:10:39.715 test_block[2704:60b] has finished task 0x1701ad200, context id 96
2014-04-02 17:10:39.715 test_block[2704:60b] has finished task 0x1701ad3c0, context id 97
2014-04-02 17:10:39.716 test_block[2704:60b] has finished task 0x1701ab520, context id 98
2014-04-02 17:10:39.716 test_block[2704:60b] has finished task 0x1701accc0, context id 99

这不是我期望看到的。如您所见，超过 5 个数据任务已启动，直到至少一个先前启动的任务完成。

我在 iOS7.1 和 iOS7.0.3 的 iPhone 上测试

任何人都可以解释为什么会发生这种情况以及如何让这些任务通过 NSURLSession 排队(或者至少让它们在一定限制后失败)？

谁能证实或反驳这种行为？

附言我已经实现了自己的排队代码，所以这个问题不是关于如何排队请求，而是如何让它与 NSURLSession 一起工作。

Answer 1

文档指出它将限制每个主机的连接数。这与创建的任务数量无关。

您可以轻松记录通知增量更新的委托(delegate)回调，以查看哪些任务代表实际连接。其他任务在队列中，等待轮到它们连接到主机。