iOS 多个异步网络请求全部返回后再执行具体逻辑的方法

先看一段错误的方法：

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    NSURLSession *session = [NSURLSession sharedSession];

    dispatch_queue_t dispatchQueue = dispatch_queue_create("test.queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_t dispatchGroup = dispatch_group_create();
    dispatch_group_async(dispatchGroup, dispatchQueue, ^(){

        NSURLSessionDataTask *task = [session dataTaskWithURL:[NSURL URLWithString:@"https://www.baidu.com"] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
            NSLog(@"got data from internet1");
        }];
        [task resume];

    });
    dispatch_group_async(dispatchGroup, dispatchQueue, ^(){
        NSURLSessionDataTask *task = [session dataTaskWithURL:[NSURL URLWithString:@"https://www.baidu.com"] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
            NSLog(@"got data from internet2");
        }];
        [task resume];
    });

    dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(), ^(){
        NSLog(@"end");
    });
}

看看log的输出

2016-07-13 17:42:55.170 aaaa[4797:295528] end
2016-07-13 17:42:55.322 aaaa[4797:295574] got data from internet2
2016-07-13 17:42:55.375 aaaa[4797:295574] got data from internet1

完全没有达到效果。这是因为这里的网络请求是个异步的方法，没有等待具体的数据返回，放入的dispatch queue的 block就执行完毕了。所以没收到2个网络数据，就提前调用了dispatch_group_notify指定的结束方法。

再看正确的方法：

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    NSURLSession *session = [NSURLSession sharedSession];

    dispatch_queue_t dispatchQueue = dispatch_queue_create("test.queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_t dispatchGroup = dispatch_group_create();
    // dispatch_group_async(dispatchGroup, dispatchQueue, ^(){

    dispatch_group_enter(dispatchGroup);

    NSURLSessionDataTask *task = [session dataTaskWithURL:[NSURL URLWithString:@"https://www.baidu.com"] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        NSLog(@"got data from internet1");

        dispatch_group_leave(dispatchGroup);
    }];
    [task resume];

    //  });
    //  dispatch_group_async(dispatchGroup, dispatchQueue, ^(){
    dispatch_group_enter(dispatchGroup);

    NSURLSessionDataTask *task2 = [session dataTaskWithURL:[NSURL URLWithString:@"https://www.baidu.com"] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        NSLog(@"got data from internet2");

        dispatch_group_leave(dispatchGroup);
    }];
    [task2 resume];
    // });

    dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(), ^(){
        NSLog(@"end");
    });
}

看正确是输出结果：

2016-07-13 17:46:10.282 aaaa[4847:300370] got data from internet1
2016-07-13 17:46:10.501 aaaa[4847:300370] got data from internet2
2016-07-13 17:46:10.502 aaaa[4847:300341] end

我们看看关于dispatch_group_enter 的具体说明

Calling this function increments the current count of outstanding tasks in the group. Using this function (with dispatch_group_leave) allows your application to properly manage the task reference count if it explicitly adds and removes tasks from the group by a means other than using the dispatch_group_async function. A call to this function must be balanced with a call to dispatch_group_leave. You can use this function to associate a block with more than one group at the same time.

简单的说，就是dispatch_group_enter会对group的内部计数加一，dispatch_group_leave会对group的内部计数减一，就类似以前的retain和release方法。说白了也是维护了一个计数器，只不过系统将这个计数器封装了起来，不用我们自己实现了，代码看起来更简洁，高大上了。