等待多个网络请求一起返回

// 网络请求模拟
- (void)httpRequest:(NSString *)method param:(NSDictionary *)param completion:(void(^)(id response))block {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSString *command = [param objectForKey:@"commandKey"];
        NSLog(@"request: %@ run in thread: %@", command, [NSThread currentThread]);
        NSTimeInterval sleepInterval = arc4random() % 10;
        [NSThread sleepForTimeInterval:sleepInterval];

        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"request: %@ done!", command);
            block(nil);
        });
    });
}

- (void)testUsingSemphore {
    dispatch_semaphore_t sem = dispatch_semaphore_create(0);

    NSArray *commandArray = @[@"requestcommand1", @"requestcommand2", @"requestcommand3", @"requestcommand4", @"requestcommand5"];
    __block NSInteger httpFinishedCount = 0;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        [commandArray enumerateObjectsUsingBlock:^(id  _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
            [self httpRequest:nil param:@{@"commandKey": obj} completion:^(id response) {
                NSLock *lock = [[NSLock alloc] init];
                [lock lock];
                if (++httpFinishedCount == [commandArray count]) {
                    dispatch_semaphore_signal(sem);
                }
                [lock unlock];
            }];
        }];

        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        NSLog(@"所有任务都已经完成了!");
    });

}

2017-06-01 17:23:15.405 Test456[17812:2801931] request: requestcommand1 run in thread: <NSThread: 0x608000268100>{number = 5, name = (null)}
2017-06-01 17:23:15.405 Test456[17812:2802100] request: requestcommand3 run in thread: <NSThread: 0x60800007f480>{number = 7, name = (null)}
2017-06-01 17:23:15.405 Test456[17812:2802099] request: requestcommand2 run in thread: <NSThread: 0x618000263d80>{number = 6, name = (null)}
2017-06-01 17:23:15.405 Test456[17812:2802101] request: requestcommand4 run in thread: <NSThread: 0x60800046c700>{number = 8, name = (null)}
2017-06-01 17:23:15.405 Test456[17812:2802102] request: requestcommand5 run in thread: <NSThread: 0x618000266ac0>{number = 9, name = (null)}
2017-06-01 17:23:15.405 Test456[17812:2801803] request: requestcommand1 done!
2017-06-01 17:23:16.409 Test456[17812:2801803] request: requestcommand5 done!
2017-06-01 17:23:19.410 Test456[17812:2801803] request: requestcommand2 done!
2017-06-01 17:23:22.406 Test456[17812:2801803] request: requestcommand4 done!
2017-06-01 17:23:23.410 Test456[17812:2801803] request: requestcommand3 done!
2017-06-01 17:23:23.411 Test456[17812:2801929] 所有任务都已经完成了!