1. RAC

// 处理多个请求，都返回结果的时候，统一做处理.
RACSignal *request1 = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
    // 发送请求1
    [subscriber sendNext:@"发送请求1"];
    return nil;
}];
RACSignal *request2 = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
    // 发送请求2
    [subscriber sendNext:@"发送请求2"];
    return nil;
}];
// 使用注意：几个信号，参数一的方法就几个参数，每个参数对应信号发出的数据。
[self rac_liftSelector:@selector(updateUIWithR1:r2:) withSignalsFromArray:@[request1,request2]];
// 更新UI
- (void)updateUIWithR1:(id)data r2:(id)data1
{
    NSLog(@"更新UI%@  %@",data,data1);
}

2. dispatch_group_t

dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    //请求1
    NSLog(@"Request_1");
});
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    //请求2
    NSLog(@"Request_2");
});
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    //请求3
    NSLog(@"Request_3");
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    //界面刷新
    NSLog(@"任务均完成，刷新界面");
});

3. dispatch_semaphore_signal

dispatch_semaphore_t sem = dispatch_semaphore_create(0);
dispatch_async(dispatch_get_global_queue(0, 0), ^{

    NSLog(@"任务1:%@",[NSThread currentThread]);
    dispatch_semaphore_signal(sem);
});

dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);

dispatch_async(dispatch_get_global_queue(0, 0), ^{
    NSLog(@"任务2:%@",[NSThread currentThread]);
    dispatch_semaphore_signal(sem);
});

dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);

dispatch_async(dispatch_get_global_queue(0, 0), ^{
    NSLog(@"任务3:%@",[NSThread currentThread]);
});

4. dispatch_group_enter&dispatch_group_leave

// 请求和参数为我项目里面的不用在意
dispatch_group_t group = dispatch_group_create();
// 计数＋1
dispatch_group_enter(group);
[JDApiService getActivityDetailWithActivityId:self.activityId Location:stockAddressId SuccessBlock:^(NSDictionary *userInfo) {
    // 数据返回后一些处理
    ...
    // 计数－1
    dispatch_group_leave(group);
} FailureBlock:^(NSError *error) {
    // 数据返回后一些处理
    ...
    // 计数－1
    dispatch_group_leave(group);
}];

// 计数＋1
dispatch_group_enter(group)
[JDApiService getAllCommentWithActivityId:self.activityId PageSize:3 PageNum:self.commentCurrentPage SuccessBlock:^(NSDictionary *userInfo) {
    // 数据返回后一些处理
    ...
    // 计数－1
    dispatch_group_leave(group);
} FailureBlock:^(NSError *error) {
    // 数据返回后一些处理
    ...
    // 计数－1
    dispatch_group_leave(group);
}];

// 其实用计数的说法可能不太对，但是就这么理解吧。会在计数为0的时候执行dispatch_group_notify的任务。
dispatch_group_notify(group, mainQueue, ^{
    // 一般为回主队列刷新UI
    ...
});