多个接口，一部分并行，剩下的串行

e.g.：一开始先执行任务一、任务二，当任务一或任务二其中一个执行完毕，再执行任务三，任务三完毕再执行任务四...。（只是觉得可以，还没有试过）

dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);

dispatch_queue_t quene = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

//任务1

dispatch_async(quene, ^{

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

[self loadDetailData];  //请求A

dispatch_semaphore_signal(semaphore);

});

//任务2

dispatch_async(quene, ^{

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

[self loadHotCommentData]; //请求B

dispatch_semaphore_signal(semaphore);

});

//任务3

dispatch_async(quene, ^{

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

[self loadHotCommentData]; //请求C

dispatch_semaphore_signal(semaphore);

});

//任务4

dispatch_async(quene, ^{

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

[self loadHotCommentData]; //请求D

dispatch_semaphore_signal(semaphore);

});