异步任务两两嵌套依赖的信号量实现

Contents

1. 1. 目标
2. 2. 实现

目标

(A)  (B)  (C)
 |    |    |
 |    |    |
 |    |    | 
 +++++ +++++
   |     |
   |     |
   |     |
  (D)   (E)

ABC 三个异步任务两两组合，双双完成之后才触发 D E 两个任务。
使用信号量，完成任务。

实现

dispatch_semaphore_t semaphore1 = dispatch_semaphore_create(0);
dispatch_semaphore_t semaphore2 = dispatch_semaphore_create(0);
dispatch_semaphore_t semaphore3 = dispatch_semaphore_create(0);
dispatch_semaphore_t semaphore4 = dispatch_semaphore_create(0);
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
    sleep(2);
    NSLog(@"mission A");
    dispatch_semaphore_signal(semaphore1); //++1
});

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
    sleep(3);
    NSLog(@"mission B");
    dispatch_semaphore_signal(semaphore2); //++1
    dispatch_semaphore_signal(semaphore3); //++1
});

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
    sleep(5);
    NSLog(@"mission C");
    dispatch_semaphore_signal(semaphore4); //++1
});

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
    dispatch_semaphore_wait(semaphore1, DISPATCH_TIME_FOREVER);
    dispatch_semaphore_wait(semaphore2, DISPATCH_TIME_FOREVER);
    NSLog(@"mission D");
});

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
    dispatch_semaphore_wait(semaphore3, DISPATCH_TIME_FOREVER);
    dispatch_semaphore_wait(semaphore4, DISPATCH_TIME_FOREVER);
    NSLog(@"mission E");
});

在任务 D 里 wait A B 的信号量，在任务 E 里 wait B C 的信号量，B 发送两个信号量