iOS实现多个网络请求ABC执行完再执行D

方案1：使用group和semaphore
方案2：group_enter和group_leave也可以实现

方案1实现例子

 dispatch_semaphore_t semaphore = dispatch_semaphore_create(2);
        dispatch_queue_t quene = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
         
        //任务A
        dispatch_async(quene, ^{
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
           [self requestA:^{
               NSLog(@"---执行A任务结束---");
           }];
            dispatch_semaphore_signal(semaphore);
        });
        //任务B
        dispatch_async(quene, ^{
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
             [self requestB:^{
                          NSLog(@"---执行B任务结束---");
                      }];
            dispatch_semaphore_signal(semaphore);
        });
        //任务C
        dispatch_async(quene, ^{
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
             [self requestC:^{
                 NSLog(@"---执行C任务结束---");
            }];
            dispatch_semaphore_signal(semaphore);
        });
        //任务D
        dispatch_async(quene, ^{
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
             [self requestD:^{
                 NSLog(@"---执行D任务结束---");
            }];
            dispatch_semaphore_signal(semaphore);
        });

方案2实现例子

  dispatch_queue_t globalQueue = dispatch_get_global_queue(0, 0);
    dispatch_group_t group = dispatch_group_create();
    
    dispatch_group_enter(group);
    [self requestA:^{
        NSLog(@"---执行A任务结束---");
        dispatch_group_leave(group);
    }];
    
    dispatch_group_enter(group);
    [self requestB:^{
        NSLog(@"---执行B任务结束---");
        dispatch_group_leave(group);
    }];
    
    dispatch_group_enter(group);
    [self requestC:^{
        NSLog(@"---执行C任务结束---");
        dispatch_group_leave(group);
    }];
    
    dispatch_group_notify(group, globalQueue, ^{
        [self requestD:^{
            NSLog(@"---执行D任务结束---");
        }];
    });

- (void)requestA:(void(^)(void))block{
      NSLog(@"---执行A任务开始---");
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        block();
    });
}
- (void)requestB:(void(^)(void))block{
      NSLog(@"---执行B任务开始---");
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        block();
    });
}
- (void)requestC:(void(^)(void))block{
      NSLog(@"---执行C任务开始---");
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        block();
    });
    
}
- (void)requestD:(void(^)(void))block{
    NSLog(@"---执行D任务开始---");
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(4 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        block();
    });
}