1.使用NSOperation

NSOperationQueue *queue = [[NSOperationQueue alloc]init];
    
    NSBlockOperation *task1 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"任務1完成");
    }];
    [queue addOperation:task1];
    
    NSBlockOperation *task2 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"任務2完成");
    }];
    [task2 addDependency:task1];
    [queue addOperation:task2];
    
    
    NSBlockOperation *task3 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"任務3完成");
    }];
    [task3 addDependency:task2];
   [queue addOperation:task3];

打印輸出

2018-07-25 17:17:46.994811+0800 CoreAnimationLearn[20897:305719] 任務1完成
2018-07-25 17:17:46.995069+0800 CoreAnimationLearn[20897:305716] 任務2完成
2018-07-25 17:17:46.995889+0800 CoreAnimationLearn[20897:305716] 任務3完成

有一種情況要注意下，任務是異步執(zhí)行的情況,不會等到異步的任務完成再執(zhí)行依賴的任務。

NSOperationQueue *queue = [[NSOperationQueue alloc]init];
    
    NSBlockOperation *task1 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"任務1完成");
    }];
    [queue addOperation:task1];
    
    NSBlockOperation *task2 = [NSBlockOperation blockOperationWithBlock:^{
       /// 使用dispatch_after模仿異步操作
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"任務2完成");
        });
    }];
    [task2 addDependency:task1];
    [queue addOperation:task2];
    
    
    NSBlockOperation *task3 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"任務3完成");
    }];
    [task3 addDependency:task2];
    [queue addOperation:task3];

打印輸出

2018-07-25 17:37:21.965293+0800 CoreAnimationLearn[21085:315994] 任務1完成
2018-07-25 17:37:21.965594+0800 CoreAnimationLearn[21085:315990] 任務3完成
2018-07-25 17:37:23.965703+0800 CoreAnimationLearn[21085:315933] 任務2完成

2.針對這種情況，可以使用信號量dispatch_semaphore_t解決

 dispatch_queue_t queue = dispatch_queue_create("test", DISPATCH_QUEUE_CONCURRENT);
    ///初始化信號量為1，可以繼續(xù)執(zhí)行任務
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
    dispatch_async(queue, ^{
        ///信號量-1，1-1=0
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        NSLog(@"任務1完成");
        ///信號量+1，0+1=1，可以繼續(xù)執(zhí)行任務
        dispatch_semaphore_signal(semaphore);
    });
    dispatch_async(queue, ^{
        ///信號量-1，1-1=0
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
       /// 使用dispatch_after模仿異步操作
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"任務2完成");
            //信號量+1，0+1=1，可以繼續(xù)執(zhí)行任務
             dispatch_semaphore_signal(semaphore);
        });
    });
    dispatch_async(queue, ^{
        ///信號量-1，1-1=0
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        NSLog(@"任務3完成");
        //信號量+1，0+1=1，可以繼續(xù)執(zhí)行任務
        dispatch_semaphore_signal(semaphore);
    });

打印輸出

2018-07-25 17:45:34.546537+0800 CoreAnimationLearn[21163:320756] 任務1完成
2018-07-25 17:45:36.735362+0800 CoreAnimationLearn[21163:320706] 任務2完成
2018-07-25 17:45:36.735520+0800 CoreAnimationLearn[21163:320759] 任務3完成