iOS中多线程任务之间的同步

当我们使用iOS的多线程并发执行一些任务的时候,有时候需要考虑任务之间的同步问题,比如任务1和任务2执行完之后,才可以执行任务3,原因可能是任务3需要任务1和任务2执行得到的结果。

部分内容参考: Waiting until two async blocks are executed before starting another block

dispatch_group_notify

当group中所有的block都执行完之后,dispatch_group_notify对应的block才会执行。并且放在同一个group中的block可以是不同queue里面的block。

例子:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

dispatch_group_async(group, queue, ^ {
    // block1
    NSLog(@"Block1 Start");
    [NSThread sleepForTimeInterval:2.0];
    NSLog(@"Block1 End");
});

dispatch_group_async(group, queue, ^ {
    // block2
    NSLog(@"Block2 Start");
    [NSThread sleepForTimeInterval:1.0];
    NSLog(@"Block2 End");
});

dispatch_group_notify(group, queue, ^ {
    // block3
    NSLog(@"Block3");
});

输出:

1
2
3
4
5
2016-10-31 11:14:28.076 OCPlayground[82366:5830581] Block2 Start
2016-10-31 11:14:28.076 OCPlayground[82366:5830578] Block1 Start
2016-10-31 11:14:29.081 OCPlayground[82366:5830581] Block2 End
2016-10-31 11:14:30.078 OCPlayground[82366:5830578] Block1 End
2016-10-31 11:14:30.078 OCPlayground[82366:5830578] Block3

dispatch_barrier_async

dispatch_barrier_async函数的作用与barrier的意思相同,在线程任务管理中起到一个栅栏的作用,它等待所有位于barrier函数之前的操作执行完毕后执行,并且在barrier函数执行之后,barrier函数之后的操作才会得到执行,该函数需要同dispatch_queue_create函数生成的并行队列一起使用,不能同dispatch_get_global_queue返回的并行队列一起使用。

例子:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
// dispatch_barrier_async can not be used together with dispatch_get_global_queue
// dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_queue_t queue = dispatch_queue_create("com.company.app.queue", DISPATCH_QUEUE_CONCURRENT);

dispatch_async(queue, ^{
    // block1
    NSLog(@"Block1 Start");
    [NSThread sleepForTimeInterval:2.0];
    NSLog(@"Block1 End");
});

dispatch_async(queue, ^{
    // block2
    NSLog(@"Block2 Start");
    [NSThread sleepForTimeInterval:1.0];
    NSLog(@"Block2 End");
});

dispatch_barrier_async(queue, ^{
    // block3
    NSLog(@"Block3");
});

dispatch_async(queue, ^{
    // block4
    NSLog(@"Block4 Start");
    [NSThread sleepForTimeInterval:3.0];
    NSLog(@"Block4 End");
});

输出:

1
2
3
4
5
6
7
2016-10-31 12:02:08.468 OCPlayground[83918:5867777] Block1 Start
2016-10-31 12:02:08.468 OCPlayground[83918:5867807] Block2 Start
2016-10-31 12:02:09.474 OCPlayground[83918:5867807] Block2 End
2016-10-31 12:02:10.469 OCPlayground[83918:5867777] Block1 End
2016-10-31 12:02:10.469 OCPlayground[83918:5867777] Block3
2016-10-31 12:02:10.470 OCPlayground[83918:5867777] Block4 Start
2016-10-31 12:02:13.473 OCPlayground[83918:5867777] Block4 End

addDependency

苹果对GCD进行了封装,有了自己的NSOperationQueueNSBlockOperation。其中NSBlockOperation中添加了addDepencency:方法可以指定operation block之间的依赖关系。

例子:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
NSOperationQueue *queue = [[NSOperationQueue alloc] init];

NSOperation *completionOperation = [NSBlockOperation blockOperationWithBlock:^{
    // block3
    NSLog(@"Block3");
}];

NSOperation *operation;

operation = [NSBlockOperation blockOperationWithBlock:^{
    // block1
    NSLog(@"Block1 Start");
    [NSThread sleepForTimeInterval:2.0];
    NSLog(@"Block1 End");
}];

[completionOperation addDependency:operation];
[queue addOperation:operation];

operation = [NSBlockOperation blockOperationWithBlock:^{
    // block2
    NSLog(@"Block2 Start");
    [NSThread sleepForTimeInterval:1.0];
    NSLog(@"Block2 End");
}];

[completionOperation addDependency:operation];
[queue addOperation:operation];

[queue addOperation:completionOperation];

输出:

1
2
3
4
5
2016-10-31 12:22:49.774 OCPlayground[84061:5887605] Block1 Start
2016-10-31 12:22:49.774 OCPlayground[84061:5887572] Block2 Start
2016-10-31 12:22:50.776 OCPlayground[84061:5887572] Block2 End
2016-10-31 12:22:51.848 OCPlayground[84061:5887605] Block1 End
2016-10-31 12:22:51.849 OCPlayground[84061:5887605] Block3

dispatch_semaphore

关于dispatch_semaphore的使用讲解的很详细。

dispatch_semaphore是GCD用来同步的一种方式,与他相关的共有三个函数,分别是dispatch_semaphore_createdispatch_semaphore_signaldispatch_semaphore_wait

下面我们逐一介绍三个函数:

(1)dispatch_semaphore_create的声明为:dispatch_semaphore_t dispatch_semaphore_create(long value); 返回一个dispatch_semaphore_t类型且值为value的信号量。这里的传入的参数value必须大于或等于0,否则dispatch_semaphore_create会返回NULL

(2)dispatch_semaphore_signal的声明为:long dispatch_semaphore_signal(dispatch_semaphore_t dsema);这个函数会使传入的信号量dsema的值加1。(至于返回值,参考关于dispatch_semaphore的使用。)

(3) dispatch_semaphore_wait的声明为:long dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout);这个函数会使传入的信号量dsema的值减1。这个函数的作用是这样的:

  • 如果dsema信号量的值大于0,该函数所处线程就继续执行下面的语句,并且将信号量的值减1;
  • 如果dsema的值为0,那么这个函数就阻塞当前线程等待timeout,如果等待期间dsema的值被dispatch_semaphore_signal函数加1了,且该函数(即dispatch_semaphore_wait)所处线程获得了信号量,那么就继续向下执行并将信号量减1。
  • 如果等待期间没有获取到信号量或者信号量的值一直为0,那么等到timeout时,其所处线程自动执行其后语句。

例子:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
dispatch_queue_t queue = dispatch_queue_create("my.conQ", DISPATCH_QUEUE_CONCURRENT);
dispatch_semaphore_t mySem = dispatch_semaphore_create(0);

dispatch_async(queue, ^{
    // block1
    NSLog(@"Block1 Start");
    [NSThread sleepForTimeInterval:2.0];
    NSLog(@"Block1 End");
    dispatch_semaphore_signal(mySem);
});

dispatch_async(queue, ^{
    // block2
    NSLog(@"Block2 Start");
    [NSThread sleepForTimeInterval:1.0];
    NSLog(@"Block2 End");
    dispatch_semaphore_signal(mySem);
});

dispatch_async(queue, ^{
    dispatch_semaphore_wait(mySem, DISPATCH_TIME_FOREVER);
    dispatch_semaphore_wait(mySem, DISPATCH_TIME_FOREVER);
    // block3
    NSLog(@"Block3");
});

输出:

1
2
3
4
5
2016-10-31 22:16:34.402 OCPlayground[85158:6133417] Block1 Start
2016-10-31 22:16:34.402 OCPlayground[85158:6133423] Block2 Start
2016-10-31 22:16:35.405 OCPlayground[85158:6133423] Block2 End
2016-10-31 22:16:36.403 OCPlayground[85158:6133417] Block1 End
2016-10-31 22:16:36.404 OCPlayground[85158:6133426] Block3

dispatch_group_enter和dispatch_group_leave

当我们碰到block里面有嵌套的block的时候,简单地使用dispatch_group_notify就不能解决问题了:dispatch_group_notify不会等到嵌套的block执行完再执行。对于这种情况,如果还坚持用dispatch_group_notify,可以有两种解决方案:

方案1:

对于嵌套的block,仍然使用dispatch_group_async将block放入对应的group。比如下面的例子,对于Block1里面的Block4,我们仍然使用dispatch_group_async将Block4放入对应的group,即可以保证Block3在Block4之后执行。也就是说,只要是在dispatch_group_notify对应的Block3执行之前加入group的block,都需要执行,然后才能执行dispatch_group_notify对应的Block3,即,当dispatch_group_notify对应的Block3执行的时候,group中不能有任何没有执行的其它block存在。

1
2
3
4
5
6
dispatch_group_async(group, queue, ^{
    // block4 inside block 1
    NSLog(@"Block4 inside Block1 Start");
    [NSThread sleepForTimeInterval:1.0];
    NSLog(@"Block4 inside Block1 End");
});

如果嵌套的block对应的是服务器请求的callback(比如onSuccess:onFailure:),就不方便将其加入对应的group中。这时候就需要使用方案2。

方案2: 通过dispatch_group_enterdispatch_group_leave手动管理group关联的block的运行状态(或计数)。但是需要注意:进入和退出group次数必须匹配,不仅是code书写方面一致,code执行也要保证一致,否则dispatch_group_notify对应的Block3就不会执行到。

例子:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

dispatch_group_async(group, queue, ^ {
    // block1
    NSLog(@"Block1 Start");
    [NSThread sleepForTimeInterval:2.0];
    NSLog(@"Block1 End");

    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        // block4 inside block 1
        NSLog(@"Block4 inside Block1 Start");
        [NSThread sleepForTimeInterval:1.0];
        NSLog(@"Block4 inside Block1 End");
        dispatch_group_leave(group);
    });
});


dispatch_group_async(group, queue, ^ {
    // block2
    NSLog(@"Block2 Start");
    [NSThread sleepForTimeInterval:1.0];
    NSLog(@"Block2 End");
});

dispatch_group_notify(group, queue, ^ {
    // block3
    NSLog(@"Block3");
});

输出:

1
2
3
4
5
6
7
2016-10-31 13:51:23.512 OCPlayground[84646:5947063] Block2 Start
2016-10-31 13:51:23.512 OCPlayground[84646:5947080] Block1 Start
2016-10-31 13:51:24.518 OCPlayground[84646:5947063] Block2 End
2016-10-31 13:51:25.518 OCPlayground[84646:5947080] Block1 End
2016-10-31 13:51:25.518 OCPlayground[84646:5947080] Block4 inside Block1 Start
2016-10-31 13:51:26.521 OCPlayground[84646:5947080] Block4 inside Block1 End
2016-10-31 13:51:26.522 OCPlayground[84646:5947063] Block3