等到两个异步块被执行后再开始另一个块

Waiting until two async blocks are executed before starting another block(等到两个异步块被执行后再开始另一个块)

问题描述

在使用 GCD 时，我们希望等到两个异步块执行完毕后再继续执行下一步.最好的方法是什么?

When using GCD, we want to wait until two async blocks are executed and done before moving on to the next steps of execution. What is the best way to do that?

我们尝试了以下方法，但似乎不起作用:

We tried the following, but it doesn't seem to work:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
    // block1
});


dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
    // block2
});

// wait until both the block1 and block2 are done before start block3
// how to do that?

dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
    // block3
});

推荐答案

使用调度组:参见 这里 例如，调度队列"一章中的等待队列任务组"Apple 的 iOS 开发者库的并发编程指南

Use dispatch groups: see here for an example, "Waiting on Groups of Queued Tasks" in the "Dispatch Queues" chapter of Apple's iOS Developer Library's Concurrency Programming Guide

您的示例可能如下所示:

Your example could look something like this:

dispatch_group_t group = dispatch_group_create();

dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
    // block1
    NSLog(@"Block1");
    [NSThread sleepForTimeInterval:5.0];
    NSLog(@"Block1 End");
});


dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
    // block2
    NSLog(@"Block2");
    [NSThread sleepForTimeInterval:8.0];
    NSLog(@"Block2 End");
});

dispatch_group_notify(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
    // block3
    NSLog(@"Block3");
});

// only for non-ARC projects, handled automatically in ARC-enabled projects.
dispatch_release(group);

并且可以产生这样的输出:

and could produce output like this:

2012-08-11 16:10:18.049 Dispatch[11858:1e03] Block1
2012-08-11 16:10:18.052 Dispatch[11858:1d03] Block2
2012-08-11 16:10:23.051 Dispatch[11858:1e03] Block1 End
2012-08-11 16:10:26.053 Dispatch[11858:1d03] Block2 End
2012-08-11 16:10:26.054 Dispatch[11858:1d03] Block3

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