在启动另一个块之前等待直到执行两个异步块 [英] Waiting until two async blocks are executed before starting another block

问题描述

使用GCD时，我们要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，但它似乎不起作用：

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：参见此处例如，在Apple的iOS开发人员库的并发编程指南的调度队列章节中等待排队任务组

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

您的示例可能看起来像这个：

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);

并且可以产生如下输出：

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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