在启动另一个块之前等待直到执行两个异步块

Question

在启动另一个块之前等待直到执行两个异步块使用GCD时，我们希望等到两个异步块执行完成后再继续执行下一步。最好的方法是什么？我们尝试了以下方法，但它似乎不起作用：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});

Answer 1

使用调度组：看到这里的一个例子，在苹果的iOS开发者库的并发编程指南“调度队列”章“等待上排队任务组”

您的示例可能如下所示：

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);

并可以产生这样的输出：

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