处理块，完成处理程序，dispatch_async与dispatch_sync [英] Dealing with Blocks, completion handlers, dispatch_async vs dispatch_sync

问题描述

我正在线程中执行在线数据获取，并且我想在执行块后立即执行某项操作.

I'm executing an online fetch of data in a thread and I want to do something immediately after the block is executed.

这是我的代码:

- (IBAction)refresh:(UIBarButtonItem *)sender {
    NSLog(@"checking");
    [self editToolbar];
    dispatch_queue_t fetchQ = dispatch_queue_create("Refreshing", NULL);
    dispatch_async(fetchQ, ^{
        [self setupFetchedResultsController];
        [self fetchImonggoItemsDataIntoDocument: self.itemDatabase];
    });
    dispatch_release(fetchQ); 

    NSLog(@"done checking");
    //do something here
}

事情是dispatch_async立即返回，并且即使在该块执行完毕之前，也会立即打印完成检查".我该如何解决?

The thing is dispatch_async returns immediately and "done checking" prints immediately even before the block is done executing. How do I solve this?

推荐答案

我认为这是一个体系结构问题.任务类似于:

I think it's an architectural issue. The tasks are something like:

1. 编辑工具栏
2. fetchImonggoItemsDataIntoDocument
3. 做其他事情

如果必须按顺序正确完成这些操作，那么我对块或队列的使用不太了解.只需一个接一个地运行这些语句，您就会被设置.

If these must be done exactly in order then I don't quite understand the use of blocks or queues; just run the statements after each other and you'll be set.

否则，备选方案1是使用dispatch_sync而不是dispatch_async.再说一次，我不太确定使用队列的好处是什么，但是确实有.

Otherwise, alternative #1 would be to use dispatch_sync rather than dispatch_async. Again, I'm not quite sure what the benefit of using a queue would be but there it is.

替代方法2是使用该块中的回调.像这样:

Alternative #2 would be to use a callback from the block. Something like:

- (IBAction)refresh:(UIBarButtonItem *)sender {
    NSLog(@"checking");
    [self editToolbar];
    dispatch_queue_t fetchQ = dispatch_queue_create("Refreshing", NULL);
    dispatch_async(fetchQ, ^{
        [self setupFetchedResultsController];
        [self fetchImonggoItemsDataIntoDocument: self.itemDatabase];
        [self doneChecking]; // <-- NOTE! call the callback
    });
    dispatch_release(fetchQ); 
}

// NOTE! refresh: has been split up into two methods
- (void)doneChecking:
    NSLog(@"done checking");
    //do something here
}

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