了解dispatch_async [英] Understanding dispatch_async

问题描述

我身边有这样code问题

I have question around this code

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    NSData* data = [NSData dataWithContentsOfURL: 
      kLatestKivaLoansURL];
    [self performSelectorOnMainThread:@selector(fetchedData:) 
      withObject:data waitUntilDone:YES];
});

这code的第一个参数是

The first parameter of this code is

dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0) 

我们是否问这个code对全局队列，其定义本身是返回给定优先级的全局并发队列进行串行任务？

Are we asking this code to perform serial tasks on global queue whose definition itself is that it returns global concurrent queue of a given priority level?

什么是优势，使用 dispatch_get_global_queue 在主队列？

What is advantage of using dispatch_get_global_queue over the main queue?

我很困惑。你能帮我明白这更好。

I am confused. Could you please help me to understand this better.

推荐答案

您使用在主队列默认队列是运行在后台任务的主要原因。

The main reason you use the default queue over the main queue is to run tasks in the background.

例如，如果我从互联网下载一个文件，我想更新的下载进度的用户，我会运行下载的优先级默认队列，并更新主队列异步的UI。

For instance, if I am downloading a file from the internet and I want to update the user on the progress of the download, I will run the download in the priority default queue and update the UI in the main queue asynchronously.

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
    //Background Thread
    dispatch_async(dispatch_get_main_queue(), ^(void){
        //Run UI Updates
    });
});

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