iOS 发送异步请求

//发送async异步请求
    NSURL *url = [NSURL URLWithString:@"http://www.baidu.com"];
    NSURLRequest *requset = [NSURLRequest requestWithURL:url];
    [NSURLConnection sendAsynchronousRequest:requset queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse * _Nullable response, NSData * _Nullable data, NSError * _Nullable connectionError) {
        //这里使用主队列 [NSOperationQueue mainQueue]
        
        //response          服务器返回的相应头
        //data              服务器返回的响应体
        //connectionError   链接错误
        
        //判断请求是否有错误
        if (!connectionError) {
            //把二进制数据转换成NSString
            NSString *html = [[NSString alloc]initWithData:data encoding:NSUTF8StringEncoding];
            NSLog(@"html的字符串=%@",html);
        }else{
            NSLog(@"连接错误");
        }
    }];