C ++ / Qt下载文件同步 [英] C++/Qt download file in sync

问题描述

您好
我想实现在一个单独的线程下载一个文件，并存储该文件，
，但我不能找到一个合适的方式来实现这没有邪恶的延迟（相当频繁的下载小文件，所以信号+插槽太慢）。
我想实现的：
（伪代码）

 请求文件; 
等待下载完成，超时或错误; 
保存下载的文件; 
  

如果可能，我更喜欢使用QNetworkAccessManager的示例。
感谢任何提示。

编辑：为了清楚，我想要的信号和插槽不是因为设计，因为缺乏速度。 >

Edit2：这个下载只是关于同步部分的下载文件，线程没有问题。
问题是，QT不提供一个api这样做，我不热衷于热情等待。

Edit3：示例代码，它应该工作，但不会：

  QNetworkAccessManager net; 
 QNetworkReply * re（net.get（QNetworkRequest（QUrl（Qstring（www.blah.org/key）））））; 
 if（re> waitForReadyRead（-1））//！ @bug这不工作，假设，waitForRead返回false，并返回INSTANTLY！ 
 qDebug（）<< ReadyRead yeha !!!; 
 if（re> error（））{
 qDebug（）< 无法下载<< re> url（）。toString（）
<< ：<< re> errorString（）; 
} else {
 img-> load（re> readAll（））; 
 qDebug（）<< Savin IMG; 
} 
 delete re; 
  

解决方案

由于QHttp和QNetworkAccessManager都是异步的，你可以做的是使用单独的事件循环，基于QHttp的完整示例可以找到此处。它不应该太难以修改它为QNetworkAccessManager。

值得一提的是，你的印象是信号/插槽是慢可能是错误的。你实际上是否已经对你的代码进行了剖析来确定这一点？

无论你为信号/插槽支付的处罚是多少，看一下单个文件下载的时间。更重要的是，这是非常非Qt这样做。

在这一天结束如果，你确实患有信号/槽（这是再次，可疑），我建议不要使用Qt这个特定的任务，也许是老的C套接字是一个更好的主意（或围绕他们的薄包装，以节省错误处理，可能需要一些额外的工作）。

Hi I want to achieve to download a file in a separate thread and store that file, but I was not able to find an appropriate way to achieve this without evil delay (quite frequent download of small files, so signal+slots are too slow). What I want to achieve: (Pseudo Code)

request file;
wait for download finishing, timeout or error;
save downloaded file;

I'd prefer an example with QNetworkAccessManager if possible. Thanks for any tipp.

Edit: Just to be clear, I want signal and slots not because of design aswell as the lack of speed.

Edit2: This download is only about the download file in sync part, threading is no problem. The problem is that QT does not provide an api for doing that and I am not keen on hotspinning wait.

Edit3: Example code like it should work, but does not:

QNetworkAccessManager net;
QNetworkReply *re (net.get( QNetworkRequest( QUrl( Qstring("www.blah.org/key") ) ) ));
if (re->waitForReadyRead(-1)) //! @bug this does not work as supposed, waitForRead returns false and returns INSTANTLY!!
    qDebug() << "ReadyRead yeha!!!";
if (re->error()) {
    qDebug() << "Can't download" << re->url().toString()
            << ":" << re->errorString();
} else {
    img->load(re->readAll());
    qDebug() << "Savin IMG";
}
delete re;

解决方案

I needed something similar but for different reasons. Since QHttp and QNetworkAccessManager are both async what you could do is use a separate event loop, a full example based on QHttp can be found here. It shouldn't be too difficult to modify it for QNetworkAccessManager.

It's worth mentioning that your impression that signals/slots are "slow" is probably wrong. Have you actually profiled your code to determine this?

Whatever penalty you might be paying for signals/slots it's probably negligible when looking at the amount of time a single file download takes. More so, it's very "non Qt" to do things this way. These classes were designed like this for a reason.

At the end of the day if you are indeed suffering from signals/slots (which is again, doubtful), I would recommend not to use Qt for this particular task, maybe plain old C sockets are a better idea (or a thin wrapper around them to save the error handling which might require some extra work).

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