在多个线程中使用QNetworkAccessManager

经研究后发现这是一个线程问题： Tangram会在多个线程发送URL request， 但是在实现时， 却没有考虑线程问题， 程序在运行时得到如下warning：

QObject: Cannot create children <span style="color: #0000ff">for</span> a parent that <span style="color: #0000ff">is</span> <span style="color: #0000ff">in</span><span style="color: #000000"> a different thread.
(Parent </span><span style="color: #0000ff">is</span> QNetworkAccessManager(<span style="color: #800080">0x21dcba0</span>), parent<span style="color: #800000">‘</span><span style="color: #800000">s thread is QThread(0x20be1c0), current thread is QThread(0x7f48e0044730)</span>

在移植Tangram到Qt平台时， 网络请求使用的是QNetworkAccessManager， QNetworkAccessManager的使用很简单：

  QNetworkAccessManager *manager = <span style="color: #0000ff">new</span> QNetworkAccessManager(<span style="color: #0000ff">this</span><span style="color: #000000">);
  connect(manager, SIGNAL(finished(QNetworkReply</span>*<span style="color: #000000">)), th</span><span style="color: #0000ff">is</span>, SLOT(replyFinished(QNetworkReply*<span style="color: #000000">)));
  manager</span>-><span style="color: #0000ff">get</span>(QNetworkRequest(QUrl(<span style="color: #800000">"</span><span style="color: #800000">http://qt-project.org</span><span style="color: #800000">"</span>)));

这里碰到的问题是：Tangram可能在多个线程发送URL， 但是QNetworkAccessManager只能在生成它所在的线程中工作， 解决办法是使用Qt的SIGNAL/SLOT机制， 把该URL request post到正确的线程中：

<span style="color: #0000ff">class</span><span style="color: #000000"> URLTaskWorker{
</span><span style="color: #0000ff">public</span><span style="color: #000000">:
    URLTaskWorker(){
        connect(</span><span style="color: #0000ff">this</span>,SIGNAL(startRequest()),<span style="color: #0000ff">this</span><span style="color: #000000">, SLOT(onRequest()), Qt::QueuedConnection);
    }
</span><span style="color: #0000ff">public</span><span style="color: #000000">:
    </span><span style="color: #0000ff">void</span><span style="color: #000000"> handleTask(QUrl url){
        </span><span style="color: #0000ff">this</span>->url =<span style="color: #000000"> url;
        emit StartNewRequest();
    }
signals:
    </span><span style="color: #0000ff">void</span><span style="color: #000000"> startRequest();
    
</span><span style="color: #0000ff">private</span><span style="color: #000000"> slots:
    </span><span style="color: #0000ff">void</span><span style="color: #000000"> onRequest(){
        qnam.</span><span style="color: #0000ff">get</span><span style="color: #000000">(url);
    }
</span><span style="color: #0000ff">private</span><span style="color: #000000">:
    QNetworkAccessManager   qnam;
    QUrl                    url;
};</span>

这里要注意的是， 在connect startRequest和onRequest时， 要使用Qt::QueuedConnection， 使用默认的Qt::AutoConnection不工作， 参考Qt帮助文档：

<span style="color: #000000">Qt::QueuedConnection
</span><span style="color: #000000">
The slot </span><span style="color: #0000ff">is</span> invoked when control returns to the <span style="color: #0000ff">event</span> loop of the receiver<span style="color: #800000">‘</span><span style="color: #800000">s thread. The slot is executed in the receiver</span><span style="color: #800000">‘</span>s thread.

这也是一个Qt的线程安全的在不同线程间传递消息的方法。

在多个线程中使用QNetworkAccessManager

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