QNetworkAccessManager的异步与线程

Qt版本5.1.1

以HTTP操作为例

Qt中的HTTP操作都是异步的. 内部通过线程实现

创建线程的时机在QNetworkReplyHttpImplPrivate::postRequest()

void QNetworkReplyHttpImplPrivate::postRequest()
{
    Q_Q(QNetworkReplyHttpImpl);

    QThread *thread = ;
    if (synchronous) {
        // A synchronous HTTP request uses its own thread
        thread = new QThread();
        thread->setObjectName(QStringLiteral("Qt HTTP synchronous thread"));
        QObject::connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
        thread->start();
    } else if (!managerPrivate->httpThread) {
        // We use the manager-global thread.
        // At some point we could switch to having multiple threads if it makes sense.
        managerPrivate->httpThread = new QThread();
        managerPrivate->httpThread->setObjectName(QStringLiteral("Qt HTTP thread"));
        managerPrivate->httpThread->start();

        thread = managerPrivate->httpThread;
    } else {
        // Asynchronous request, thread already exists
        thread = managerPrivate->httpThread;
    }

..........

    // Move the delegate to the http thread
    delegate->moveToThread(thread);
    // This call automatically moves the uploadDevice too for the asynchronous case.

...........
}

分为两种情况:
(1) synchronous == true 每次HTTP请求创建自己的线程, 并在finished后自动退出线程

在QNetworkRequest设置QNetworkRequest::SynchronousRequestAttribute 属性为真时, synchronous = true, 然而SynchronousRequestAttribute被Qt标记为internal. 以防止外部创建synchronous HTTP请求.
我在Qt的源码中找到一点说明, QNetworkReplyHttpImpl的构造函数中.

........
   // Internal code that does a HTTP reply for the synchronous Ajax
    // in Qt WebKit.
    QVariant synchronousHttpAttribute = request.attribute(
            static_cast<QNetworkRequest::Attribute>(QNetworkRequest::SynchronousRequestAttribute));
    if (synchronousHttpAttribute.isValid()) {
        d->synchronous = synchronousHttpAttribute.toBool();
........

webkit的ajax请求使用

(2) synchronous == false 则把所有http请求放置在一个线程中.
并且该线程在
QNetworkAccessManagerPrivate对象析构(即QNetworkAccessManager析构)或者调用QNetworkAccessManagerPrivate::clearCache 时退出

QNetworkAccessManagerPrivate::~QNetworkAccessManagerPrivate()
{
    if (httpThread) {
        httpThread->quit();
        httpThread->wait();
        if (httpThread->isFinished())
            delete httpThread;
        else
            QObject::connect(httpThread, SIGNAL(finished()), httpThread, SLOT(deleteLater()));
        httpThread = ;
    }
}

void QNetworkAccessManagerPrivate::clearCache(QNetworkAccessManager *manager)
{
    manager->d_func()->objectCache.clear();
    manager->d_func()->authenticationManager->clearCache();

    if (manager->d_func()->httpThread) {
        manager->d_func()->httpThread->quit();
        manager->d_func()->httpThread->wait();
        if (manager->d_func()->httpThread->isFinished())
            delete manager->d_func()->httpThread;
        else
            QObject::connect(manager->d_func()->httpThread, SIGNAL(finished()), manager->d_func()->httpThread, SLOT(deleteLater()));
        manager->d_func()->httpThread = ;
    }
}

否则会一直HTTP 线程会一直存在. 另外, 每个QNetworkAccessManager对象对应自己的HTTP thread.