Qt 5.13 | Qt WebEngine | Porting from Qt WebKit to Qt WebEngine

Qt 5.13.2 Reference Documentation

# Porting from Qt WebKit to Qt WebEngine

The following sections contain information about porting an application that uses the Qt WebKit QWebView API to use the Qt WebEngine QWebEngineView.

## Architecture

Chromium provides its own network and painting engines, which Qt WebEngine uses. This, among other things, allows Qt WebEngine to provide better and more reliable support for the latest HTML5 specification than Qt WebKit. However, Qt WebEngine is thus also heavier than Qt WebKit and does not provide direct access to the network stack and the HTML document through C++ APIs.

## Class Names

The Qt WebEngine equivalent of Qt WebKit C++ classes are prefixed by "QWebEngine" instead of "QWeb".

Qt WebKit

      #include <QWebHistory>
      #include <QWebHistoryItem>
      #include <QWebPage>
      #include <QWebView>

      QWebHistory
      QWebHistoryItem
      QWebPage
      QWebView

Qt WebEngine

      #include <QWebEngineHistory>
      #include <QWebEngineHistoryItem>
      #include <QWebEnginePage>
      #include <QWebEngineView>

      QWebEngineHistory
      QWebEngineHistoryItem
      QWebEnginePage
      QWebEngineView

## Qt Module Name

### In qmake Project Files

Qt WebKit

      QT += webkitwidgets

Qt WebEngine

      QT += webenginewidgets

### Including the Module in Source Files

Qt WebKit

      #include <QtWebKit/QtWebKit>
      #include <QtWebKitWidgets/QtWebKitWidgets> // With Qt >= 4.8

Qt WebEngine

      #include <QtWebEngineWidgets/QtWebEngineWidgets>

## QWebFrame Has Been Merged into QWebEnginePage

HTML frames can be used to divide web pages into several areas where the content can be represented individually.

In Qt WebKit, QWebFrame represents a frame inside a web page. Each QWebPage object contains at least one frame, the main frame, obtained using QWebPage::mainFrame(). Additional frames will be created for the HTML `<frame>` element, which defines the appearance and contents of a single frame, or the `<iframe>` element, which inserts a frame within a block of text.

In Qt WebEngine, frame handling has been merged into the QWebEnginePage class. All child frames are now considered part of the content, and only accessible through JavaScript. Methods of the QWebFrame class, such as `load()` are now available directly through the QWebEnginePage itself.

Qt WebKit

      QWebPage page;
      connect(page.mainFrame(), SIGNAL(urlChanged(const QUrl&)), SLOT(mySlotName()));
      page.mainFrame()->load(url);

Qt WebEngine

      QWebEnginePage page;
      connect(&page, SIGNAL(urlChanged(const QUrl&)), SLOT(mySlotName()));
      page.load(url);

## Some Methods Now Return Their Result Asynchronously

Because Qt WebEngine uses a multi-process architecture, calls to some methods from applications will return immediately, while the results should be received asynchronously via a callback mechanism. A function pointer, a functor, or a lambda expression must be provided to handle the results when they become available.

Qt WebKit

      QWebPage *page = new QWebPage;
      QTextEdit *textEdit = new QTextEdit;
      // *textEdit is modified immediately.
      textEdit->setPlainText(page->toHtml());
      textEdit->setPlainText(page->toPlainText());

Qt WebEngine (with a lambda function in C++11)

      QWebEnginePage *page = new QWebEnginePage;
      QTextEdit *textEdit = new QTextEdit;
      // *textEdit must remain valid until the lambda function is called.
      page->toHtml([textEdit](const QString &result){ textEdit->setPlainText(result); });
      page->toPlainText([textEdit](const QString &result){ textEdit->setPlainText(result); });

Qt WebEngine (with a functor template wrapping a member function)

      template<typename Arg, typename R, typename C>
      struct InvokeWrapper {
          R *receiver;
          void (C::*memberFun)(Arg);
          void operator()(Arg result) {
              (receiver->*memberFun)(result);
          }
      };

      template<typename Arg, typename R, typename C>
      InvokeWrapper<Arg, R, C> invoke(R *receiver, void (C::*memberFun)(Arg))
      {
          InvokeWrapper<Arg, R, C> wrapper = {receiver, memberFun};
          return wrapper;
      }

      QWebEnginePage *page = new QWebEnginePage;
      QTextEdit *textEdit = new QTextEdit;
      // *textEdit must remain valid until the functor is called.
      page->toHtml(invoke(textEdit, &QTextEdit::setPlainText));
      page->toPlainText(invoke(textEdit, &QTextEdit::setPlainText));

Qt WebEngine (with a regular functor)

      struct SetPlainTextFunctor {
          QTextEdit *textEdit;
          SetPlainTextFunctor(QTextEdit *textEdit) : textEdit(textEdit) { }
          void operator()(const QString &result) {
              textEdit->setPlainText(result);
          }
      };

      QWebEnginePage *page = new QWebEnginePage;
      QTextEdit *textEdit = new QTextEdit;
      // *textEdit must remain valid until the functor is called.
      page->toHtml(SetPlainTextFunctor(textEdit));
      page->toPlainText(SetPlainTextFunctor(textEdit));

## Qt WebEngine Does Not Interact with QNetworkAccessManager

Some classes of Qt Network such as QAuthenticator were reused for their interface but, unlike Qt WebKit, Qt WebEngine has its own HTTP implementation and cannot go through a QNetworkAccessManager.

The signals and methods of QNetworkAccessManager that are still supported were moved to the QWebEnginePage class.

Qt WebKit

      QNetworkAccessManager qnam;
      QWebPage page;
      page.setNetworkAccessManager(&qnam);
      connect(&qnam, SIGNAL(authenticationRequired(QNetworkReply*,QAuthenticator*)), this, SLOT(authenticate(QNetworkReply*,QAuthenticator*)));

Qt WebEngine

      QWebEnginePage page;
      connect(&page, SIGNAL(authenticationRequired(QNetworkReply*,QAuthenticator*)), this, SLOT(authenticate(QNetworkReply*,QAuthenticator*)));

Note: In Qt WebEngine, the QAuthenticator must be explicitly set to null to cancel authentication:

      *authenticator = QAuthenticator();

Omitting the `QNetworkAccessManager` also affects the way in which certificates are managed. For more information, see Managing Certificates.

## Notes About Individual Methods

### evaluateJavaScript

QWebFrame::evaluateJavaScript was moved and renamed as QWebEnginePage::runJavaScript. It is currently only possible to run JavaScript on the main frame of a page and the result is returned asynchronously to the provided functor.

Qt WebKit

      QWebPage *page = new QWebPage;
      qDebug() << page->mainFrame()->evaluateJavaScript("'Java' + 'Script'");

Qt WebEngine (with lambda expressions in C++11)

      QWebEnginePage *page = new QWebEnginePage;
      page->runJavaScript("'Java' + 'Script'", [](const QVariant &result){ qDebug() << result; });

### setHtml and setContent

QWebEnginePage::setHtml and QWebEnginePage::setContent perform asynchronously the same way as a normal HTTP load would, unlike their QWebPage counterparts.

### setContentEditable

QWebPage::setContentEditable has no equivalent since any document element can be made editable through the contentEditable attribute in the latest HTML standard. Therefore, QWebEnginePage::runJavaScript is all that is needed.

Qt WebKit

      QWebPage page;
      page.setContentEditable(true);

Qt WebEngine

      QWebEnginePage page;
      page.runJavaScript("document.documentElement.contentEditable = true");

## Unavailable Qt WebKit API

The Qt WebKit classes and methods in this list will not be available in Qt WebEngine.

QGraphicsWebView | Qt WebEngine is designed for being used with hardware acceleration. Because we could not support a web view class in a QGraphicsView unless it would be attached to a QGLWidget viewport, this feature is out of scope.
QWebElement | Qt WebEngine uses a multi-process architecture and this means that any access to the internal structure of the page has to be done asynchronously, any query result must be returned through callbacks. The QWebElement API was designed for synchronous access and this would require a complete redesign.
QWebDatabase | The Web SQL Database feature that this API was wrapping in Qt WebKit was dropped from the HTML5 standard.
QWebPluginDatabase, QWebPluginFactory, QWebPluginInfo, QWebPage::setPalette, QWebView::setRenderHints | Qt WebEngine renders web pages using Skia and is not using QPainter or Qt for this purpose. The HTML5 standard also now offers much better alternatives that were not available when native controls plugins were introduced in Qt WebKit.
QWebHistoryInterface | Visited links are persisted automatically by Qt WebEngine.
QWebPage::setContentEditable | In the latest HTML standard, any document element can be made editable through the `contentEditable` attribute. So `runJavaScript` is all that is needed: `page->runJavaScript("document.documentElement.contentEditable = true")`

QWebPage::setLinkDelegationPolicy

There is no way to connect a signal to run C++ code when a link is clicked. However, link clicks can be delegated to the Qt application instead of having the HTML handler engine process them by overloading the QWebEnginePage::acceptNavigationRequest() function. This is necessary when an HTML document is used as part of the user interface, and not to display external data, for example, when displaying a list of results.

Note: acceptNavigationRequest() starts the loading process and emits the loadStarted() signal before the request is accepted or rejected. Therefore, a loadFinished() signal that returns `false` is to be expected even after delegating the request.