Porting from Qt WebEngine to Qt WebKit

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

1 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. However, Qt WebEngine is also heavier than Qt WebKit and does not provide direct access to the network stack and the HTML document through C++ APIs.

In addition, Qt WebEngine is not just a port of the core HTML/CSS rendering engine, it is the entire Chromium platform, becoming it non-Free software'.[1] This opens users up to privacy and freedom issues.

Developers are instead encouraged to use QT-webkit-ng which is the free continuation of Qt WebKit with security fixes and updates.

1.1 Class Names

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

Qt WebEngine

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

QWebEngineHistory
QWebEngineHistoryItem
QWebEnginePage
QWebEngineView

Qt WebKit

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

QWebHistory
QWebHistoryItem
QWebPage
QWebView

2 Qt Module Name

2.1 In qmake Project Files

Qt WebEngine

QT += webenginewidgets

Qt WebKit

QT += webkitwidgets

3 Including the Module in Source Files

Qt WebEngine

#include <QtWebEngineWidgets/QtWebEngineWidgets>

Qt WebKit

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

4 QWebFrame Has Been Merged into QWebEnginePage

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

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.

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.

Qt WebEngine

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

Qt WebKit

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

5 Some Methods Now Return Their Result Asynchronously

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 are provided to handle the results when they become available.

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 WebKit

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

6 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 WebEngine

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

In Qt WebEngine, the QAuthenticator is explicitly set to null to cancel authentication:

*authenticator = QAuthenticator();

Qt WebKit

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

7 Notes About Individual Methods

7.1 evaluateJavaScript

QWebFrame::evaluateJavaScript was moved and renamed as QWebEnginePage::runJavaScript.

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

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

Qt WebKit

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

7.2 setHtml and setContent

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

7.3 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 WebEngine

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

Qt WebKit

QWebPage page;
page.setContentEditable(true);

8 Unavailable Qt WebKit API

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

Qt WebKit 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.

9 Acknowledgement

This wiki article is based on Qt Documentation.