Qt 5.13 | Qt WebEngine | WebEngine Markdown Editor Example

Qt 5.13.2 Reference Documentation

# WebEngine Markdown Editor Example

Demonstrates how to integrate a web engine in a hybrid desktop application.

Markdown Editor demonstrates how to use QWebChannel and JavaScript libraries to provide a rich text preview tool for a custom markup language.

Markdown is a lightweight markup language with a plain text formatting syntax. Some services, such as github, acknowledge the format, and render the content as rich text when viewed in a browser.

The Markdown Editor main window is split into an editor and a preview area. The editor supports the Markdown syntax and is implemented by using QPlainTextEdit. The document is rendered as rich text in the preview area, which is implemented by using QWebEngineView. To render the text, the Markdown text is converted to HTML format with the help of a JavaScript library inside the web engine. The preview is updated from the editor through QWebChannel.

#### Running the Example

To run the example from Qt Creator, open the Welcome mode and select the example from Examples. For more information, visit Building and Running an Example.

#### Exposing Document Text

Because we expose the current Markdown text to be rendered to the web engine through QWebChannel, we need to somehow make the current text available through the Qt metatype system. This is done by using a dedicated `Document` class that exposes the document text as a `Q_PROPERTY`:

      class Document : public QObject
      {
          Q_OBJECT
          Q_PROPERTY(QString text MEMBER m_text NOTIFY textChanged FINAL)
      public:
          explicit Document(QObject *parent = nullptr) : QObject(parent) {}

          void setText(const QString &text);

      signals:
          void textChanged(const QString &text);

      private:
          QString m_text;
      };

The `Document` class wraps a QString to be set on the C++ side with the `setText()` method and exposes it at runtime as a `text` property with a `textChanged` signal.

We define the `setText` method as follows:

      void Document::setText(const QString &text)
      {
          if (text == m_text)
              return;
          m_text = text;
          emit textChanged(m_text);
      }

#### Previewing Text

We implement our own `PreviewPage` class that publicly inherits from `QWebEnginePage`:

      class PreviewPage : public QWebEnginePage
      {
          Q_OBJECT
      public:
          explicit PreviewPage(QObject *parent = nullptr) : QWebEnginePage(parent) {}

      protected:
          bool acceptNavigationRequest(const QUrl &url, NavigationType type, bool isMainFrame);
      };

We reimplement the virtual `acceptNavigationRequest` method to stop the page from navigating away from the current document. Instead, we redirect external links to the system browser:

      bool PreviewPage::acceptNavigationRequest(const QUrl &url,
                                                QWebEnginePage::NavigationType /*type*/,
                                                bool /*isMainFrame*/)
      {
          // Only allow qrc:/index.html.
          if (url.scheme() == QString("qrc"))
              return true;
          QDesktopServices::openUrl(url);
          return false;
      }

#### Creating the Main Window

The `MainWindow` class inherits the QMainWindow class:

      class MainWindow : public QMainWindow
      {
          Q_OBJECT

      public:
          explicit MainWindow(QWidget *parent = nullptr);
          ~MainWindow();

          void openFile(const QString &path);

      private slots:
          void onFileNew();
          void onFileOpen();
          void onFileSave();
          void onFileSaveAs();
          void onExit();

      private:
          bool isModified() const;

          Ui::MainWindow *ui;
          QString m_filePath;
          Document m_content;
      };

The class declares private slots that match the actions in the menu, as well as the `isModified()` helper method.

The actual layout of the main window is specified in a `.ui` file. The widgets and actions are available at runtime in the `ui` member variable.

`m_filePath` holds the file path to the currently loaded document. `m_content` is an instance of the `Document` class.

The actual setup of the different objects is done in the `MainWindow` constructor:

      MainWindow::MainWindow(QWidget *parent) :
          QMainWindow(parent),
          ui(new Ui::MainWindow)
      {
          ui->setupUi(this);
          ui->editor->setFont(QFontDatabase::systemFont(QFontDatabase::FixedFont));
          ui->preview->setContextMenuPolicy(Qt::NoContextMenu);

The constructor first calls `setupUi` to construct the widgets and menu actions according to the UI file. The text editor font is set to one with a fixed character width, and the QWebEngineView widget is told not to show a context menu.

          PreviewPage *page = new PreviewPage(this);
          ui->preview->setPage(page);

Here the constructor makes sure our custom `PreviewPage` is used by the QWebEngineView instance in `ui->preview`.

          connect(ui->editor, &QPlainTextEdit::textChanged,
                  [this]() { m_content.setText(ui->editor->toPlainText()); });

          QWebChannel *channel = new QWebChannel(this);
          channel->registerObject(QStringLiteral("content"), &m_content);
          page->setWebChannel(channel);

Here the `textChanged` signal of the editor is connected to a lambda that updates the text in `m_content`. This object is then exposed to the JS side by `QWebChannel` under the name `content`.

          ui->preview->setUrl(QUrl("qrc:/index.html"));

Now we can actually load the index.html file from the resources. For more information about the file, see Creating an Index File.

          connect(ui->actionNew, &QAction::triggered, this, &MainWindow::onFileNew);
          connect(ui->actionOpen, &QAction::triggered, this, &MainWindow::onFileOpen);
          connect(ui->actionSave, &QAction::triggered, this, &MainWindow::onFileSave);
          connect(ui->actionSaveAs, &QAction::triggered, this, &MainWindow::onFileSaveAs);
          connect(ui->actionExit, &QAction::triggered, this, &MainWindow::onExit);

          connect(ui->editor->document(), &QTextDocument::modificationChanged,
                  ui->actionSave, &QAction::setEnabled);

The menu items are connected to the corresponding member slots. The Save item is activated or deactivated depending on whether the user has edited the content.

          QFile defaultTextFile(":/default.md");
          defaultTextFile.open(QIODevice::ReadOnly);
          ui->editor->setPlainText(defaultTextFile.readAll());
      }

Finally, we load a default document default.md from the resources.

#### Creating an Index File

      <!doctype html>
      <html lang="en">
      <meta charset="utf-8">
      <head>
        <link rel="stylesheet" type="text/css" href="3rdparty/markdown.css">
        <script src="3rdparty/marked.js"></script>
        <script src="qrc:/qtwebchannel/qwebchannel.js"></script>
      </head>
      <body>
        <div id="placeholder"></div>
        <script>
        'use strict';

        var placeholder = document.getElementById('placeholder');

        var updateText = function(text) {
            placeholder.innerHTML = marked(text);
        }

        new QWebChannel(qt.webChannelTransport,
          function(channel) {
            var content = channel.objects.content;
            updateText(content.text);
            content.textChanged.connect(updateText);
          }
        );
        </script>
      </body>
      </html>

In the index.html, we load a custom stylesheet and two JavaScript libraries. markdown.css is a markdown-friendly stylesheet created by Kevin Burke. marked.js is a markdown parser and compiler designed for speed written by Christopher Jeffrey and qwebchannel.js is part of the QWebChannel module.

In the `<body>` element we first define a `placeholder` element, and make it available as a JavaScript variable. We then define the `updateText` helper method that updates the content of `placeholder` with the HTML that the JavaScript method `marked()` returns.

Finally, we set up the web channel to access the `content` proxy object and make sure that `updateText()` is called whenever `content.text` changes.

#### Files and Attributions

The example bundles the following code with third-party licenses:

Marked | MIT License
Markdown.css | Apache License 2.0

Example project @ code.qt.io