WebEngine Markdown 编辑器范例

演示如何在混合桌面应用程序中集成 Web 引擎。

Markdown 编辑器 演示如何使用 QWebChannel 和 JavaScript 库以提供用于自定义标记语言的富文本预览工具。

Markdown 是采用纯文本格式句法的轻量标记语言。某些服务，譬如 github ，承认格式，并将内容渲染成富文本当在浏览器中查看时。

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 .

运行范例

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曝光文档文本

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 信号。

定义 setText method as follows:

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

预览文本

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);
};

重实现虚拟 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;
}

创建主窗口

The MainWindow 类继承 QMainWindow 类：

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 类。

The actual setup of the different objects is done in the MainWindow 构造函数：

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);

构造函数首先调用 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 创建索引文件 .

    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.

创建索引文件

<!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>

在 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 属于 QWebChannel 模块。

在 <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 改变。

文件和归属

范例绑定采用第三方许可的以下代码：

Marked	MIT 许可
Markdown.css	Apache 许可 2.0

范例工程 @ code.qt.io