Kate plugin tutorial

Learn how to write a Kate plugin

The plugin we will write will basically be a Markdown previewer. It will do something like

  • Once a file opens, check if the file is a Markdown file
  • If it is, then create and show a preview for it in right the sidebar

We'll call this plugin Markdown Previewer.

This tutorial assumes that you have a basic understanding of C++ and Qt concepts and that you have development versions of Qt and KDE Framework libraries installed, as well as extra-cmake-modules.

Initial directory structure:

myplugin
├─ CMakeLists.txt  # the build file, this is needed to build the plugin
├─ plugin.cpp      # the actual plugin code
├─ plugin.h
└─ plugin.json     # plugin type, description and name

Let's start by writing our cmake file:

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cmake_minimum_required(VERSION 3.16)
project(markdownpreview VERSION 1.0)

find_package(ECM ${KF5_DEP_VERSION} QUIET REQUIRED NO_MODULE)

list(APPEND CMAKE_MODULE_PATH ${ECM_MODULE_PATH})

include(KDEInstallDirs)
include(KDECMakeSettings)

find_package(Qt${QT_MAJOR_VERSION}Widgets CONFIG REQUIRED)

set(KF5_DEP_VERSION "5.90")
find_package(KF5 ${KF5_DEP_VERSION}
    REQUIRED COMPONENTS
        CoreAddons # Core addons on top of QtCore
        I18n # For localization
        TextEditor # The editor component
)

# This line defines the actual target
kcoreaddons_add_plugin(markdownpreview # your plugin name here
    INSTALL_NAMESPACE "ktexteditor")

target_sources(
  markdownpreview
  PRIVATE
  plugin.h
  plugin.cpp
)

# This makes the plugin translatable
target_compile_definitions(markdownpreview PRIVATE TRANSLATION_DOMAIN="markdownpreview")

target_link_libraries(markdownpreview
    PRIVATE
    KF5::CoreAddons KF5::I18n KF5::TextEditor
)

Ok, CMake is done. Let's write the plugin.json file:

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{
    "KPlugin": {
        "Description": "Displays preview for markdown files",
        "Description[ca]": "Mostra una vista prèvia dels fitxers Markdown",
        "Description[cs]": "Zobrazí náhledy souborů Markdown",
        "Description[eo]": "Montras antaŭrigardon por markdown dosieroj",
        "Description[es]": "Muestra una vista previa de los archivos Markdown",
        "Description[fi]": "Näyttää esikatselun Markdown-tiedostoista.",
        "Description[fr]": "Affiche un aperçu de fichiers « Markdown »",
        "Description[gl]": "Amosa unha vista previa de ficheiros Markdown.",
        "Description[it]": "Visualizza un'anteprima dei file markdown",
        "Description[ja]": "markdown ファイルのプレビューを表示する",
        "Description[nl]": "Toont voorbeeld van markdown-bestanden",
        "Description[pt]": "Apresenta uma antevisão dos ficheiros Markdown",
        "Description[sl]": "Prikaže predogled datotek markdown",
        "Description[sv]": "Förhandsvisar Markdown-filer",
        "Description[tr]": "Markdown dosyaları için önizlemeler görüntüler",
        "Description[uk]": "Показує попередній перегляд файлів Markdown",
        "Description[x-test]": "xxDisplays preview for markdown filesxx",
        "Description[zh_TW]": "顯示 Markdown 檔案的預覽",
        "Name": "Markdown Previewer",
        "Name[ca]": "Previsualitzador del Markdown",
        "Name[cs]": "Náhledy Markdown",
        "Name[eo]": "Markdown-Antaŭrigardilo",
        "Name[es]": "Vista previa de Markdown",
        "Name[fi]": "Markdown-esikatselu",
        "Name[fr]": "Aperçu pour langage « Markdown »",
        "Name[gl]": "Xerador de vista previa de Markdown",
        "Name[it]": "Strumento di anteprima Markdown",
        "Name[ja]": "Markdown ビューアー",
        "Name[nl]": "Markdown vooraf bekijken",
        "Name[pt]": "Antevisão de Markdown",
        "Name[sl]": "Predogledovalnik Markdown",
        "Name[sv]": "Markdown förhandsvisning",
        "Name[tr]": "Markdown Önizleyicisi",
        "Name[uk]": "Засіб перегляду Markdown",
        "Name[x-test]": "xxMarkdown Previewerxx",
        "Name[zh_TW]": "Markdown 預覽器"
    }
}

Finally, let's start writing the actual code for the plugin.

Before I start, let's go through a couple of basic things first. Every Kate plugin consists of at least two classes

  • Plugin class
  • Plugin View class

The plugin class creates a global instance of the plugin only once. The plugin view class will be created once for each new MainWindow. Your UI code will always go into the plugin view class. The plugin class usually stores stuff that will be same across multiple plugin views for e.g., the configuration.

Below is the code for both of the classes. At this point, the classes are empty and don't do anything at all

plugin.h

#pragma once

#include <KTextEditor/Document>
#include <KTextEditor/MainWindow>
#include <KTextEditor/Plugin>
#include <KTextEditor/View>
#include <KXMLGUIClient>
#include <QTextBrowser>

class MarkdownPreviewPlugin : public KTextEditor::Plugin
{
    Q_OBJECT
public:
    explicit MarkdownPreviewPlugin(QObject *parent, const QList<QVariant> & = QList<QVariant>())
        : KTextEditor::Plugin(parent)
    {
    }

    QObject *createView(KTextEditor::MainWindow *mainWindow) override;
};

class MarkdownPreviewPluginView : public QObject, public KXMLGUIClient
{
    Q_OBJECT
public:
    explicit MarkdownPreviewPluginView(MarkdownPreviewPlugin *plugin, KTextEditor::MainWindow *mainwindow);

private:
    KTextEditor::MainWindow *m_mainWindow = nullptr;
};

plugin.cpp

#include "plugin.h"

#include <KPluginFactory>
#include <QIcon>
#include <QLayout>
#include <KLocalizedString>

K_PLUGIN_FACTORY_WITH_JSON(MarkdownPreviewPluginFactory, "plugin.json", registerPlugin<MarkdownPreviewPlugin>();)

QObject *MarkdownPreviewPlugin::createView(KTextEditor::MainWindow *mainWindow)
{
    return new MarkdownPreviewPluginView(this, mainWindow);
}

MarkdownPreviewPluginView::MarkdownPreviewPluginView(MarkdownPreviewPlugin *plugin, KTextEditor::MainWindow *mainwindow)
    : m_mainWindow(mainwindow)
{
}

#include "plugin.moc"

With this, we should now be able to compile and install it. For testing purposes, we will be installing it to our home directory.

cmake -B build/ -D CMAKE_INSTALL_PREFIX=$HOME/kde/usr
cmake --build build/
cmake --install build/

You should now see the installed plugin in ~/kde/usr/lib64/plugins/ktexteditor/markdowpreview.so.

To test the local plugin with our system-installed Kate, we can use the generated prefix.sh inside our build folder:

source build/prefix.sh
kate

With Kate now running, go to Settings, Configure Kate..., Plugins, and verify that the "Markdown Previewer" plugin is present.

Next we will create a toolview in the right sidebar. This toolview will be the GUI component that appears on the right side of Kate that, when clicked, opens the Markdown preview.

First add two new member variables to the MarkdownPreviewPluginView class in plugin.h:

    // The top level toolview widget
    std::unique_ptr<QWidget> m_toolview;

    // The widget which will show the actual preview
    QTextBrowser *m_previewer = nullptr;

Now let's create the toolview by adding the following to the constructor in plugin.cpp:

MarkdownPreviewPluginView::MarkdownPreviewPluginView(MarkdownPreviewPlugin *plugin, KTextEditor::MainWindow *mainwindow)
    : m_mainWindow(mainwindow)
{
    m_toolview.reset(
        m_mainWindow->createToolView(plugin,                        // pointer to plugin
                                      "markdownpreview",            // just an identifier for the toolview
                                      KTextEditor::MainWindow::Right, // we want to create a toolview on the right side
                                      QIcon::fromTheme("preview"), // icon,
                                      i18n("Markdown Preview"))); // User visible name of the toolview, i18n means it will be available for translation

    m_previewer = new QTextBrowser(m_toolview.get());
    // Add the preview widget to our toolview
    m_toolview->layout()->addWidget(m_previewer);
}

Before proceeding further, make sure the code compiles. Then install the plugin, enable it in Kate and verify that the toolview is visible in the right sidebar. If the toolview isn't visible, make sure the plugin is enabled and recheck your code.

Now to the actual previewing. To be able to tell when the active document changes the MainWindow class emits a viewChanged() signal. We will connect to this signal and then check if the new document is of Markdown type. If it is, we will load the document's text into the preview widget which will take care of the rest.

So, in the constructor of the MarkdownPreviewPluginView class add the following:

    // Connect the view changed signal to our slot
    connect(m_mainWindow, &KTextEditor::MainWindow::viewChanged, this, &MarkdownPreviewPluginView::onViewChanged);

Next, we will define the onViewChanged() function. Let's add its declaration in MarkdownPreviewPluginView in plugin.h:

void onViewChanged(KTextEditor::View *v);

And definition in the plugin.cpp file:

void MarkdownPreviewPluginView::onViewChanged(KTextEditor::View *v)
{
    if (!v || !v->document()) {
        return;
    }

    if (v->document()->highlightingMode().toLower() == "markdown") {
        m_previewer->setMarkdown(v->document()->text());
    }
}

Compile and install it. You should now be able to see the preview of markdown files in the toolview we created.

Full code

CMakeLists.txt
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cmake_minimum_required(VERSION 3.16)
project(markdownpreview VERSION 1.0)

find_package(ECM ${KF5_DEP_VERSION} QUIET REQUIRED NO_MODULE)

list(APPEND CMAKE_MODULE_PATH ${ECM_MODULE_PATH})

include(KDEInstallDirs)
include(KDECMakeSettings)

find_package(Qt${QT_MAJOR_VERSION}Widgets CONFIG REQUIRED)

set(KF5_DEP_VERSION "5.90")
find_package(KF5 ${KF5_DEP_VERSION}
    REQUIRED COMPONENTS
        CoreAddons # Core addons on top of QtCore
        I18n # For localization
        TextEditor # The editor component
)

# This line defines the actual target
kcoreaddons_add_plugin(markdownpreview # your plugin name here
    INSTALL_NAMESPACE "ktexteditor")

target_sources(
  markdownpreview
  PRIVATE
  plugin.h
  plugin.cpp
)

# This makes the plugin translatable
target_compile_definitions(markdownpreview PRIVATE TRANSLATION_DOMAIN="markdownpreview")

target_link_libraries(markdownpreview
    PRIVATE
    KF5::CoreAddons KF5::I18n KF5::TextEditor
)

plugin.json
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{
    "KPlugin": {
        "Description": "Displays preview for markdown files",
        "Description[ca]": "Mostra una vista prèvia dels fitxers Markdown",
        "Description[cs]": "Zobrazí náhledy souborů Markdown",
        "Description[eo]": "Montras antaŭrigardon por markdown dosieroj",
        "Description[es]": "Muestra una vista previa de los archivos Markdown",
        "Description[fi]": "Näyttää esikatselun Markdown-tiedostoista.",
        "Description[fr]": "Affiche un aperçu de fichiers « Markdown »",
        "Description[gl]": "Amosa unha vista previa de ficheiros Markdown.",
        "Description[it]": "Visualizza un'anteprima dei file markdown",
        "Description[ja]": "markdown ファイルのプレビューを表示する",
        "Description[nl]": "Toont voorbeeld van markdown-bestanden",
        "Description[pt]": "Apresenta uma antevisão dos ficheiros Markdown",
        "Description[sl]": "Prikaže predogled datotek markdown",
        "Description[sv]": "Förhandsvisar Markdown-filer",
        "Description[tr]": "Markdown dosyaları için önizlemeler görüntüler",
        "Description[uk]": "Показує попередній перегляд файлів Markdown",
        "Description[x-test]": "xxDisplays preview for markdown filesxx",
        "Description[zh_TW]": "顯示 Markdown 檔案的預覽",
        "Name": "Markdown Previewer",
        "Name[ca]": "Previsualitzador del Markdown",
        "Name[cs]": "Náhledy Markdown",
        "Name[eo]": "Markdown-Antaŭrigardilo",
        "Name[es]": "Vista previa de Markdown",
        "Name[fi]": "Markdown-esikatselu",
        "Name[fr]": "Aperçu pour langage « Markdown »",
        "Name[gl]": "Xerador de vista previa de Markdown",
        "Name[it]": "Strumento di anteprima Markdown",
        "Name[ja]": "Markdown ビューアー",
        "Name[nl]": "Markdown vooraf bekijken",
        "Name[pt]": "Antevisão de Markdown",
        "Name[sl]": "Predogledovalnik Markdown",
        "Name[sv]": "Markdown förhandsvisning",
        "Name[tr]": "Markdown Önizleyicisi",
        "Name[uk]": "Засіб перегляду Markdown",
        "Name[x-test]": "xxMarkdown Previewerxx",
        "Name[zh_TW]": "Markdown 預覽器"
    }
}

plugin.h
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#pragma once

#include <KTextEditor/Document>
#include <KTextEditor/MainWindow>
#include <KTextEditor/Plugin>
#include <KTextEditor/View>
#include <KXMLGUIClient>

#include <QTextBrowser>

// Forward declare
class MarkdownPreviewPluginView;

class MarkdownPreviewPlugin : public KTextEditor::Plugin
{
    Q_OBJECT
public:
    explicit MarkdownPreviewPlugin(QObject *parent, const QList<QVariant> & = QList<QVariant>())
        : KTextEditor::Plugin(parent)
    {
    }

    QObject *createView(KTextEditor::MainWindow *mainWindow) override;
};

class MarkdownPreviewPluginView : public QObject, public KXMLGUIClient
{
    Q_OBJECT
public:
    explicit MarkdownPreviewPluginView(MarkdownPreviewPlugin *plugin, KTextEditor::MainWindow *mainwindow);

    void onViewChanged(KTextEditor::View *v);

private:
    KTextEditor::MainWindow *m_mainWindow = nullptr;
    std::unique_ptr<QWidget> m_toolview;
    QTextBrowser *m_previewer = nullptr;
};

plugin.cpp
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#include "plugin.h"

#include <KLocalizedString>
#include <KPluginFactory>
#include <QHBoxLayout>
#include <QIcon>

K_PLUGIN_FACTORY_WITH_JSON(MarkdownPreviewPluginFactory, "plugin.json", registerPlugin<MarkdownPreviewPlugin>();)

QObject *MarkdownPreviewPlugin::createView(KTextEditor::MainWindow *mainWindow)
{
    return new MarkdownPreviewPluginView(this, mainWindow);
}

MarkdownPreviewPluginView::MarkdownPreviewPluginView(MarkdownPreviewPlugin *plugin, KTextEditor::MainWindow *mainwindow)
    : m_mainWindow(mainwindow)
{
    m_toolview.reset(m_mainWindow->createToolView(plugin, // pointer to plugin
                                                  "markdownpreview", // just an identifier
                                                  KTextEditor::MainWindow::Right, // we want to create a toolview on the right side
                                                  QIcon::fromTheme("preview"),
                                                  i18n("Markdown Preview"))); // Name of the toolview

    m_previewer = new QTextBrowser(m_toolview.get());
    m_toolview->layout()->addWidget(m_previewer);

    // Connect the view changed signal to our slot
    connect(m_mainWindow, &KTextEditor::MainWindow::viewChanged, this, &MarkdownPreviewPluginView::onViewChanged);
}

void MarkdownPreviewPluginView::onViewChanged(KTextEditor::View *v)
{
    if (!v || !v->document()) {
        return;
    }

    if (v->document()->highlightingMode().toLower() == "markdown") {
        m_previewer->setMarkdown(v->document()->text());
    }
}

#include "plugin.moc"

Since this is a very basic tutorial and likely doesn't explain a lot of things or shows other APIs that we have, the following resources may be of more help: