Architecture

All KDE software is built using the Qt toolkit.

You will be using different components depending on whether you are developing an application or Plasma and the nature of your application. The interaction patterns of the HIG apply regardless of the components you use.

KDE Applications

Qt offers two ways of defining the application UI. Which one to choose depends on the nature of your application. The application logic is written in C++ (or other supported languages) regardless of that.

Qt Widgets

Qt Widgets is the traditional way of writing Qt applications. It is best suited for traditional desktop applications with complex UI, e.g. KDevelop.

QML/QtQuick/Kirigami

QML/QtQuick is the modern way of developing Qt applications. It features a declarative approach to writing touch and mobile friendly UIs with fluent gestures. It is best suited for mobile and convergent applications.

Kirigami builds on top of QtQuick and helps you write convergent applications. It features controls that adapt their presentation according to the device's form factor.

Discover, a convergent application built using Kirigami

Plasma

Plasma is built out of widgets (also called Plasmoids), allowing you to move, mix, add, and remove just about everything to perfect your personal workflow. Those are built using Plasma Components 3 , which are based on Qt Quick Controls 2.

Plasma desktop and mobile.

Common Components

The KDE HIG defines a set of common components which are independent of any device type .

  • Workspace: The top-level container of the whole user interface. Often called “desktop”, “home screen”, or “shell”, it shows the wallpaper and allows users to add widgets, app launchers, files or folders.
  • Application Launcher: Provides an overview of installed applications and allows the user to launch one of them.
  • Application Shortcuts: Provides quick access to frequently-used applications.
  • Active Application Overview: Provides an overview of the active applications that are directly used by the user.
  • Workspace Tools: Provides quick access to functionality integrated into the workspace that is both highly visible to the user and frequently changed, like enabling/disabling WiFi and Bluetooth, or whether or not to show notifications.
  • Application-Workspace Interaction: Displays information about each application’s windows, and provides ways to move or close them and change how they run within the workspace.
  • Application: The top-level container of a single application.
  • Application Tools: Provides access to an application's commonly-used functionality in an always-accessible toolbar or menubar. These tools should not change depending on what the application is displaying.
  • Application Content: The actual content of an application. This depends on the application itself, but conformance to the KDE HIG should make it easier to allow convergence for this component. This part of the application can also contain contextually-appropriate tools that operate directly on the active or selected content.

Theme

There are three different kinds of themes influencing the the look-and-feel of KDE applications and the Plasma workspace.

  • Workspace
  • Application
  • Window decoration

The default for all there of them is Breeze.

Overview of breeze controls