Qt Fundamentals

Qt is an open source, multiplatform toolkit for creating graphical user interfaces (GUIs). It is a complete development framework that offers utilities for building applications, and has extensions for networking, Bluetooth, charts, 3D rendering, and navigation, among others.

To develop a GUI, we start with the core building blocks—graphical elements known as widgets—and arrange them using layouts, themselves contained within other widgets and ultimately application windows. Then, we interconnect these widgets using special functions and methods (called slots) that are triggered by receiving connected signals. In turn, actions are the common building blocks of toolbars and menu items representing a specific option the user can select.

Basic Qt Components

According to the official Qt documentation: “a component is a reusable building block for a UI”. Qt has some preset components that can include simple representations (shapes, text or images) or complex user interface controls (sliders or dropdowns). Each type of Qt component is a class starting with the letter Q, followed by the name of the component (such as QSlider or QCombobox). Much of Spyder plugin development consists of extending the functionality of its Qt-based graphical interface.

Common Qt components are summarized below in the following sections, and linked to their full reference in the Qt documentation.

Widgets

Widgets are the main building blocks for creating a UI in Qt. They can display information, receive user input (mouse, keyboard, and other events from the window system), and provide a container for grouping other widgets.

QWidget is the fundamental class in Qt that provides the capability to handle user input events and renders its representation on the screen. All UI elements provided by Qt are subclasses of QWidget or are used in connection with a QWidget subclass. You can define your own custom widget class by subclassing QWidget, and adding or reimplementing the methods that you need.

Windows and MainWindows

A Qt window is a widget that is not embedded in a parent widget. The MainWindow class provides a main application window with its own layout in which you can include toolbars (QToolBar), menus (QMenuBar), status bars (QStatusBar), and dockable widgets (QDockWidget).

Layouts

Qt’s layout system provides a way for a widget to arrange and automatically distribute child widgets, taking advantage of the available space. It can automatically position and resize widgets when the space available for them changes, ensuring that the UI remains usable. You can use the built-in layout managers to organize your widgets:

• QHBoxLayout

• QVBoxLayout

• QGridLayout

• QFormLayoutClass, and

• QStackedLayout

Actions, Menus & Toolbars

User interfaces of desktop applications often rely on menus and toolbars to display groups of commands and options to the user, for which Qt provides the QToolBar and QMenu classes. Menus present a list of items to the user when clicked and may be either global or context-specific. Toolbars display commonly-used controls as a ribbon of buttons (and often other interactive elements). Since the same individual items can be included in multiple places in the UI, items only need to be created once as a QAction that can be added to any menu or toolbar. Their state will then be automatically synchronized: for example, if the user toggles the “Bold” toolbar button, the “Bold” menu item will also be (un)checked.

Dialogs

A dialog is a new, smaller window for temporary interactions and messages, such as a configuration panel or a warning notification. The QDialog class is used to create dialogs, which can be set as either modal or modeless. A modal dialog blocks interaction with other windows in the same application until it is dismissed, useful for an error message or requesting key information to complete a requested action. A modeless dialog behaves as an independent window without blocking the user from doing anything else, such as for a find and replace feature where a user needs to interact with other windows. Modal dialogs should only be used where truly necessary, as they can harm user productivity and create frustration if a user wants to interact with the rest of the application before dismissing the dialog.

Qt provides some special dialogs for common use-cases, such as file open/save, font selection, error messages, color choosing, printing, among others.

Signals & Slots

GUI programming requires a way to notify and communicate with other objects in the application, which Qt provides via signals and slots.

A Signal is a notification emitted by a widget (as well as non-graphical Qt objects) when a particular event occurs. For example, this might include pressing a button, changing text in an input box, or moving a slider. Many signals are initiated by an user action, but they can also be issued for other events, like completing a process. Qt’s widgets have different predetermined signals, and you can also add your own.

A Slot is a function or method that is invoked in response to a specific signal. Functions and methods can be used as slots by connecting a signal to them. If a signal sends data, the receiving callable will receive it as arguments. You can add your own functions as slots to manage particular signals (built-in or custom) that you are interested in.