Building a Shopping List App with Zig and Qt: A Deep Dive into High-Performance GUI Development

Developing graphical user interfaces (GUIs) often involves choosing between convenience and performance. For developers keen on performance, languages like Zig offer exciting possibilities, especially when paired with powerful frameworks like Qt. This article explores the journey of building a simple yet functional shopping list application using Zig with the libqt6zig bindings, demonstrating how a low-level language can tackle desktop application development with surprising ease.

Why Zig and Qt?

The author’s enthusiasm for Zig and the robust Qt GUI framework naturally led to an exploration of their synergy. While many Qt bindings lean on QML, libqt6zig offers direct access to the Qt C++ API, promising higher performance and a more direct connection to the framework’s core. A shopping list application served as the perfect testbed, simple enough for quick development but complex enough to cover essential GUI concepts like user input, data display, event handling, and dynamic data manipulation.

Understanding libqt6zig

libqt6zig stands out as a direct binding to the Qt C++ API, a choice that appeals to developers prioritizing performance and fine-grained control. The experience of building the shopping list app revealed the bindings to be remarkably well-implemented and practical. Despite the inherent complexities of direct bindings (and a few expected segfaults that added a touch of nostalgia), the author was able to create a fully functional application quickly and with minimal friction. The maintainer, rcalixte, was also noted for his supportive and enthusiastic approach, further enhancing the positive experience.

Setting Up Your Zig Project

Integrating libqt6zig into a Zig project is a streamlined process. After installing necessary dependencies, initializing a new Zig project with zig init, you can easily add the bindings using zig fetch --save git+https://github.com/rcalixte/libqt6zig`. This command not only adds the dependency to yourbuild.zig.zonfile, locking it to a specific commit hash for consistent builds, but also caches the repository contents locally, eliminating the need for constant refetching. The final step involves linking the library within yourbuild.zigfile, a well-documented process in thelibqt6zig` README.

Crafting the User Interface

The foundation of any GUI application is its window and layout. With libqt6zig, opening a window involves creating a QWidget and instructing Qt to display it. The application window was configured with a title and a minimum size.

Qt’s layout system is crucial for arranging widgets intuitively. A QVBoxLayout was used for the main vertical arrangement, establishing spacing and margins for visual appeal. Within this, a QHBoxLayout organized the input row, comprising a QLineEdit for item entry and a QPushButton for adding items. Explicit calls for each widget and layout operation provided clear control, a welcome alternative to implicit behaviors often found in other frameworks.

Further down the layout, a QLabel introduced the “Items:” section, followed by a QListWidget to display the shopping list items. At the very bottom, another QHBoxLayout housed control buttons: “Clear All” and “Clear Bought” QPushButtons, each with a fixed width for a uniform look. This hierarchical structure ensured a clean and organized UI.

The Context System: Bridging Zig and Qt

One of the more innovative aspects of this application is its custom context system. Because Qt’s signals are C-style callbacks lacking built-in state, a global hash map (AppCtxMap) was employed to associate each widget pointer with its corresponding Zig state Context struct. This Context held references to key widgets, enabling callbacks to retrieve and manipulate the application’s state. While this approach leveraged opaque pointers, sacrificing some compile-time type safety for convenience, it proved effective for the experimental nature of the project. Developers building more serious applications would be advised to seek more type-safe alternatives.

Bringing the Application to Life: Logic and Interactivity

With the UI and context system in place, the next phase involved connecting signals to slots, making the application interactive.

• Adding Items: When the “Add” button is clicked (or Enter is pressed in the input field), the on_add_clicked function retrieves text from the QLineEdit, creates a new QListWidgetItem if the input is not empty, adds it to the QListWidget, and clears the input field. The add_item helper function centralizes item creation, setting Qt::ItemIsUserCheckable to display a checkbox and defaulting it to unchecked.
• Clearing Items: The “Clear All” button’s on_clear_all function simply clears the entire QListWidget. For “Clear Bought,” the on_clear_bought function iterates backward through the list, removing items marked as bought. Iterating backward prevents issues with shifting indices during removal and optimizes the process by stopping once an unbought item is encountered (since bought items are sorted to the bottom).
• Marking Items as Bought: The on_item_changed function handles checkbox toggles. When an item’s check state changes, it applies or removes a strikethrough effect to the item’s text via font manipulation and then reorders the item. Checked items are moved to the bottom of the list, while unchecked items are inserted at the top, ensuring unbought items remain visible. This reordering is achieved by temporarily taking the item out of the list and re-inserting it at the desired position.
• Keyboard Input: For user convenience, pressing the Enter key in the input field also triggers the add item functionality, leveraging the QLineEdit‘s OnReturnPressed signal.

Styling with Qt Style Sheets (QSS)

To enhance the application’s visual appeal, Qt Style Sheets (QSS) were utilized. QSS provides a CSS-like syntax for styling widgets, and libqt6zig exposes this feature directly. A comprehensive style sheet was defined to customize backgrounds, text colors, borders, padding, and button states (hover/pressed) for various widgets like QWidget, QListWidget, QLineEdit, QPushButton, and QLabel. This style sheet could be applied directly to the main window or loaded from an external .qss file, offering flexibility for design iterations without recompilation.

Concluding the Application Journey

The final steps involved displaying the main window and initiating Qt’s event loop with qapplication.Exec(). This brought the fully functional shopping list application to life, a testament to Zig’s capabilities and the robust libqt6zig bindings. Users could now add items, mark them as bought (with visual feedback and reordering), and clear lists with dedicated buttons.

The author’s overall reflection highlighted the eye-opening experience of working with libqt6zig. The bindings faithfully expose Qt’s C++ API, offering a high degree of control over GUI elements. The explicit nature of widget and layout operations, while initially daunting, proved empowering, allowing direct interaction with the system’s core.

However, the experience also brought to light certain limitations, such as the reliance on opaque pointers, which compromised type safety, and the absence of high-level Zig features like generics or compile-time reflection for more abstract patterns. Despite these trade-offs, the library’s robustness, consistency, and the dedication of its single maintainer make it a compelling choice for Zig developers venturing into GUI development.

In conclusion, the journey of building a shopping list application with Zig and libqt6zig proved that Zig is a viable and powerful language for cross-platform GUI development. It offers a path to creating lightweight, high-performance desktop applications with a rewarding level of control and explicitness. The library, still evolving, holds immense potential for the growing Zig ecosystem, promising exciting future developments in GUI programming.

Explore Further

For those inspired to delve into Zig GUI development with Qt, here are some valuable resources:

• libqt6zig GitHub Repository: The primary source for the bindings.
• rcalixte GitHub Profile: Learn more about the dedicated maintainer.
• Qt Documentation: Official guides for widgets, layouts, and styling.
• Zig Programming Language: Official site to learn more about Zig.
• QSS Styling Guide: Comprehensive guide to Qt’s CSS-like styling.