Week 8 recap GSoC 2025 - Adding Dragging Interaction

Intro

This week I focused on making the Selection Action Bar draggable directly on the canvas. This would let the user reposition the toolbar that is best for their workflow.

Obstacles to Implementation

Up to this point adding elements on Krita using Qt was straightforward however making these elements interact and respond to mouse click events needed some deeper diving. The goal of this part of the Selection Action Bar was to allow users to click and drag the tool to reposition it anywhere within the canvas. In order to implement this I asked myself some questions: Where does Qt handle click events? How can the UI elements dynamically update their position?

Event Filters and Mouse Events

Documentation Links https://doc.qt.io/qt-6/eventsandfilters.html
https://doc.qt.io/qt-5.15/events.html
https://doc.qt.io/qt-5/qobject.html#installEventFilter

My research led me to Qt’s event system. One of the key concepts I learned about was event filters. Event filters are able to intercept events that are sent to another object. In our case, we can intercept the click events that are sent to the canvas and process that event to update the positions of our floating toolbar.

Since canvas was passed in through the KisSelectionAssistantsDecoration class, we can install an event filter on the canvas object, specifically the canvasWidget to intercept the mouse events.

// install event filter on canvasWidget, so KisSelectionAssistantsDecoration class (aka 'this') can look at event first
canvasWidget->installEventFilter(this);

After learning about event filters, I needed to learn how to handle the mouse events. By distinguishing user inputs of MouseButtonPress, MouseMove, and MouseButtonRelease, I was able to update the position of the floating bar. To keep things simple, imagine a rectangle on a 2D grid. Between the top left point of the rectangle and a point we click inside the rectangle is called an offset. This offset is to make sure that when we click and drag from within the rectangle, we are updating the position of the rectangle. Get offset = mouse position (click) - rectangle position
Update rectangle position = mouse position (click + drag) - offset
Mouse button release = stop updating rectangle

Working concept

With this major change, the Selection Action Bar is starting to feel like a full feature. Currently, we are able to activate selection actions, move the toolbar to a desirable position on canvas, and toggle it on and off via Krita settings.

Conclusion

This week really pushed me to explore how to handle events and how to make elements move in Krita! As I continue building out the Selection Action Bar every week, I start to understand how complex a ‘simple’ change could be. Next week I plan to improve on the movement of the toolbar by restricting its dragging area within the canvas.

Contact

To anyone reading this, please feel free to reach out to me. I’m always open to suggestions and thoughts on how to improve as a developer and as a person. Email: ross.erosales@gmail.com Matrix: @rossr:matrix.org