gtk-layers

This project demonstrates how to use the haskell-gi GTK 4 bindings to implement a system of layers, like one finds in typical image editing software such as Photoshop, Inkscape or GIMP.

Running

To run, clone the repository and run cabal run exe:gtk-layers.

This requires the GTK 4 C library to be installed, version 4.10 or above. See the haskell-gi repository for installation instructions for various platforms.

Overview

The gtk-layers executable demonstrates key some implementation details of a layer system in GTK:

• The GTK 4 TreeListModel API, which separates the underlying model hierarchy from its visual display (it replaces the now deprecated TreeView API).
  In particular, the application illustrates:

  • how to create a TreeListModel with child models, avoiding common pitfalls;
  • how a TreeListModel can be edited by keeping references to underlying child list stores;
  • how to store custom data in a list model, and what data to choose to facilitate editing the model;
  • how to set up the ListView that displays the model, including what to do in the setup and bind signal handlers of the SignalListItemFactory which is used to create widgets that visualise the underlying data.

• The GTK drag-and-drop API, which includes:

  • how to communicate Haskell data from the drag source to the drop target;
  • full CSS styling to animate the drag and drop effect, using whether one is hovering on the top of bottom of a layer to style the destination differently;
  • updating the selection after a move operation.

• Undo/redo functionality.

• Other details such as the handling of ownership or the use of STM to ensure atomicity of updates.

The code contains extensive comments that detail the implementation and justify the approach taken.

Acknowledgements

I extend my thanks to Iñaki García Etxebarria, the author and maintainer of the haskell-gi library, for his patient help with diagnosing and fixing bugs in early versions of this project.