Crystal MPD libui

A desktop MPD client written in Crystal using the UIng (libui-ng) GUI library.

Screenshots

Features

• Playback controls: play/pause, previous, next
• Shuffle and repeat toggle buttons
• Seek slider with elapsed / total time display
• Album cover art fetched from MPD (readpicture / albumart)
• Integrated playlist with Artist, Title, Duration columns
• Currently playing track highlighted and auto-scrolled into view
• Window title updates to reflect the current track
• Settings window for MPD host / port configuration
• About dialog with MPD server stats (version, artists, albums, songs, uptime)

Requirements

• Crystal >= 1.19.1
• libui-ng shared library installed
• GTK3 (required by libui-ng on Linux)
• A running MPD server

Installation

git clone https://github.com/mamantoha/mpd-libui
cd mpd-libui
shards install
crystal build src/mpd_ui.cr --release -o mpd-libui
./mpd-libui

Dependencies

Platform support

Custom components

ToggleButton (src/mpd_ui/toggle_button.cr)

UIng does not provide a toggle button widget. ToggleButton is a custom widget built on top of UIng::Area (a libui-ng raw drawing surface).

• Renders a filled rectangle with a centered emoji/text label using the UIng::Area::Draw API (paths, brushes, text layouts)
• Active state shown as a blue highlight; hover tracked for a subtle gray tint
• Minimum size enforced via uiControlHandle + gtk_widget_set_size_request because uiNewArea has no natural GTK size hint
• Font loaded once via UIng::FontDescriptor (uiLoadControlFont) and cached as an instance variable to avoid per-frame allocation and Pango errors
• AttributedString and TextLayout created and freed deterministically each draw using the open { } block API to prevent libui leak warnings at shutdown

PlaylistView (src/mpd_ui/playlist_view.cr)

A UIng::Table-backed playlist widget embedded directly in the main window.

• Custom UIng::Table::Model::Handler backed by a mutable Array(Song) that is updated in-place (clear + concat) so the model handler closure always sees the live array — replacing the array reference caused a uiTableModel_get_value: stamp assertion crash
• Per-row background color column (type Color) highlights the active track with a blue tint
• Auto-scrolls to the playing row using raw GTK calls: uiControlHandle → gtk_bin_get_child (GtkScrolledWindow → GtkTreeView) → gtk_tree_view_scroll_to_cell
• Proper cleanup on window close: table.destroy before model.free, matching libui-ng's required teardown order

Architecture notes

• Dual MPD clients: a regular MPD::Client for commands and a second client with with_callbacks: true running in a background Thread for push events (song change, state, elapsed, random, repeat, playlist)
• UI thread safety: all UI mutations from the callback thread go through UIng.queue_main { }
• Cover art: fetched in a third Thread using a dedicated MPD connection; decoded in pure Crystal (no subprocess) via StumpyJPEG/StumpyPNG to avoid Process.run deadlock inside threads (Crystal's waitpid signal handler is occupied by UIng.main)
• Stale result guard: @current_file is compared before applying a fetched cover image to discard results that arrived after the track changed

License

MIT