musicmouse/slint-frontend

Musik Delfin — the Slint front-end

A native front-end for the music player, next to ../web/. Same backend, same layout, same German copy; no browser. It exists because the React UI is sluggish on the Pi 5 kiosk, and the things making it sluggish are browser-shaped: a compositor deciding what gets its own layer, a requestAnimationFrame loop that repaints the viewport, and a JPEG decode per album card per cold start.

Only the music player is here. The typing game (Tippen), the room-lights page, parent mode and the IR-remote assignment are all still web-only.

Running it

Start the backend, then:

nix-shell                 # cargo, the GL/X11/Wayland libs, and Nunito
cargo run                 # windowed, FemtoVG
cargo run -- --server http://musicdolphin:8080

Checks:

cargo test && cargo clippy --all-targets && cargo fmt --check

tools/headless-shots.sh renders the UI and screenshots it inside a nested headless compositor, for checking a layout change on a machine with no display (or a locked one). A Slint window needs a real compositor - under bare Xvfb nothing maps at all.

Build profiles

	Backend	Renderer	For
cargo run	winit	FemtoVG	development here
cargo run --features skia	winit	Skia	checking what the device will show
cargo build --release --no-default-features --features kiosk	linuxkms + libinput	Skia	the Pi

The device must not use FemtoVG. On a Pi 5 (Mesa V3D), FemtoVG draws every runtime-loaded Image as solid black - slint#11785, open as of this writing - which for this app means every album cover. Skia and the software renderer are both unaffected. FemtoVG stays the default here because it builds in seconds and is fine on desktop Mesa; kiosk is the profile that ships.

--no-default-features is not optional on that last one: the features are additive, so leaving desktop on would link winit and linuxkms into the same binary.

kiosk takes the framebuffer directly through KMS/DRM: no X, no Wayland, no compositor. (backend-linuxkms-noseat is deprecated upstream in favour of the libinput variant used here.) SLINT_BACKEND=winit-software forces the software renderer without a rebuild.

Where the speed comes from

Four things, in rough order of how much they matter on the device:

Covers are downscaled once, ever. The backend serves one size - 640 px on the long edge - and has no ?size=. A grid card is 180 px. src/covers.rs fetches each cover once on a worker thread, resizes it to the tier that was asked for, and writes a JPEG thumbnail to ~/.cache/musicmouse-slint/covers/<album-id>@<px>.jpg. Every later start reads that instead. There are two tiers and only two, which is what makes the cache worth having: a per-widget pixel size would give each album a dozen near-identical files and a fresh decode for each.

The cache is dropped wholesale when the websocket announces a rescan ({"type": "library"}), because that is the one moment the backend rewrites the art behind an unchanged cover URL. That is the bug the web client had to learn about the hard way - see the comment at python-backend/musicmouse/services/web/api.py:106.

The grid is virtualized. Rust hands the UI a [GridRow] - the album list pre-chunked into rows - and the view puts those in a ListView, which only instantiates the rows on screen. A flat list of 660 cards gives it no rows to skip, hence the chunking. This is the counterpart of content-visibility: auto on .grid > .card in the web UI, which exists for exactly the same measurement: 340 live cards saturated the Pi's renderer so completely that the main thread stopped scheduling anything.

The progress bar animates instead of ticking. Position arrives twice a second. The web client interpolates it onto animation frames with a JS clock and a render budget; here the fill is a Slint animate width between the pushed values, so interpolation costs a property evaluation per frame on the render side and there is no clock at all. It is suppressed for the one frame a track changes on, so a new track jumps rather than sliding across two unrelated positions.

Nothing on screen animates by itself. The web UI's per-track ambient canvas is not here. Halving its resolution and its frame rate took it from 53.5% of a core to 25.0% and 25% was still too much: a loop repainting the viewport is a floor you cannot get under while it runs at all. The stage keeps its gradient, which is what ?pi=1 already does on the device this replaces.

What is deliberately different from the web UI

No glass blur. Slint has no backdrop-filter, so the panels use a flat translucent fill - exactly the web app's own data-blur="off" fallback. Some depth is genuinely lost. Faking it by compositing a pre-blurred copy would reintroduce the per-frame cost that dropping the blur was meant to avoid.

Worth knowing if you ever port the blur back: on the Pi, backdrop-filter measured five times better than no blur (25% of a core against 118%), because it is what promotes each panel to its own compositing layer. That is a browser-compositor artifact and does not transfer here - but it is why the web UI keeps the panel blur under ?pi=1.

No dolphin. The mascot and its swim/bob loops are not ported.

Layout is Slint's, not a transcription of the CSS: fixed row heights instead of clamp() and vh, a 1080p-shaped fixed measure for the track list rather than max-width. The colours are the same - ui/theme.slint carries the web UI's oklch tokens converted to sRGB, with the oklch source kept beside each one, because oklch is the form the design is actually maintained in.

How it is put together

The split that matters is the web UI's, unchanged: what is playing comes from the backend, what you are looking at lives here. The buttons on the mouse, a figurine on the reader and Home Assistant all move playback too, so this follows it rather than assuming it is in charge.

Three worker threads and no async runtime - the whole client makes one library request, one websocket connection and a trickle of covers.

Path	What it is
src/api.rs	The wire types and the HTTP calls. Mirrors services/web/schemas.py.
src/ws.rs	The state websocket, with reconnect-and-reseed.
src/library.rs	The search index and the browse hierarchy. Port of web/src/lib/search.ts.
src/keymap.rs	The keyboard model, as a pure function. Port of web/src/lib/keyboard.ts.
src/covers.rs	The thumbnail cache, and the generated stand-in art.
src/oklch.rs, src/format.rs	Colour conversion; durations.
src/main.rs	Wiring: worker channels in, Slint models out.
ui/*.slint	Layout only. Nothing here formats a number or picks a word.

src/keymap.rs and src/library.rs are ports of modules that were already pure functions with their own tests, and they keep them: 43 tests, no window required.

Keyboard

The same map as the web UI.

Key
SPACE	play / pause (types a space mid-query)
SHIFT+H / SHIFT+L	previous / next
SHIFT+J / SHIFT+K	quieter / louder
SHIFT+M	mute
SHIFT+← / SHIFT+→	seek 15 s
A-Z, 0-9	search albums as you type
?	search individual tracks
TAB	cycle Musik / Hörbücher / Podcasts
← ↑ ↓ →	move the selection while browsing; transport and volume while playing
CTRL+H/J/K/L	the same, without leaving the home row
CTRL+D / CTRL+U	half a page
ENTER	play the selection
SHIFT+ENTER	open the track list
ESC	back out one layer at a time
/	back to browsing

Everything is clickable too.

The special keys are named in ui/app.slint, where the Key.* constants live, and reach Rust as "escape", "left", "enter" and so on - so src/keymap.rs is testable without a window and without hardcoding keysym values.

Fonts

Nunito, via SLINT_DEFAULT_FONT, which shell.nix sets. Slint cannot read the woff2 the web app self-hosts, so this wants the TTF; a deployment can drop one at fonts/Nunito.ttf instead. With neither, everything still renders in the system sans.