moonegui

An immediate mode graphical interface for MoonBit, after egui.

An interface here is a function of your state, called once a frame. There is no widget tree to keep in step with anything, no callbacks and no subscriptions: you ask for a button and are told whether it was clicked, on the line below.

if @widget.button(ui, "Press me").clicked {
  clicks = clicks + 1
}
clicks_shown = @widget.slider(ui, clicks_shown, low=0, high=10).0
name = @widget.edit(ui, name, hint="your name").0

What is here

Package	What it does
emath	Vec2, Rect, Align, Margin, and the interpolation everything else is built from
id	Ids derived from a parent and a piece of salt, which is what connects a widget to the same widget last frame
input	The events a host hands over, folded once a frame into the state widgets ask questions of
mem	What survives a frame: focus, the widget being dragged, and a value per widget, tidied at the end of every frame
paint	Colours, strokes, fonts, and the five shapes a frame is made of
layout	Placers: where the next widget goes, in four directions, aligned or stretched across the flow
hit	What the pointer is over, and the Response a widget answers with
text	Breaking, aligning, eliding, and turning a click into a caret position
ctx	The frame itself, and Ui — the value you spend the frame with
widget	Label, heading, separator, button, checkbox, radio, slider and text field
back	The backend contract: three ways to measure, nine to draw, and a recorder that writes them down
back/web	What both browser backends share: the surface, the text metrics, and the input coming the other way
back/canvas	A frame painted on a 2D canvas
back/html	The same frame as elements in the document, for pages that want selectable text and the rest of what H5 already has

Design

Three decisions shape everything else.

The core is synchronous. wasm-gc has no async, and a browser cannot be blocked, so the frame is a function the host calls — ctx.run(raw, ui) — rather than a loop the library owns.

A backend measures and draws, nothing more. Twelve functions, all of which a plain 2D canvas already has. There is no triangulation, no glyph atlas and no shader anywhere in the library, and adding a backend is an afternoon — which is how the same frame reaches both a canvas and the document itself, measured by one piece of code so that the two lay out identically.

Hit testing uses the previous frame's rectangles. This frame's widgets do not exist yet when the pointer has to be matched to one. That is where immediate mode's one frame delay comes from, and it is written down in the tests rather than left to be discovered.

Use it

{ "deps": { "moonbitstack/moonegui": "0.1.0" } }

A frame, in full:

let canvas : @canvas.Canvas = @canvas.Canvas::new()
let context = @ctx.Ctx::new(canvas)
canvas.listen()
// then, once a frame:
let out = context.run(canvas.raw(now, dt), fn(ui) {
  @widget.heading(ui, "moonegui") |> ignore
  if @widget.button(ui, "Press me").clicked {
    clicks = clicks + 1
  }
})
canvas.clear(context.style().background)
@back.play(out.shapes, canvas)

examples/browser is that program, with every widget in it. To run it:

moon build --target js
python -m http.server 8741
# then open http://127.0.0.1:8741/examples/browser/index.html
# and the same page as elements: .../index.html?html

Tests

177 tests, run on all four backends. The recording backend turns a frame into a list of text commands, so a test compares drawing to a string and needs no screen, no font file and no browser. Its font is deliberately regular — every character the same width — which is what lets a test work out the expected numbers by hand.

moon check --target all --deny-warn
moon test --target all

Numbers reaching a snapshot are printed as sixty-fourths, since a floating point number does not print the same on every backend.

Not in this version

Meshes and triangulation, glyph rasterisation, images, multiple windows and viewports, drag and drop, animation, and a colour picker. Text is drawn a row at a time by the backend; glyph positions are used only for carets and selections.

License

Apache-2.0.