go-widgets/mvvm

A tiny, dependency-free MVVM (Model–View–ViewModel) layer for the go-widgets ecosystem. Generics-only, no reflection, 100% statement coverage.

It provides the three MVVM primitives and the binding glue to wire them to widgets — without importing any widget package, so a single ViewModel drives both the pixel toolkit and the terminal-cell tui.

Primitive	Role
Observable[T]	a bindable property (Get/Set/Subscribe, skips equal values, re-entrancy-safe)
Command	a bindable action with CanExecute + RaiseCanExecuteChanged
ObservableList[T]	a bindable collection emitting granular insert/remove/replace/move/reset events

Binding adapters reach a widget only through pointers to its value field and its callback slot — BindField, OneWay, BindCommand, BindList.

Why it's backend-agnostic

go-widgets/toolkit and go-widgets/tui mirror the same field names and callback signatures per widget (Entry.Text + Entry.OnChange, Scale.Value + Scale.OnChange, …). So a binding written against (&w.Text, &w.OnChange) compiles and runs for either backend. This package therefore imports neither — it depends only on that field/callback shape.

Why binding is loop-free

A widget fires its change callback from inside its event handler, and leaves direct field writes silent. So the View→ViewModel edge is the callback, the ViewModel→View edge is a silent field write, and Observable.Set skips equal values — a two-way binding can echo without recursing.

A form in ~10 lines

type FormVM struct {
	Name  *mvvm.Observable[string]
	Names *mvvm.ObservableList[string]
	Save  *mvvm.Command
}

vm := &FormVM{Name: mvvm.NewObservable(""), Names: mvvm.NewObservableList[string]()}
vm.Save = mvvm.NewCommand(
	func() { vm.Names.Append(vm.Name.Get()); vm.Name.Set("") },
	func() bool { return vm.Name.Get() != "" }, // CanExecute
)
mvvm.BindCanExecute(vm.Save, vm.Name) // Save re-greys as the name changes

// View — identical for tui except the widget types + the repaint hook.
mvvm.BindField(vm.Name, &name.Text, &name.OnChange, repaint)
mvvm.BindCommand(vm.Save, &save.OnClick, setEnabled)
mvvm.BindList(vm.Names, &list.Items, func(s string) string { return s }, repaint)

The same FormVM drives the pixel and the cell form verbatim.

Backend adapters

The core package binds any widget whose value + change-callback fit (&field, &hook). Widgets with a multi-argument or oddly-named callback get a small named adapter in a per-backend subpackage — the only packages that import a backend:

• mvvm/tkbind (imports toolkit) — BindRange for the two-handle RangeSlider (OnChange(low, high)), plus BindContainer / BindCardActive to drive a toolkit.Container / CardLayout from an ObservableList / Observable (data-driven views).
• mvvm/tuibind (imports tui) — BindDropdown (OnChange(idx, value)) and BindTableSelection (OnSelect(row)).

The core mvvm package itself still imports nothing (verified with go list -deps), so a consumer who only wants observables/commands pays for no backend.

Status

v0.3.0: core + tkbind (incl. BindContainer / BindCardActive) + tuibind, all 100% coverage.

License

BSD-3-Clause — see LICENSE.