Cross-platform native UI, in pure Go

// Counter is a widget: a plain struct value describing what to show.
type Counter struct{ Start int }

func (Counter) CreateState() widget.State { return &counterState{} }

// counterState holds what changes. StateBase[Counter] gives it SetState and a
// typed W() — the current Counter value, with no cast.
type counterState struct {
	widget.StateBase[Counter]
	n int
}

func (s *counterState) Init(widget.Ctx) { s.n = s.W().Start }

func (s *counterState) Build(ctx widget.Ctx) widget.Widget {
	// Pinned to light so the still beside it matches (see the package doc).
	th := theme.Light()
	return widget.Provide[theme.Theme]{Value: th, Child: widget.Fill{Color: th.Bg,
		Child: widget.Center(widget.Column(
			widget.Text{Value: "TAPS", Size: th.Type.Caption, Color: th.Muted},
			widget.Sized{H: 4},
			widget.Text{
				Value: fmt.Sprintf("%d", s.n),
				Size:  th.Type.Display,
				Font:  theme.FontBold,
				Color: th.Text,
			},
			widget.Sized{H: 18},
			theme.Button{
				Label:   "Increment",
				Primary: true,
				OnTap:   func() { s.SetState(func() { s.n++ }) },
			},
		))}}
}