Theme selector

widget.ThemeSelector is a ready-made appearance setting. It shows a System, Light and Dark switch above a row of swatches for the eight standard primary colours.

Add the selector

The widget only reports what the user picks. It does not change the theme or save anything itself, so you decide what happens:

selector := widget.NewThemeSelector(func(sel widget.ThemeSelection) {
	sel.Apply()                                     // re-theme this app now
	widget.SaveThemeSelection(sel, a.Preferences()) // remember it
})
if saved, ok := widget.LoadThemeSelection(a.Preferences()); ok {
	selector.Selection = saved // show the saved choice without firing OnChanged
}

• OnChanged receives a widget.ThemeSelection: the variant choice (widget.ThemeVariantSystem, ThemeVariantLight or ThemeVariantDark) and the name of the primary colour.
• ThemeSelection.Apply re-themes the running app. An empty Primary leaves the primary colour as it is.
• widget.SaveThemeSelection stores the choice in the preferences you pass, normally the app’s own.

A new selector starts on System, with the app’s current primary colour (blue if none is set). To show a saved choice, assign it to the Selection field before the selector is displayed. Assigning the field does not call OnChanged. SetSelection does, when the value changes.

Restore the choice at startup

Load the saved selection and apply it before you show the first window, so the app does not open in one theme and then switch:

// Preferences need an app ID.
a := app.NewWithID("com.example.themeselector")

// Restore the saved appearance before the first window is shown.
if saved, ok := widget.LoadThemeSelection(a.Preferences()); ok {
	saved.Apply()
}

widget.LoadThemeSelection returns false when nothing was saved. The app then follows the system appearance, as it does by default.

Preferences need an app ID, so create the app with app.NewWithID (or set the ID in the app’s metadata). Without one, Preferences() logs an error and nothing is stored between runs.

What it does not do

• It does not change other apps. The choice applies to your app and is saved in your app’s preferences. Bellui’s system-wide settings, shared by all Bellui apps, are not touched.
• System is resolved when it is applied. Apply with ThemeVariantSystem reads the system appearance at that moment.
• No custom colours. The swatches are the eight named primary colours. For any other colour, build a custom theme.

The full program:

// An appearance setting built from widget.ThemeSelector, applied live and
// remembered in the app's own preferences.
package main

import (
	"github.com/belluikit/bellui"
	"github.com/belluikit/bellui/app"
	"github.com/belluikit/bellui/widget"
)

func main() {
	// Preferences need an app ID.
	a := app.NewWithID("com.example.themeselector")

	// Restore the saved appearance before the first window is shown.
	if saved, ok := widget.LoadThemeSelection(a.Preferences()); ok {
		saved.Apply()
	}

	w := a.NewWindow("Appearance")

	selector := widget.NewThemeSelector(func(sel widget.ThemeSelection) {
		sel.Apply()                                     // re-theme this app now
		widget.SaveThemeSelection(sel, a.Preferences()) // remember it
	})
	if saved, ok := widget.LoadThemeSelection(a.Preferences()); ok {
		selector.Selection = saved // show the saved choice without firing OnChanged
	}

	w.SetContent(widget.NewCard("Appearance", "Theme and accent colour", selector))
	bellui.ShowAndRun(w)
}