Modal

For modal message boxes, there is a built-in primitive called Modal:

Code:

package main

import (
	"github.com/rivo/tview"
)

func main() {
	app := tview.NewApplication()
	modal := tview.NewModal().
		SetText("Do you want to quit the application?").
		AddButtons([]string{"Quit", "Cancel"}).
		SetDoneFunc(func(buttonIndex int, buttonLabel string) {
			if buttonLabel == "Quit" {
				app.Stop()
			}
		})
	if err := app.SetRoot(modal, false).SetFocus(modal).Run(); err != nil {
		panic(err)
	}
}

If you need more functionality than the Modal class provides, you can use any other primitive instead. To center your primitive, use the Flex class with the border elements set to nil. To make it appear on top of other elements, as with the Modal class itself, use the Pages class. Here is an example:

package main

import (
	"strings"

	"github.com/gdamore/tcell/v2"
	"github.com/rivo/tview"
)

func main() {
	app := tview.NewApplication()

	// Returns a new primitive which puts the provided primitive in the center and
	// sets its size to the given width and height.
	modal := func(p tview.Primitive, width, height int) tview.Primitive {
		return tview.NewFlex().
			AddItem(nil, 0, 1, false).
			AddItem(tview.NewFlex().SetDirection(tview.FlexRow).
				AddItem(nil, 0, 1, false).
				AddItem(p, height, 1, true).
				AddItem(nil, 0, 1, false), width, 1, true).
			AddItem(nil, 0, 1, false)
	}

	background := tview.NewTextView().
		SetTextColor(tcell.ColorBlue).
		SetText(strings.Repeat("background ", 1000))

	box := tview.NewBox().
		SetBorder(true).
		SetTitle("Centered Box")

	pages := tview.NewPages().
		AddPage("background", background, true, true).
		AddPage("modal", modal(box, 40, 10), true, true)

	if err := app.SetRoot(pages, true).Run(); err != nil {
		panic(err)
	}
}

The same effect can be achieved by using the Grid class instead of Flex:

// Returns a new primitive which puts the provided primitive in the center and
// sets its size to the given width and height.
modal := func(p tview.Primitive, width, height int) tview.Primitive {
	return tview.NewGrid().
		SetColumns(0, width, 0).
		SetRows(0, height, 0).
		AddItem(p, 1, 1, 1, 1, 0, 0, true)
}

See also: https://pkg.go.dev/github.com/rivo/tview#Modal