timer.go

package main

import (
	"time"

	"github.com/rivo/tview"
)

var app = tview.NewApplication()
var view = tview.NewModal().SetText("00:00:00")

func startTimer() func() {
	done := make(chan int)

	cancel := func() {
		done <- 1
	}

	startTime := time.Now()

	go func() {
		for {
			select {
			case <-done:
				return
			case <-time.After(500 * time.Millisecond):
				t2 := time.Now()
				diff := t2.Sub(startTime)
				out := time.Time{}.Add(diff)
				app.QueueUpdateDraw(func() {
					view.SetText(out.Format("15:04:05"))
				})
			}
		}
	}()

	return cancel

}

func main() {

	var cancelFunc func()

	view.AddButtons([]string{"Start", "Reset", "Quit"}).
		SetDoneFunc(func(buttonIndex int, buttonLabel string) {
			switch buttonIndex {
			case 0:
				cancelFunc = startTimer()
			case 1:
				cancelFunc()
				view.SetText("00:00:00")
			case 2:
				app.Stop()
			}
		})

	err := app.SetRoot(view, false).EnableMouse(true).Run()

	if err != nil {
		panic(err)
	}

}