Watches templates and reinitializes them on changes without fully restarting the app (excerpt from real app)

0_render.go

package main

import (
	"fmt"
	"html/template"
	"time"

	"github.com/pkg/errors"
	"github.com/rjeczalik/notify"
)

var templateFuncs = map[string]interface{}{}

func initTemplating() error {
	err = reinitHtmlTemplates()
	if err != nil {
		return err
	}

	if DEVELOPMENT_MODE {
		htmlEvents, err := watchHtmlTemplates()
		if err != nil {
			return err
		}
		go reinitHtmlTemplatesOnChange(htmlEvents)
	}

	return nil
}

func reinitHtmlTemplates() error {
	templates := template.New("")
	templates.Funcs(template.FuncMap(templateFuncs))
	_, err := templates.ParseGlob("templates/html/*")
	env.htmlTemplates = templates // Writing a pointer is atomic.
	return errors.WithStack(err)
}

func reinitHtmlTemplatesOnChange(events chan notify.EventInfo) {
	for range events {
		t0 := time.Now()
		err := reinitHtmlTemplates()
		if err != nil {
			fmt.Printf("HTML template error: %+v\n", err)
		} else {
			t1 := time.Now()
			fmt.Printf("reinited HTML templates in %v\n", t1.Sub(t0))
			notifyInternalSubscriptions(InternalNotification{Type: InternalNotificationTypeReload})
		}
	}
}

func watchHtmlTemplates() (chan notify.EventInfo, error) {
	events := make(chan notify.EventInfo, 1)
	err := notify.Watch("templates/html", events, notify.All)
	return events, errors.WithStack(err)
}

1_head.html

<script>
  {
    function reinit() {
      const url    = 'ws://' + window.location.host + '/ws'
      const ws     = new WebSocket(url)
      ws.onmessage = onmessage
      ws.onclose   = onclose
    }

    reinit()

    function onmessage({data}) {
      let message
      try {message = JSON.parse(data)}
      catch (__) {return}
      if (message && message.type === 'reload') {
        window.location.reload()
      }
    }

    function onclose() {setTimeout(reinit, 1000)}
  }
</script>

Notes

env.htmlTemplates is a global which is used to render templates by name.

The subscription notification (notifyInternalSubscriptions) broadcasts a message to websocket clients. The HTML pages include a snippet which connects to the server via a websocket, and reloads the page when receiving the appropriate message.