Hot-reloading / hot-swapping live template files in Go

index.gohtml

My name is {{.Name}} and I'm {{.Age}} years old!!

reload.go

package main

import (
	"fmt"
	"net/http"
	"sync"
	"text/template"

	"golang.org/x/exp/inotify"
)

var (
	// TemplateExt is the extension for the physical template files. Failure
	// to set this to the same extension your physical template files have
	// will result in the failure to reload the files.
	TemplateExt = ".gohtml"

	// TemplatePath is the path to the directory containing the template files.
	// It defaults to the current directory, provided you call r.Watch("./")
	TemplatePath = ".//"
)

// Reloader is a struct used to contain the parsed template files as well
// as provide a sane API for locking and watching for events.
//
// Common usage would be inside a RenderTemplate sort of function similar to
//     r := reloader.New("my_path")
//     r.Watch()
//
//     ...
//
//     func RenderTemplate(w http.ResponseWriter, r *http.Request, name string, data interface{}) (err error) {
//         tmpl := r.Templates[name]
//         err = tmpl.Execute(w, data)
//         if err != nil {
//             // handle error.
//         }
//         return
//     }
type Reloader struct {
	Templates map[string]*template.Template

	*inotify.Watcher
	*sync.Mutex
}

// New returns an initialized Reloader that starts watching the given
// directories for all events.
func New(dirs ...string) *Reloader {
	watcher, err := inotify.NewWatcher()
	if err != nil {
		panic(err)
	}

	for _, path := range dirs {
		watcher.AddWatch(path, inotify.IN_ALL_EVENTS)
	}

	return &Reloader{
		Watcher: watcher,
		RWMutex: &sync.Mutex{},
	}
}

func (r *Reloader) Watch() {
	go func() {
		for {
			select {
			case evt := <-r.Watcher.Event:
				if eventIsWanted(evt.Mask) {
					fmt.Printf("File: %s Event: %s. Hot reloading.\n",
						evt.Name, evt.String())

					if err := r.reload(evt.Name); err != nil {
						fmt.Println(err)
					}
				}
			case err := <-r.Watcher.Error:
				fmt.Println(err)
			}
		}
	}()
}

func eventIsWanted(mask uint32) bool {
	return mask&inotify.IN_MODIFY != 0 ||
		mask&inotify.IN_CREATE != 0
}

func (r *Reloader) reload(name string) error {

	// Just for example purposes, and sssuming 'index.gohtml' is in the
	// same directory as this file.
	if name == TemplatePath+"reload.go" {
		return nil
	}

	if len(name) >= len(TemplateExt) &&
		name[len(name)-len(TemplateExt):] == TemplateExt {

		tmpl := template.Must(template.ParseFiles(name))

		// Gather what would be the key in our template map.
		// 'name' is in the format: "path/identifier.extension",
		// so trim the 'path/' and the '.extension' to get the
		// name (minus new extension) used inside of our map.
		key := name[len(TemplatePath) : len(name)-len(TemplateExt)]

		r.Lock()
		r.Templates[key] = tmpl
		r.Unlock()

		return nil
	}

	return fmt.Errorf("Unable to reload file %s\n", name)

}

func main() {
	r := New("./")
	r.Templates = map[string]*template.Template{
		"index": template.Must(template.ParseFiles("index.gohtml")),
	}

	r.Watch()

	render := func(w http.ResponseWriter, name string, data interface{}) (err error) {
		tmpl := r.Templates[name]
		if err = tmpl.Execute(w, data); err != nil {
			panic(err)
		}
		return
	}

	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Content-Type", "text/plain")
		data := struct {
			Name string
			Age  int
		}{
			"Eric",
			21,
		}
		render(w, "index", data)
	})

	http.Handle("/", handler)
	http.ListenAndServe("0.0.0.0:8000", nil)
}

zlicense.txt

The MIT License (MIT)

Copyright (c) 2015, Eric Lagergren

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.