Add option to specify file system (#80)

* Add option to specify a file system

This option can be used to load files other than the local file system.
This is useful if templates are stored within the binary.

* Make default file system public

* Update readme with new option

README.md

@@ -81,6 +81,7 @@ Render comes with a variety of configuration options _(Note: these are not the d
 // ...
 r := render.New(render.Options{
     Directory: "templates", // Specify what path to load the templates from.
+    FileSystem: &LocalFileSystem{}, // Specify filesystem from where files are loaded.
     Asset: func(name string) ([]byte, error) { // Load from an Asset function instead of file.
       return []byte("template content"), nil
     },
@@ -117,6 +118,7 @@ r := render.New()
 
 r := render.New(render.Options{
     Directory: "templates",
+    FileSystem: &LocalFileSystem{},
     Asset: nil,
     AssetNames: nil,
     Layout: "",

fs.go

@@ -0,0 +1,21 @@
+package render
+
+import (
+	"io/ioutil"
+	"path/filepath"
+)
+
+type FileSystem interface {
+	Walk(root string, walkFn filepath.WalkFunc) error
+	ReadFile(filename string) ([]byte, error)
+}
+
+type LocalFileSystem struct{}
+
+func (LocalFileSystem) Walk(root string, walkFn filepath.WalkFunc) error {
+	return filepath.Walk(root, walkFn)
+}
+
+func (LocalFileSystem) ReadFile(filename string) ([]byte, error) {
+	return ioutil.ReadFile(filename)
+}

render.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"html/template"
 	"io"
-	"io/ioutil"
 	"log"
 	"net/http"
 	"os"
@@ -51,6 +50,8 @@ type Delims struct {
 type Options struct {
 	// Directory to load templates. Default is "templates".
 	Directory string
+	// FileSystem to access files
+	FileSystem FileSystem
 	// Asset function to use in place of directory. Defaults to nil.
 	Asset func(name string) ([]byte, error)
 	// AssetNames function to use in place of directory. Defaults to nil.
@@ -149,6 +150,9 @@ func (r *Render) prepareOptions() {
 	if len(r.opt.Directory) == 0 {
 		r.opt.Directory = "templates"
 	}
+	if r.opt.FileSystem == nil {
+		r.opt.FileSystem = &LocalFileSystem{}
+	}
 	if len(r.opt.Extensions) == 0 {
 		r.opt.Extensions = []string{".tmpl"}
 	}
@@ -186,7 +190,7 @@ func (r *Render) compileTemplatesFromDir() {
 	r.templates.Delims(r.opt.Delims.Left, r.opt.Delims.Right)
 
 	// Walk the supplied directory and compile any files that match our extension list.
-	filepath.Walk(dir, func(path string, info os.FileInfo, err error) error {
+	r.opt.FileSystem.Walk(dir, func(path string, info os.FileInfo, err error) error {
 		// Fix same-extension-dirs bug: some dir might be named to: "users.tmpl", "local.html".
 		// These dirs should be excluded as they are not valid golang templates, but files under
 		// them should be treat as normal.
@@ -207,7 +211,7 @@ func (r *Render) compileTemplatesFromDir() {
 
 		for _, extension := range r.opt.Extensions {
 			if ext == extension {
-				buf, err := ioutil.ReadFile(path)
+				buf, err := r.opt.FileSystem.ReadFile(path)
 				if err != nil {
 					panic(err)
 				}