html/template: add examples of loading templates from files

Adds examples showing loading templates from files and
executing them.

Shows examples:
- Using ParseGlob.
- Using ParseFiles.
- Using helper functions to share and use templates
in different contexts by adding them to an existing
bundle of templates.
- Using a group of driver templates with distinct sets
of helper templates.

Almost all of the code was directly copied from text/template.

Fixes #8500

Change-Id: Ic3d91d5232afc5a1cd2d8cd3d9a5f3b754c64225
Reviewed-on: https://go-review.googlesource.com/21854Reviewed-by: Andrew Gerrand <adg@golang.org>

src/html/template/examplefiles_test.go

+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template_test
+
+import (
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+	"path/filepath"
+	"text/template"
+)
+
+// templateFile defines the contents of a template to be stored in a file, for testing.
+type templateFile struct {
+	name     string
+	contents string
+}
+
+func createTestDir(files []templateFile) string {
+	dir, err := ioutil.TempDir("", "template")
+	if err != nil {
+		log.Fatal(err)
+	}
+	for _, file := range files {
+		f, err := os.Create(filepath.Join(dir, file.name))
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer f.Close()
+		_, err = io.WriteString(f, file.contents)
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+	return dir
+}
+
+// The following example is duplicated in text/template; keep them in sync.
+
+// Here we demonstrate loading a set of templates from a directory.
+func ExampleTemplate_glob() {
+	// Here we create a temporary directory and populate it with our sample
+	// template definition files; usually the template files would already
+	// exist in some location known to the program.
+	dir := createTestDir([]templateFile{
+		// T0.tmpl is a plain template file that just invokes T1.
+		{"T0.tmpl", `T0 invokes T1: ({{template "T1"}})`},
+		// T1.tmpl defines a template, T1 that invokes T2.
+		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
+		// T2.tmpl defines a template T2.
+		{"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
+	})
+	// Clean up after the test; another quirk of running as an example.
+	defer os.RemoveAll(dir)
+
+	// pattern is the glob pattern used to find all the template files.
+	pattern := filepath.Join(dir, "*.tmpl")
+
+	// Here starts the example proper.
+	// T0.tmpl is the first name matched, so it becomes the starting template,
+	// the value returned by ParseGlob.
+	tmpl := template.Must(template.ParseGlob(pattern))
+
+	err := tmpl.Execute(os.Stdout, nil)
+	if err != nil {
+		log.Fatalf("template execution: %s", err)
+	}
+	// Output:
+	// T0 invokes T1: (T1 invokes T2: (This is T2))
+}
+
+// Here we demonstrate loading a set of templates from files in different directories
+func ExampleTemplate_parsefiles() {
+	// Here we create different temporary directories and populate them with our sample
+	// template definition files; usually the template files would already
+	// exist in some location known to the program.
+	dir1 := createTestDir([]templateFile{
+		// T1.tmpl is a plain template file that just invokes T2.
+		{"T1.tmpl", `T1 invokes T2: ({{template "T2"}})`},
+	})
+
+	dir2 := createTestDir([]templateFile{
+		// T2.tmpl defines a template T2.
+		{"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
+	})
+
+	// Clean up after the test; another quirk of running as an example.
+	defer func(dirs ...string) {
+		for _, dir := range dirs {
+			os.RemoveAll(dir)
+		}
+	}(dir1, dir2)
+
+	// Here starts the example proper.
+	// Let's just parse only dir1/T0 and dir2/T2
+	paths := []string{
+		filepath.Join(dir1, "T1.tmpl"),
+		filepath.Join(dir2, "T2.tmpl"),
+	}
+	tmpl := template.Must(template.ParseFiles(paths...))
+
+	err := tmpl.Execute(os.Stdout, nil)
+	if err != nil {
+		log.Fatalf("template execution: %s", err)
+	}
+	// Output:
+	// T1 invokes T2: (This is T2)
+}
+
+// The following example is duplicated in text/template; keep them in sync.
+
+// This example demonstrates one way to share some templates
+// and use them in different contexts. In this variant we add multiple driver
+// templates by hand to an existing bundle of templates.
+func ExampleTemplate_helpers() {
+	// Here we create a temporary directory and populate it with our sample
+	// template definition files; usually the template files would already
+	// exist in some location known to the program.
+	dir := createTestDir([]templateFile{
+		// T1.tmpl defines a template, T1 that invokes T2.
+		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
+		// T2.tmpl defines a template T2.
+		{"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
+	})
+	// Clean up after the test; another quirk of running as an example.
+	defer os.RemoveAll(dir)
+
+	// pattern is the glob pattern used to find all the template files.
+	pattern := filepath.Join(dir, "*.tmpl")
+
+	// Here starts the example proper.
+	// Load the helpers.
+	templates := template.Must(template.ParseGlob(pattern))
+	// Add one driver template to the bunch; we do this with an explicit template definition.
+	_, err := templates.Parse("{{define `driver1`}}Driver 1 calls T1: ({{template `T1`}})\n{{end}}")
+	if err != nil {
+		log.Fatal("parsing driver1: ", err)
+	}
+	// Add another driver template.
+	_, err = templates.Parse("{{define `driver2`}}Driver 2 calls T2: ({{template `T2`}})\n{{end}}")
+	if err != nil {
+		log.Fatal("parsing driver2: ", err)
+	}
+	// We load all the templates before execution. This package does not require
+	// that behavior but html/template's escaping does, so it's a good habit.
+	err = templates.ExecuteTemplate(os.Stdout, "driver1", nil)
+	if err != nil {
+		log.Fatalf("driver1 execution: %s", err)
+	}
+	err = templates.ExecuteTemplate(os.Stdout, "driver2", nil)
+	if err != nil {
+		log.Fatalf("driver2 execution: %s", err)
+	}
+	// Output:
+	// Driver 1 calls T1: (T1 invokes T2: (This is T2))
+	// Driver 2 calls T2: (This is T2)
+}
+
+// The following example is duplicated in text/template; keep them in sync.
+
+// This example demonstrates how to use one group of driver
+// templates with distinct sets of helper templates.
+func ExampleTemplate_share() {
+	// Here we create a temporary directory and populate it with our sample
+	// template definition files; usually the template files would already
+	// exist in some location known to the program.
+	dir := createTestDir([]templateFile{
+		// T0.tmpl is a plain template file that just invokes T1.
+		{"T0.tmpl", "T0 ({{.}} version) invokes T1: ({{template `T1`}})\n"},
+		// T1.tmpl defines a template, T1 that invokes T2. Note T2 is not defined
+		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
+	})
+	// Clean up after the test; another quirk of running as an example.
+	defer os.RemoveAll(dir)
+
+	// pattern is the glob pattern used to find all the template files.
+	pattern := filepath.Join(dir, "*.tmpl")
+
+	// Here starts the example proper.
+	// Load the drivers.
+	drivers := template.Must(template.ParseGlob(pattern))
+
+	// We must define an implementation of the T2 template. First we clone
+	// the drivers, then add a definition of T2 to the template name space.
+
+	// 1. Clone the helper set to create a new name space from which to run them.
+	first, err := drivers.Clone()
+	if err != nil {
+		log.Fatal("cloning helpers: ", err)
+	}
+	// 2. Define T2, version A, and parse it.
+	_, err = first.Parse("{{define `T2`}}T2, version A{{end}}")
+	if err != nil {
+		log.Fatal("parsing T2: ", err)
+	}
+
+	// Now repeat the whole thing, using a different version of T2.
+	// 1. Clone the drivers.
+	second, err := drivers.Clone()
+	if err != nil {
+		log.Fatal("cloning drivers: ", err)
+	}
+	// 2. Define T2, version B, and parse it.
+	_, err = second.Parse("{{define `T2`}}T2, version B{{end}}")
+	if err != nil {
+		log.Fatal("parsing T2: ", err)
+	}
+
+	// Execute the templates in the reverse order to verify the
+	// first is unaffected by the second.
+	err = second.ExecuteTemplate(os.Stdout, "T0.tmpl", "second")
+	if err != nil {
+		log.Fatalf("second execution: %s", err)
+	}
+	err = first.ExecuteTemplate(os.Stdout, "T0.tmpl", "first")
+	if err != nil {
+		log.Fatalf("first: execution: %s", err)
+	}
+
+	// Output:
+	// T0 (second version) invokes T1: (T1 invokes T2: (T2, version B))
+	// T0 (first version) invokes T1: (T1 invokes T2: (T2, version A))
+}