present: Set the background using CSS

This allowed me to better match the background image to the size of
the slides.

Change-Id: Ieaae93cd78582a3059ed6c3e64e740dea9088af5
Reviewed-on: https://go-review.googlesource.com/47130
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

cmd/present/static/styles.css

@@ -391,18 +391,9 @@ article > .video {
   margin-top: 40px;
 }
 
-article > .background {
-  position: absolute;
-  top: 0;
-  left: 0;
-  right: 0;
-  bottom: 0;
-  z-index: -1;
-}
-
-article > .background > img {
-  max-height: 100%;
-  max-width: 100%;
+article.background {
+  background-size: contain;
+  background-repeat: round;
 }
 
 table {

cmd/present/templates/slides.tmpl

@@ -55,7 +55,7 @@
 
   {{range $i, $s := .Sections}}
   <!-- start of slide {{$s.Number}} -->
-      <article>
+      <article {{$s.HTMLAttributes}}>
       {{if $s.Elem}}
         <h3>{{$s.Title}}</h3>
         {{range $s.Elem}}{{elem $.Template .}}{{end}}

present/background.go

@@ -1,25 +0,0 @@
-// 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 present
-
-import (
-	"strings"
-)
-
-func init() {
-	Register("background", parseBackground)
-}
-
-type Background struct {
-	URL string
-}
-
-func (i Background) TemplateName() string { return "background" }
-
-func parseBackground(ctx *Context, fileName string, lineno int, text string) (Elem, error) {
-	args := strings.Fields(text)
-	background := Background{URL: args[1]}
-	return background, nil
-}

present/parse.go

@@ -96,12 +96,32 @@ func (p *Author) TextElem() (elems []Elem) {
 // Section represents a section of a document (such as a presentation slide)
 // comprising a title and a list of elements.
 type Section struct {
-	Number []int
-	Title  string
-	Elem   []Elem
-	Notes  []string
+	Number  []int
+	Title   string
+	Elem    []Elem
+	Notes   []string
+	Classes []string
+	Styles  []string
 }
 
+// HTMLAttributes for the section
+func (s Section) HTMLAttributes() template.HTMLAttr {
+	if len(s.Classes) == 0 && len(s.Styles) == 0 {
+		return ""
+	}
+
+	var class string
+	if len(s.Classes) > 0 {
+		class = fmt.Sprintf(`class=%q`, strings.Join(s.Classes, " "))
+	}
+	var style string
+	if len(s.Styles) > 0 {
+		style = fmt.Sprintf(`style=%q`, strings.Join(s.Styles, " "))
+	}
+	return template.HTMLAttr(strings.Join([]string{class, style}, " "))
+}
+
+// Sections contained within the section.
 func (s Section) Sections() (sections []Section) {
 	for _, e := range s.Elem {
 		if section, ok := e.(Section); ok {
@@ -366,6 +386,11 @@ func parseSections(ctx *Context, name string, lines *Lines, number []int) ([]Sec
 				}
 			case strings.HasPrefix(text, "."):
 				args := strings.Fields(text)
+				if args[0] == ".background" {
+					section.Classes = append(section.Classes, "background")
+					section.Styles = append(section.Styles, "background-image: url('"+args[1]+"')")
+					break
+				}
 				parser := parsers[args[0]]
 				if parser == nil {
 					return nil, fmt.Errorf("%s:%d: unknown command %q\n", name, lines.line, text)