Add TODO list support for Blackfriday

* Add CSS class to TODO list and list items
* Add a flag to turn task list support off

Fixes #2269

3 files changed, 81 insertions, 0 deletions

diff --git a/helpers/content.go b/helpers/content.go
index 49d3469c5..53176de64 100644
--- a/helpers/content.go
+++ b/helpers/content.go
@@ -51,6 +51,7 @@ type Blackfriday struct {
 	HrefTargetBlank                  bool
 	SmartDashes                      bool
 	LatexDashes                      bool
+	TaskLists                        bool
 	PlainIDAnchors                   bool
 	SourceRelativeLinksEval          bool
 	SourceRelativeLinksProjectFolder string
@@ -68,6 +69,7 @@ func NewBlackfriday(c ConfigProvider) *Blackfriday {
 		"smartDashes":                      true,
 		"latexDashes":                      true,
 		"plainIDAnchors":                   true,
+		"taskLists":                        true,
 		"sourceRelativeLinks":              false,
 		"sourceRelativeLinksProjectFolder": "/docs/content",
 	}
diff --git a/helpers/content_renderer.go b/helpers/content_renderer.go
index 02a9e9c83..6bd2212e1 100644
--- a/helpers/content_renderer.go
+++ b/helpers/content_renderer.go
@@ -72,6 +72,44 @@ func (renderer *HugoHTMLRenderer) Image(out *bytes.Buffer, link []byte, title []
 	}
 }
 
+// ListItem adds task list support to the Blackfriday renderer.
+func (renderer *HugoHTMLRenderer) ListItem(out *bytes.Buffer, text []byte, flags int) {
+	if !renderer.Config.TaskLists {
+		renderer.Renderer.ListItem(out, text, flags)
+		return
+	}
+
+	switch {
+	case bytes.HasPrefix(text, []byte("[ ] ")):
+		text = append([]byte(`<input type="checkbox" disabled class="task-list-item">`), text[3:]...)
+
+	case bytes.HasPrefix(text, []byte("[x] ")) || bytes.HasPrefix(text, []byte("[X] ")):
+		text = append([]byte(`<input type="checkbox" checked disabled class="task-list-item">`), text[3:]...)
+	}
+
+	renderer.Renderer.ListItem(out, text, flags)
+}
+
+// List adds task list support to the Blackfriday renderer.
+func (renderer *HugoHTMLRenderer) List(out *bytes.Buffer, text func() bool, flags int) {
+	if !renderer.Config.TaskLists {
+		renderer.Renderer.List(out, text, flags)
+		return
+	}
+	marker := out.Len()
+	renderer.Renderer.List(out, text, flags)
+	if out.Len() > marker {
+		list := out.Bytes()[marker:]
+		if bytes.Contains(list, []byte("task-list-item")) {
+			// Rewrite the buffer from the marker
+			out.Truncate(marker)
+			// May be either dl, ul or ol
+			list := append(list[:4], append([]byte(` class="task-list"`), list[4:]...)...)
+			out.Write(list)
+		}
+	}
+}
+
 // HugoMmarkHTMLRenderer wraps a mmark.Renderer, typically a mmark.html
 // Enabling Hugo to customise the rendering experience
 type HugoMmarkHTMLRenderer struct {
diff --git a/helpers/content_renderer_test.go b/helpers/content_renderer_test.go
index f96cf0ad5..7baaadb20 100644
--- a/helpers/content_renderer_test.go
+++ b/helpers/content_renderer_test.go
@@ -88,3 +88,44 @@ func TestCodeFence(t *testing.T) {
 		}
 	}
 }
+
+func TestBlackfridayTaskList(t *testing.T) {
+	for i, this := range []struct {
+		markdown        string
+		taskListEnabled bool
+		expect          string
+	}{
+		{`
+TODO:
+
+- [x] On1
+- [X] On2
+- [ ] Off
+
+END
+`, true, `<p>TODO:</p>
+
+<ul class="task-list">
+<li><input type="checkbox" checked disabled class="task-list-item"> On1</li>
+<li><input type="checkbox" checked disabled class="task-list-item"> On2</li>
+<li><input type="checkbox" disabled class="task-list-item"> Off</li>
+</ul>
+
+<p>END</p>
+`},
+		{`- [x] On1`, false, `<ul>
+<li>[x] On1</li>
+</ul>
+`},
+	} {
+		blackFridayConfig := NewBlackfriday(viper.GetViper())
+		blackFridayConfig.TaskLists = this.taskListEnabled
+		ctx := &RenderingContext{Content: []byte(this.markdown), PageFmt: "markdown", Config: blackFridayConfig}
+
+		result := string(RenderBytes(ctx))
+
+		if result != this.expect {
+			t.Errorf("[%d] got \n%v but expected \n%v", i, result, this.expect)
+		}
+	}
+}