Added ability to manipulate HTML-attributes with AST

[temapavloff]

Purpose: make possible to change output HTML attributes without re-implementing whole renderer.

For example, I want to add extra class to paragraphs which contains images. Now I can do it simply analyzing and changing AST before rendering content:

func TestASTModification(t *testing.T) {
	input := "\nPicture signature\n![alt text](/p.jpg)\n"
	expected := "<p class=\"img\">Picture signature\n<img src=\"/p.jpg\" alt=\"alt text\" /></p>\n"

	r := NewHTMLRenderer(HTMLRendererParameters{
		Flags: CommonHTMLFlags,
	})
	var buf bytes.Buffer
	optList := []Option{
		WithRenderer(r),
		WithExtensions(CommonExtensions)}
	parser := New(optList...)
	ast := parser.Parse([]byte(input))
	r.RenderHeader(&buf, ast)
	ast.Walk(func(node *Node, entering bool) WalkStatus {
                // Where the magic happens
		if node.Type == Image && entering && node.Parent.Type == Paragraph {
			node.Parent.Attributes.Add("class", "img")
		}
		return GoToNext
	})
	ast.Walk(func(node *Node, entering bool) WalkStatus {
		return r.RenderNode(&buf, node, entering)
	})
	r.RenderFooter(&buf, ast)
	actual := buf.String()

	if actual != expected {
		t.Errorf("Expected: %s\nActual: %s\n", expected, actual)
	}
}

[rtfb]

I love the idea! Let me sit on it for a few days to let the API design to sink in...

Thanks!

[bep]

While it would be cool to do something like the above, I don't think changing the AST is a good idea.

What I want to do with Blackfriday in Hugo is to parse once, render many. Adding HTML specific AST mods will break that. I think your approach could work if you somehow managed to create a copy of the node you want to "override".

Also, note that you don't have implement a full renderer to manipulate single nodes, as shown in my quick test repo below:

that https://github.com/bep/blackfriday-v2-test

You will end up with byte manipulation, which is less clean, but certainly possible.