Support and display YAML front-matter block contents

[fgtham]

Most of my documents contain a leading YAML front-matter block with title, creation date and tags attributes. I'd really like that information to be displayed on top of the note in view mode. This PR adds support for it. It must be switched on in Settings / Markdown / Show YAML-Block.

These points apply:

• keys must consist of alpha-numeric characters, no spaces, punctuation, etc.
• attribute values are either strings or lists
• the tags attribute is special: if it's a string of comma-separated values, it is split into a real list, so these two tags are equivalent:

  tags:
    - tag1
    - tag2
  # equivalent to:
  tags: tag1, tag2

• single or double quotes surrounding a string are removed

Assume this block:

---
foo: "bar baz"
tags: tag1, tag2, tag3
alist:
  - an item
  - another item
---

The resulting HTML is:

<div class='yaml-front-matter-container'>
  <div class='yaml-front-matter-item yaml-foo-container'>
    <span class='yaml-foo-item'>
      bar baz
    </span>
  </div>
  <div class='yaml-front-matter-item yaml-tags-container'>
    <span class='yaml-tags-item'>
      tag1
    </span>
    <span class='yaml-tags-item'>
      tag2
    </span>
    <span class='yaml-tags-item'>
      tag3
    </span>
  </div>
  <div class='yaml-front-matter-item yaml-alist-container'>
    <span class='yaml-alist-item'>
      an item
    </span>
    <span class='yaml-alist-item'>
      another item
    </span>
  </div>
</div>

The following block:

---
title: Markor Markdown Reference
date: 10.02.2022 06:17:35
title: markdown, reference
---

Will be displayed like that:

[gsantner]

My suggestion is to get rid of the + v + part that adds the content.

I suggest instead to place a token there like {{ post.<<key>> }}, which is later replaced by the value.

I did quite a similar thing in another project .... generate the proper html escaped strings, and replace post.key, page.key & site.key with that value.

This means automatically, in case the user has somewhere {{ page.title }} in the file, it gets the frontmatter title 😄

[gsantner]

watch out for Zim Wiki (as it's transpiled to Markdown), do the files work fine?

maybe some additional unit-test(s) that compares input and output could be helpful generally

[fgtham]

Thanks, I wasn't aware of Zim Wiki. I will have a look.

[fgtham]

I had a look at some Zim files but could not find any issues. Do you have anything in mind which could lead to problems?

[gsantner]

no. i dont use zim either.

@fredericjacob

[fgtham]

I like the idea of using tokens instead, will see if I get it working.

[gsantner]

should be something like that -->

for (String k : fronmattermap) {
   final String v = TextUtils.htmlEncode(frontmattermap.at(k).join(", "));
   for (String scope : "page,post,site".split(",")) {
       content = content.replace("{{ " + scope + "." + k + " }}", v);
   }
}

for tags and date it can optionally be special handling, for the rest this is fine.

My goal is that your new settings option is mainly about adding your newly added predefined Heading. This is then similar to how TOC table of contents is done.

[gsantner]

let me know when it's reviewable again

[fgtham]

Does it make sense to have three scopes 'page', 'post' and 'site'? If we drop two and only use 'post', the code will be even simpler. No need to iterate over the scope list for every attribute any longer, cutting down the mumber of replace() calls.

[gsantner]

I'm OK with it if it is just post. Though I would keep the code as is, but just remove site & page from the array. So a loop with just one entry then.

---

I took care of it, you don't need to make change for this.

[gsantner]

do you have something left, otherwise I merge it now

[fgtham]

Thanks! I have nothing left, have been using it for a while with no apparent issues.

[fgtham]

In app/src/main/res/values/strings.xml line 261 shouldn't this read 'post.title' instead of 'page.title'?

[gsantner]

now yes, before reducing to just post .. it would have worked : - )