Type setting for theme metadata

[yhatt]

We are considering to support type setting for theme metadata specified in the metaType option in Theme#fromCSS, ThemeSet#add, and ThemeSet.metaType member.

Currently theme metadata only allows a single value. This behavior is very natural because Marpit only requires @theme metadata.

But in the extended class as like as Marp Core, there may be the meta key want to allow multi-time definitions. Thus, we should accept metaType option when creating Theme instance.

It is an object that has a key as the name of metadata and a value as the specified type which of String or Array. We will parse meta as String by default, included undefined meta key. The user has to set Array if the theme allows multi-time definitions in same meta key.

/**
 * @theme example
 * @foo Single value
 * @foo allows only one string
 * @bar Multiple value 1
 * @bar Multiple value 2
 * @bar Multiple value 3
 */

const themeSet = new ThemeSet()

themeSet.metaType = {
  foo: String,
  bar: Array,
}

themeSet.add(css)

console.log(themeSet.getThemeMeta('example', 'foo'))
// => 'allows only one string'

console.log(themeSet.getThemeMeta('example', 'bar'))
// => ['Multiple value 1', 'Multiple value 2', 'Multiple value 3']

It requires in proposed marp-team/marp-core#91: The proposed @size metadata requires multi-time definitions.

/**
 * @theme foobar
 * @size 4:3 960px 720px
 * @size 16:9 1280px 720px
 */

ToDo

• Allow metaType option value in Theme#fromCSS
• [ ] Allow metaType option value in ThemeSet#add
  • Typically using theme with different type definitions would make confusion. Option argument to override metatype had better not allow.
• Add metaType member in ThemeSet class to set the default type setting for ThemeSet#add
• Reserve @theme metadata as String and disallow changing type
• [ ] Support merging array in ThemeSet#getThemeProp
  • It will merge if the all of defined metadata that includes imported theme are Array.
  • There is concern reported in https://github.com/marp-team/marpit/pull/171/files#r294092779.
  • 👉 Add ThemeSet#getThemeMeta to get correct meta value with array support