# Custom Blocks

You can define custom language blocks inside *.kdu files. Loaders applied for a custom block are matched based on the lang attribute of the block, the block's tag name, and the rules in your webpack config.

If a lang attribute is specified, the custom block will be matched as a file with the lang as its extension.

You can also use resourceQuery to match a rule against a custom block with no lang. For example, to match against <foo> custom blocks:

{
  module: {
    rules: [
      {
        resourceQuery: /blockType=foo/,
        loader: 'loader-to-use'
      }
    ]
  }
}

If a matching rule is found for a custom block, it will be processed; otherwise the custom block will be silently ignored.

Additionally, if the custom block exports a function as the final result after being processed by all the matching loaders, that function will be called with the component of the *.kdu file as a parameter.

# Example

Here's an example of injecting the <docs> custom blocks into the component so that it's available during runtime.

In order for the custom block content to be injected, we'll write a custom loader:

module.exports = function (source, map) {
  this.callback(
    null,
    `export default function (Component) {
      Component.options.__docs = ${
        JSON.stringify(source)
      }
    }`,
    map
  )
}

Now we'll configure webpack to use our custom loader for <docs> custom blocks.

// wepback.config.js
module.exports = {
  module: {
    rules: [
      {
        resourceQuery: /blockType=docs/,
        loader: require.resolve('./docs-loader.js')
      }
    ]
  }
}

We are now able to access the <docs> block's content of imported components during runtime.

<!-- ComponentB.kdu -->
<template>
  <div>Hello</div>
</template>

<docs>
This is the documentation for component B.
</docs>

<!-- ComponentA.kdu -->
<template>
  <div>
    <ComponentB />
    <p>{{ docs }}</p>
  </div>
</template>

<script>
import ComponentB from './ComponentB.kdu';

export default {
  components: { ComponentB },
  data () {
    return {
      docs: ComponentB.__docs
    }
  }
}
</script>