Cache invalidation issues on custom HTML transformer

[Diegovsky]

Hi, I have a custom transformer that generates HTML. It seems that parcel doesn't do normal post-processing of HTML if it came from this transformer. Following the guide, my transformer looks like this:

export default new Transformer({
  async transform({asset,options}) {

    // generates html from source file
    let html = await runHtmeta(asset.filePath);

    let deps = html.matchAll(/@(import)|(include) "(.*?)"/g);
    
    for (let dep of deps) {
      //  Tried adding dependencies but it also doesn't work
      asset.addDependency({
        specifier: `./${dep}`,
        specifierType: 'kdl'
      });
    }
    // had to add this so the gen'd html is invalidated on source change
    asset.invalidateOnFileChange(asset.filePath)
    asset.type = 'html';
    asset.setCode(html);

    // Return the asset
    return [asset];
  }
});

I've noticed that it doesn't invalidate HTML deps like imported TS and CSS, despite knowing about them in the first build, which makes me always delete the generated files to invalidate the cache. However, the actual contents of the generated HTML do change.

I assume the generated HTML goes through the usual steps, as it does generate a dependency tree, but it seems that caching does not take this into account.

Am I doing something wrong? (Besides not setting a source map)

Originally posted by @Diegovsky in #10086

[Diegovsky]

My original discussion didn't get any answers so I'm reposting as an issue to hopefully get some insights.

Also, I would love more pointers into how I would write a custom HTML transformer, sort-of in a tutorial form. I wrote what I have reading the docs, but I think I'm still missing some bits.