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                                                                                                                                                                Play UI loaders are packages that extend the capabilities of the OOHTML Bundler. Play UI comes with certain loaders built-in and also makes it possible to provide custom loaders.


                                                                                                                                                                Loaders are functions that are called with each file during the bundling process. Multiple loaders are made cascaded and a loader will be expected to call the next. This makes for an awesome processing pipeline for each file being bundled. But it also requires thoughtfulness in the order in which these loaders are specified.

                                                                                                                                                                By default, the main Play UI bundler only handles .html files and images (.ico, .png, .jpg, .jpeg, .svg). Then it features built-in loaders that extend the list.



                                                                                                                                                                The md-loader loader is used to load .md (markdown) files into HTML exports just the way regular HTML files are. It takes an initial step of converting the markdown content into HTML using the Showdown library, then goes ahead to add it to the bundle as a module export. Markdown links are automatically resolved to better work as HTML links. A few other transformations are supported through arguments/flags. (Learn more about specifying arguments/flags for a loader here.)


                                                                                                                                                                All parameters are optional.

                                                                                                                                                                • base_url - Set this to a value that will be used as the base URL for relative links. This is similar to how the [ASSETS_PUBLIC_BASE] option works.
                                                                                                                                                                • outline_generation - Set this to a non-empty value to generate a JSON outline of the page's content. The generated outline will show up in the meta data for the file in the bundle's overall JSON outline.
                                                                                                                                                                • code_highlighting - Set this to a non-empty value to transform code blocks into stylable markup using the Showdown-Highlight utility. The transformed code blocks are highlighted in the UI on adding any of the Highlight.js CSS to the page.
                                                                                                                                                                • flavor - This equates to any of Showdown's three flavours: original, vanilla, github.


                                                                                                                                                                • Markdown Metadata - By default, md-loader automatically parses any found markdown metadata (defined at the top of the document between ««« and »»» or between --- and ---) into JSON and this will show up in the meta data for the file in the bundle's overall JSON outline. Below is an example metadata:
                                                                                                                                                                  description: Page description.
                                                                                                                                                                  # Page Title
                                                                                                                                                                • Markdown Tables - The markdown table syntax is supported by default. Below is an example table:
                                                                                                                                                                  | h1    |    h2   |      h3 |
                                                                                                                                                                  | 100   | [a][1]  | ![b][2] |
                                                                                                                                                                  | *foo* | **bar** | ~~baz~~ |


                                                                                                                                                                The md-loader loader is used by specifying default:md-loader in the [LOADERS] config option.

                                                                                                                                                                Custom Loaders

                                                                                                                                                                Custom Loaders are easy to develop. They are basic functions that recieve the current file being processed along with a few paramters, to return a fully-formed module export markup.


                                                                                                                                                                export default function loadsCSS(resource, params, args, received, meta, next) {
                                                                                                                                                                  if (received || !resource.endsWith('.css')) {
                                                                                                                                                                      // Or let the flow continue
                                                                                                                                                                      return next(received);
                                                                                                                                                                  // Return a module export format
                                                                                                                                                                  return '<style exportgroup="">...</style>';


                                                                                                                                                                • resource: String - The filename of the resource being processed - relative to the current working directory (CWD).
                                                                                                                                                                • params: Object - The options object that the Bundler was initialized with. It also has an additional property indentation - a number which represents how deep in the source directory the given resource is. This number is 0 at the root of the source directory.
                                                                                                                                                                • args: Object - The args object defined specifically for the loader.
                                                                                                                                                                • received: String - The output forwarded (that is, next()ed) by the previous loader in the list, if any.
                                                                                                                                                                • meta: Object - The meta object for the given resource. Properties can be added and they will show up in the meta data for the file in the bundle's overall JSON outline.
                                                                                                                                                                • next: Function - A function that forwards control to the next loader in the list, if any, and if none, to the default internal loader. It accpets only one parameter, which is received by the next loader on its received parameter. (When any value passed this way reaches the internal loader, it is added to the bundle as-is without any further processing of the resource.)

                                                                                                                                                                Return Value

                                                                                                                                                                Loaders are expected to return a valid markup string as the module export for the resource.

                                                                                                                                                                Error Handling

                                                                                                                                                                Loaders may throw exceptions, they will be gracefully handled, that is, formatted and printed to the console.


                                                                                                                                                                Custom loaders are used by specifying their filename in the [LOADERS] config option. Loaders installed as an npm package are used by specifying their package name.