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HOW_IT_WORKS.md

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How ngx-prefetch works

After adding a service worker to your project, as specified in the prerequisites, the modifications include the creation of a service worker configuration file called ngsw-config.json, which specifies the caching behaviors. Then the build process creates the manifest file ngsw.json in the dist folder using information from ngsw-config.json.

Create Angular CLI builder

Before all, if the mandatory options are provided and verified, the production output folder is retrieved. Then the creation of the builder consists of four steps:

  1. The first step is to read the ngsw.json file. This allows to create an array of resources based on the asset groups that have the installMode set to prefetch.
  2. The second step is to read the prefetch Mustache template file. This template is rendered in JavaScript using the object of variables that is passed as a parameter.
  3. The third step is to write the prefetch JS script using the previous template.
  4. The fourth step is to create a Compiler by bundling the JS file for usage in a browser using webpack.

When this compiler is run successfully, a new file ngxPrefetch.js is created in the production output folder.

ngxPrefetch.js file

Static content

The ngxPrefetch.js file contains a list of static resources that the Angular service worker should cache based on the ngsw-config.json file. The path where these resources are hosted can be defined in the builder options otherwise the default path is next to the ngxPrefetch.js file, on the same server.

Localization

In addition to the static content, the localization file can be prefetched in order to have the language information while performing the prefetch.

It is possible to set the language value at runtime by searching for {LANG} and replacing it on the server side. Then the localization pattern can be set in the builder options to get the relative path of the localization file.

Dynamic content

Dynamic content can also be prefetched in order to override some static content if wanted. There are two placeholders that can be replaced in the server:

  • {DYNAMIC_CONTENT_PATH}: the path where the dynamic resources are hosted ("mydynamiccontentpath" for example)
  • {DYNAMIC_CONTENT_FILES}: the path of the dynamic content files relative to the dynamic content path (expects a format like the following example: ["relativePath/myfile1", "relativePath/myfile2"])

Link elements in header

For each static resource of type string, a step is implemented to check if this same resource is present in the dynamic content. If this is the case, the full path of the static resource is overridden by the dynamic full path. Then, a link is appended to the html header of the webpage with the following properties:

  • A relationship (rel) of type prefetch
  • The type of content being loaded (as) if possible
  • Possibly the crossorigin attribute to indicate whether CORS must be used when fetching the resource
  • The URL of the linked resource (href)

Below is an example of the appended link:

<link rel="prefetch" as="type-of-content" crossorigin href="url-of-linked-resource">