Basic usage

When a user loads a page of your website in their browser, it includes a number of <link ...>, <img ...>, <script ...> and other tags that tell their browser how to fetch these additional resources from the server.

When dealing with assets on the server, our application needs to address two problems:

  1. How do we locate a Sprinkle's assets, generate an appropriate URL, and inject the appropriate reference tags when rendering a template?
  2. When the client actually loads a page and requests an asset via the URL, how do we map the URL back to a file path on the server and return it to the client?

These questions are answered in this chapter.

Referencing assets

Since the client must issue a separate request to retrieve an asset after loading a page, we need a way to generate asset URLs in our templates. To automatically build a url for a single asset in a Twig template, you may use the assets.url() helper. This helper takes a file path to an asset, and generates an appropriate absolute url:

<img src="{{ assets.url('assets://userfrosting/images/barn-owl.jpg') }}">

You'll notice that we reference the file path using the assets:// stream wrapper. Stream wrappers allow UserFrosting to define a sort of virtual mini-filesystem for a particular type of resource (in this case, assets).

When we refer to an asset using the path assets://userfrosting/images/barn-owl.jpg, UserFrosting will use the locator service to search through each loaded Sprinkle's assets/ directory, starting with the most recently loaded Sprinkle, looking for a relative match to userfrosting/images/barn-owl.jpg. As with other sorts of entities, this allows us to override assets from previously loaded Sprinkles.

For example, suppose we have:

└── assets
    └── userfrosting
        └── images
            └── barn-owl.jpg

as well as:

└── assets
    └── userfrosting
        └── images
            └── barn-owl.jpg

Assuming we've loaded the account and site Sprinkles (in that order), we can now use the uri assets://userfrosting/images/barn-owl.jpg in our code, and UserFrosting will correctly resolve it to /site/assets/userfrosting/images/barn-owl.jpg.

Notice the directory pattern used to organise the assets. Generally speaking, the basic assets that come with UserFrosting go in a userfrosting/ subdirectory in each Sprinkle's main assets directory. You should put your own custom assets in a separate subdirectory at the same level, unless you actually need to override one of assets that ship with UserFrosting. This will avoid collision as you load more sprinkles.

Public asset URLs

Custom stream uris like assets:// will be correctly interpreted in your server-side code, but cannot be understood by clients' browsers. To serve an asset like this to the client, UserFrosting must generate a public http(s) URL for use in HTML (e.g. for <img>, <link>, <script>, and other tags).

This is handled automatically by the assets.url() helper in Twig. For example, the call to assets.url('assets://userfrosting/images/barn-owl.jpg') might generate a URL

The question is, how does this URL get correctly resolved by the server when it is requested? After all, there is no assets-raw/ directory in our project's public root directory. By default, all requests made to URLs beginning with /assets-raw/ are sent to a special route defined in the core Sprinkle. This route then uses the assetLoader service to resolve the request to an asset in a Sprinkle.

For example, will be resolved to the core Sprinkle, and respond with the contents of vendor/bootstrap-3.3.6/css/bootstrap.css (if it exists).

The assetLoader service will automatically try to determine the MIME type of the asset based on the file extension, and set the appropriate Content-Type header in the response.