Asset serving

[nicolaskruchten]

There's a surprisingly deep and complicated pipeline behind the generation of the component <script> tags that Dash drops into the index.html which it returns to clients on first load.

Minimally, DashR needs to serve script tags for locally served JS bundles that match the component libraries that are loaded in the initial layout.

This is on the critical path for #45

[nicolaskruchten]

[rpkyle]

Minimally, DashR needs to serve script tags for locally served JS bundles that match the component libraries that are loaded in the initial layout.

This is currently working. However, since this is minimal, but not sufficient, I'm filling in a few more details here.

Here's the list of features that must be available for the initial release:

• Assets folder auto resolution (/assets) at the root of the project by default.
• Assert that assets folder may be passed as parameter
• Assert that JS/CSS files in /assets are served in the index
• Assert that an ignore filter exists and works for served assets
• Assert that assets are cached by the browser, and uncached when modified
• Assert that all files in /assets are served statically
• Assert that the assets URL respects routes/requests pathname prefix
• Assert that easy retrieval of assets URLs is possible: get_asset_url(asset) should return a ready-to-insert URL string
• Assert that if a favicon.ico is found in the assets folder, it be served as the title icon of the page.

The order of operations:

1. Just prior to serving the index, walk assets directory, if present, and use ressource_route (sic) function to supply static routes to local resources.
2. Ensure that the static handler supplied by routr handles caching and MIME typing properly.
3. Check out the current Python test app, and implement something similar in R, since there's a logical ordering to asset serving.

See also:
https://github.com/plotly/dash/blob/master/tests/test_resources.py
https://github.com/plotly/dash/blob/1de85b387ae56f19d70b1c148953dce6ddb0ecfc/dash/dash.py#L1107-L1170

@T4rk1n