WASM demo takes 40 seconds to load and is not interactive

[sffc]

During the 40 seconds it takes to download the latest full-data WASM demo, the page is not interactive.

Short-term solutions:

1. Go back to using testdata
2. Add a loading spiral that goes away when WASM is finished loading (this would take about 10 lines of code)
3. Print an error upon input if the WASM is not loaded

Medium-term solution: download the locale data on demand. Steps to make that work:

1. The wasm-demo datagen should generate a directory of language-specific postcard files
2. A data provider should be hooked up that informs JS when root data is being used
3. JS code that fetches the requested language pack when that callback occurs, and then re-tries the rendering

Steps 1 and 3 should already be achievable via public APIs. Step 2 is doable in Rust but may need an FFI hook. It will be easier to write once callback support is added.

[Manishearth]

Without callbacks we can possibly make it work by having an introspecting adapter like you added in #2766

[ashu26jha]

Hello, can I take this one? My WASM demo is now running

I think short-term solutions has already been implemented by @sffc and implementing the medium term solution remains

[sffc]

Yes! This should be a good starter project now that you have the wasm running in your development environment. Happy to review an implementation plan or answer any questions.

[ashu26jha]

Here is my findings so far, feel free to point anything out:

1. As we are currently using ICU4XDataProvider.create_compiled() which is not an efficient approach. We could have the following directory specification, if we want to use fixed_decimal with bn locale:

   |-generated
     |-fixed-decimal
       |-bn.json 
     |-segmenter
     |-date-time
   

   Now to download the locale, we could fetch to get locale data from something like:

   https://raw.githubusercontent.com/unicode-org/icu4x/main/provider/datagen/tests/data/json/decimal/symbols%401/bn.json
   

2. We need to keep track whether root data is being used or not. It should use an FFI hook to call a JavaScript function if root data is being used and display the message in our frontend. (I am slightly unsure about this one, apologies)

3. This one should be comparitively easy as we need to re-render

Thoughts and question about second point:

1. I am unsure about how js be informed, as diplomat is used to call into Rust libraries that too in a unidirectional fashion.
2. I am still learning about diplomat so will update this comment if I could find something.

[Manishearth]

It should use an FFI hook to call a JavaScript function if root data is being used and display the message in our frontend

We don't have callback support for Diplomat and are unlikely to have it soon. The introspection should be done on the JS side using the APIs from icu_capi, we can add more if needed.