Preload Blazor WebAssembly resources to improve startup time

[mkArtakMSFT]

We want to add preload links for all the files that are on the critical path (dotnet.js, dotnet.runtime.js, dotnet.native.js, dotnet.wasm, boot.js) so the browser can start downloading them eagerly.

In runtime sample we do https://github.com/dotnet/runtime/blob/main/src/mono/sample/wasm/browser-advanced/index.html#L13-L16.

In .NET 9 we have split list of assemblies to "core" (required to started MonoVM) and "others". Preloading core assemblies with lower priority also seems like a good fit.

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Implementation plan

• In WebAssembly SDK identify key assets and add preloading properties to endpoints (rel, as, fetchpriority, crossorigin, integrity)
  • Preload dotnet.js
  • Preload boot config
  • Preload dotnet.*.js
  • Preload dotnet.*.wasm
  • Preload core dlls with lower priority
  • Based on StaticWebAssetProjectMode=Default or OverrideHtmlAssetPlaceholders=true we can determine whether to use fingerprinted endpoint for dotnet.js and dotnet.boot.js
  • Based on measurements the best results provided inlining boot config to dotnet.js & preloading dotnet.js
• In StaticWebAssets SDK introduce a new placeholder for preloading meta tags in OverrideHtmlAssetPlaceholders=true feature
• In AspNetCore runtime (EndpointHtmlRenderer.WriteComponentHtml) read preloading properties from endpoints and supply them to the initial response as Link headers
  • Do it only for WebAssembly render mode, not in Auto mode

Additional tasks

• Consider merging boot config into dotnet.js based if preloading measurements prove it's benefitial

[alc-aardvark]

I am just commenting to support this, as when running on AWS using a static website hosted in an S3 bucket, then you cannot use the server to pre-render the first page.

I have tried pre-fetching the *.wasm assemblies using the solution from here but this does not work like the Blazor.start() as it doesn't load the assemblies into Webassembly so the Blazor.start still goes to fetch them again (and returns a 304 Not Modified). This is somewhat different behavior from a refreshing an already loaded Blazor app that was started via Blazor.start()

[alc-aardvark]

I have a statically hosted Blazor Wasm app in AWS S3 and I have used a hidden <div> in index.html to to the Blazor.start(); while the user is reading the landing page content. A button will then toggle the landing page div and the Blazor app div to show the Blazor app.

This works quite well except that on a mobile phone the UI locks up for 1-2 seconds at the end of the WASM load. It would be good if this was done in another thread so that it did not lock up the UI.

During the UI lockup, my carousel stops moving (then continues after the lock up) and button clicks aren't actioned until after the lock up.