[Blazor] Finalize behavior of JavaScript initializers for Blazor Web apps

[MackinnonBuck]

Open questions with JavaScript initializers:

• Should we update the arguments to beforeStart to make it easier to detect which .NET runtime (Server/WebAssembly) is starting?
• In the case of Blazor WebAssembly, should we wait to invoke beforeStart until we know for sure that Blazor WebAssembly is starting? Currently, beforeStart gets invoked during the Blazor WebAssembly loading procedure, but this doesn't always indicate that Blazor WebAssembly will be used (consider auto mode).
• In the case of Blazor Server, should the afterStarted callback run each time a new circuit is created? We plan on closing circuits automatically if all Blazor Server components get removed from the page.
• Should beforeStart execute immediately after Blazor.start() is called when using blazor.web.js? Currently, beforeStart only executes just before Blazor WebAssembly or Blazor Server starts up.
  • Or, alternatively, should the module be imported as soon as Blazor.start() gets called, but beforeStart only gets called when Blazor Server or WebAssembly starts?
• Should we add arguments to the afterStarted callback to indicate which .NET runtime started?

[MackinnonBuck]

After discussing offline, this is what the current thinking is:

Legacy initializers (beforeStart, afterStarted) will not get invoked in Blazor Web apps by default. However, they can be enabled by specifying a new option that gets passed to Blazor.start().

We will add new beforeRuntimeStart and afterRuntimeStarted callbacks that accept arguments allowing the callback to differentiate between the Blazor Server and Blazor WebAssembly cases. These callbacks will only be executed once per runtime (Server/WebAssembly).

We will also add a new API on the options object passed to Blazor.start() that lets developers listen to the Blazor Server circuit starting, since that can happen multiple times after Blazor Server initialization.