Support multiple event handlers for the same event on the same element

[SteveSandersonMS]

Currently, Blazor only allows a single onsomething for a given element. A consequence of this is that:

• if you're using @bind you can't also use @onchange (because the framework is using it)
• if you're using @bind:event="oninput" you can't also use @oninput (because the framework is using it)

This is a limitation we should be able to relax, and would help fix issues like #14242 (comment)

Ways we could fix it:

• At compile-time, by making the Razor compiler able to detect duplicate @onsomeevent (including ones it creates itself via @bind) and collapse them into a single @onsomeevent that calls a code-genned method that runs all the handlers.
  • Some design would be needed to decide on execution order. Do we run them in parallel, or sequentially? Do we inject StateHasChanged calls that run after each of the returned tasks complete, so as to preserve the usual updating behavior?
• At runtime, by changing the diff algorithm to be able to handle multiple values for a given-named attribute. It might be tricky to do this while preserving current perf characteristics, but I haven't attempted yet.
  • We'd also need the ts-side code to understand that, when a certain DOM event occurs, it might need to trigger multiple event handlers. If it currently stores the eventname->handler_id mappings as a dictionary keyed by eventname, it would have to be generalised.

[rynowak]

a single @onsomeevent that calls a code-genned method that runs all the handlers.

Let's be careful when we get here - this kind of solution will be really tricky because we don't want to influence evaluation order.

[SteveSandersonMS]

Agreed it's tricky.

One especially difficult bit, that might even be fatally difficult to most designs, is making this work alongside our existing AddMultipleAttributes-overriding logic. Currently we reason that, since you can only finish with one attribute with a given name, we can have a precedence rule whereby later instances overwrite earlier ones. It's unclear how this makes sense in a world where you're allowed to finish with multiple instances of a given attribute - how do we know when to overwrite, and when to duplicate?

Maybe the distinction is just "event handlers combine, other things overwrite" but I'm not confident that's desirable without a lot more careful consideration.