Event binding guidance improvements

[guardrex]

Fixes #35976

@danroth27 @javiercn ... Holding until we figure out the right guidance to provide.

I probably need a code example of the "mapping" part of the suggestion. The direction that I thought you were pitching is (and I'll use onkeyup as my example because onkeydown is probably best served by just oninput) ...

using Microsoft.AspNetCore.Components;

namespace {APP NAMESPACE};

[EventHandler("onkeyup", typeof(ChangeEventArgs),
    enableStopPropagation: true, enablePreventDefault: true)]
public static class EventHandlers
{
}

... but that with ...

<input @bind="boundValue" @bind:event="onkeyup" />

... throws an unhandled exception. Perhaps, it's because onkeyup is already mapped as a keyboard event argument.

One simple approach for this scenario is to use @on delegate event handler syntax ...

<input @onkeyup="@((KeyboardEventArgs args) => boundValue = args.Key)" />

... but that's not the ask on the issue, and it's clunky compared to @bind:event="onkeyup" syntax (if that can be made to work).

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[guardrex]

@danroth27 @javiercn @oroztocil @ilonatommy ... I'm stuck on a technical problem here. Anyone free to assist?

We describe onchange and oninput for binding. A dev asked about other events, which aren't wired up OOB to work. Dan, you said map the event to make it work. I'm trying to get the following to bind (boundValue is a field) ...

<input @bind="boundValue" @bind:event="onkeyup" />

Per the discussion on #35976, I thought that we'd just need ...

using Microsoft.AspNetCore.Components;

namespace {APP NAMESPACE};

[EventHandler("onkeyup", typeof(ChangeEventArgs),
    enableStopPropagation: true, enablePreventDefault: true)]
public static class EventHandlers
{
}

It throws an unhandled exception. Perhaps, it's because onkeyup is already mapped as a keyboard event argument.

One simple approach for this scenario is to use @on delegate event handler syntax ...

<input @onkeyup="@((KeyboardEventArgs args) => boundValue = args.Key)" />

... but that's not the ask on the issue, and it's clunky compared to @bind:event="onkeyup" syntax (if that can be made to work).

How can ...

<input @bind="boundValue" @bind:event="onkeyup" />

... be made to work?