Add method to indicate an event handler should enforce value consistency

In other words, a way to tell Blazor that a given event handler is part of a two-way binding even if you’re not using @bind.

It would probably look like this:

<input value="@MyValue" @onchange="MyHandler" @onchange:enforceConsistency />

… or maybe @onchange:sync or @onchange:twoWay. The implementation is simply to emit a builder.SetUpdatesAttributeName call for whatever is the “value” attribute for this element type (e.g., on <input type=checkbox> it would call builder.SetUpdatesAttributeName("checked");).

For more details about the scenario and justification for this design, see https://github.com/aspnet/AspNetCore/issues/17099#issuecomment-556825048

[JvanderStad]

Looking forward to this one!

[hultqvist]

I’m looking for a workaround. This one appears to work though there’s some magic in there.

async Task MyHandler(ChangeEventArgs args)
{
    var old = Text;
    Text = args.Value as string;
    await Task.Delay(1); //Magic!
    Text = old;