[Blazor] Cannot bind custom checkbox element's `checked` value

[CuddleBunny]

Describe the bug

Cannot bind custom checkbox element's checked value.

I am using the new FAST web components in a Blazor server side application and trying to bind the checkbox component any way I can, referencing <input type="checkbox" />:

<!-- Works! -->
<input type="checkbox" @bind="MyBool" />

<!-- Does not work... -->
<fast-checkbox @bind-checked="MyBool" @bind-checked:event="onchange"></fast-checkbox>
<fast-checkbox checked="@MyBool" @onchange="@((ChangeEventArgs e) => ViewModel.AcceptTerms = (bool)e.Value"></fast-checkbox>

I know the values themselves are binding because changing the native input will cause the fast components to change to match. The problem seems to lie within the event. The event mapping code has a check built in specifically for checkboxes which causes the change event value to fall back to element.value, in this case e.Value is always "on".

Rather than making an exception for checkboxes, could we simply always include both value and checked in ChangeEventArgs to match the JavaScript behavior? If this approach is acceptable I will open a PR.

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[a11delavar]

I just had a similar problem

@* Works *@
<input @bind="@Value">

@* Nope! *@
<fluent-text-field @bind="@Value"></fluent-text-field>

The value is: @Value

It seems Blazor's @bind attribute can not listen to the events dynamically.

It is a major problem, as it causes incompatibility with custom elements e.g. the new fluent's web components do not work in Blazor properly.

[CuddleBunny]

@arshia11d for custom elements you need to specific an event. Try:

<fluent-text-field @bind-value="Value" @bind-value:event="onchange"></fluent-text-field>

[a11delavar]

@CuddleBunny Thanks
Yes, the event mapping must be explicit for custom elements, and it works just fine.

I just found out what my problem was. It seems that changing value attribute is not mapped to the value property of the fluent-text-field component.

In Web Components, it is common to map public primitive properties like value to an HTML-attribute by setting a static attributes getter in the component class and listen to those changes using attributeChangedCallback to update the property, which FAST and Fluent web components don't support(?) for some reason.

So in my case Blazor actually did set the HTML attribute value but the web component didn't react to it, which causes the data-binding to work one-way, which was unexpected for me.

Could it be the same thing for you with the checked attribute?

[CuddleBunny]

@arshia11d - two-way works for my checkbox. If I bind a native checkbox input to the same value they will both update to match. Blazor just doesn't properly populate ChangeEventArgs because it only does so if the target is input type="checked":

aspnetcore/src/Components/Web.JS/src/Rendering/EventForDotNet.ts

Lines 218 to 220 in 1455aae

	function isCheckbox(element: Element | null): boolean {
	return !!element && element.tagName === 'INPUT' && element.getAttribute('type') === 'checkbox';
	}