Aspnetcore: @onevent:preventDefault does not trigger when event is not already registered

[poke]

When using the new @onevent:preventDefault directive to prevent the default event behavior for events, I noticed that in order for this to work, there needs to be a @onevent handler registered anywhere on the page.

Apparently, server-side Blazor registers every event just once on the document instead of on individual elements. So e.g. @onclick="…" causes the click event to be subscribed on the document. Just using @onclick:preventDefault on the page does not appear to register this global event handler on document, so the preventDefault() call is also never made in the handler.

My use case for this is managing focus with buttons: When you click a button, then the button stays focused after clicking it, causing the focused style to be applied. In order to avoid this, you can subscribe to the mousedown event in JavaScript and just prevent the default behavior:

<button id="test">Test</button>
<script>
    var button = document.getElementById('test')
    button.addEventListener('click', () => console.log('Clicked'));
    button.addEventListener('mousedown', (evt) => evt.preventDefault());
</script>

Doing the same in Blazor would look like this:

<button @onclick="Click" @onmousedown:preventDefault>Test</button>

@code {
    void Click() => Console.WriteLine("Click");
}

This however does not work if there isn’t also some mousedown handler registered somewhere. As soon as you add one literally anywhere, it works:

<button @onclick="Click" @onmousedown:preventDefault>Test</button>
<div @onmousedown="() => { }"></div>

@code {
    void Click() => Console.WriteLine("Click");
}

This is using server-side Blazor on ASP.NET Core 3.1.

[javiercn]

@poke thanks for contacting us.

@poke what you are mentioning makes sense.

@SteveSandersonMS do you have any thoughts here?

[javiercn]

@poke on a second look, I think the current behavior is actually fine.

I think that as a rule, for simplicity we want to keep the behavior where something:modifier attributes to only apply if you also have a something applied, like in this case mousedown.

The reason is for simplicity and consistency within the language. If we were to do so, every time we introduce a new modifier we will have to think if it makes sense to apply it in isolation or not, and that will make the rule for modifiers very complicated (as it would depend on the modifier being applied).

The alternative, even though it requires a bit more code is simpler to understand and explain. If you want to apply some modifier something:modifier you also need to apply something, even if that doesn't cause any action.

/cc: @SteveSandersonMS in case he has other thoughts.

[poke]

where something:modifier attributes to only apply if you also have a something applied

But that’s not what is the case here. Please note that in my example, I use @onmousedown:preventDefault on a button element while I have to apply @onmousedown on literally _any_ other element.

Your statement makes it sound as if you would need to put both on the same element like this:

<button @onclick="Click"
        @onmousedown="() => { }"
        @onmousedown:preventDefault>Test</button>

If that was the case, I would probably accept this (although I would really dislike it). But the thing is that you do not need @onmousedown on the element. You just need it somewhere throughout the whole app to make Blazor’s client side runtime register the event once on the document element.

The fact that this is so disconnected from the @onmousedown:preventDefault makes this very confusing, and definitely not _“simpler to understand and explain”_. And considering that this is preventDefault it is also very common to put this on events that you will usually not care about, so requiring an empty event handler just to make Blazor realize that the event handler is needed feels very redundant.

I understand the desire to keep the attribute:modifier handling simple but it shouldn’t be too problematic to make @event:modifier also trigger an empty registration of @event.

[SteveSandersonMS]

There's been a bit of confusion here. I think there is a bug to be addressed here.

I think that as a rule, for simplicity we want to keep the behavior where something:modifier attributes to only apply if you also have a something applied, like in this case mousedown.

That's not how this was designed - see #14517:

For each @oneventname, there's also @oneventname:preventDefault and @oneventname:stopPropagation. These can be used either alongside @oneventname or independently of it.

I do agree there are valid use cases for using preventDefault/stopPropagation independently of handlers for the same event on the same element; that's something we always intended to support and in fact do support today. The bug is when you happen not to have any handlers for a given event type anywhere in the document.

[poke]

@SteveSandersonMS Thanks a lot for the clarification :)

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