如何实现异步 ValidationAttribute.IsValid()

Question

I have a use case where, when the user enters an email in a form, I need to verify it's unique. This is for registering as a new user and every user email in our app must be unique.

The call to do this takes time as it's hitting a database. Therefore I'd like it to be async. So would the following work and how do I accomplish the below problematic steps.

What if I use CustomValidation instead of ValidationAttribute. This works great for synchronous complex validation. But making it async...

<EditForm EditContext="_editContext" OnValidSubmit="HandleValidSubmitAsync" OnInvalidSubmit="HandleInvalidSubmitAsync" Context="EditFormContext">
    <DataAnnotationsValidator />
    <CustomValidation @ref="_customValidation" />

When it is called for validation it kicks off a background Task and returns an error of "checking email..."

Now here's where I don't know exactly what to do. When the task completes:

1. How do I update the CustomValidation object, either saying the email is valid or updating the message to now say the email is a duplicate?
2. If I can simply update _customValidation.ClearErrors()/DisplayErrors(), can I call it in the thread the task completes in? Or do I need to do something similar to WinForms where I invoke a method that will be called in the main/UI thread?
3. How do I then tell the form that the CustomValidation object has changed, possibly now allowing a submit?

If all I can do when the task completes is update an object and then force the form to re-validate the email, I can make that work. But in that case I still have #3 above, how do I make that happen?

Answer 1

You can implement custom validation like this:

EditContext editContext;
ValidationMessageStore messageStore;

protected override void OnInitialized()
{
    editContext = new(yourModel);
    messageStore = new(editContext);

    editContext.OnValidationRequested += Validate;
}

void Validate(object sender, ValidationRequestedEventArgs args)
{
    messageStore.Clear();

    if (yourModel.RequiredObject == null)
        messageStore.Add(() => yourModel.RequiredObject, "This object is required!");

    editContext.NotifyValidationStateChanged();
}

Then in your method used for data submission you can call editContext.Validate() , which returns a bool based on whether validation succeeded or not, or use the editContext object in your EditForm component.

There are of course ways, but you need to ditch EditContext for validation.

Answer 2

The Validation process is synchronous. It's based on standard events.

"Submitting" the form calls Validate on the EditContext which looks like this:

    public bool Validate()
    {
        OnValidationRequested?.Invoke(this, ValidationRequestedEventArgs.Empty);
        return !GetValidationMessages().Any();
    }

This invokes OnValidationRequested and then checks for validation messages. This is the standard "fire and forget" event process. It doesn't await any of the handlers, so will always complete before any yielding handler completes, and potentially provide the wrong result.

You need to write your own validation if you want async behaviour.