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Extensibility

The validation engine in DotVVM offers several extensibility points. You can use your own validation attributes, and you can interact with the validation engine on the client-side.

Custom validation attributes

You can build your own validation attributes by implementing the IValidationAttribute class, or inheriting from the ValidationAttribute base class. See Custom attributes page in the official docs.

Client-side validation

Providing a client-side validation for custom attributes is possible by providing a custom implementation of the IValidationRuleTranslator interface, and then by registering a custom rule into dotvvm.validation.rules collection, but the process is quite complicated. We plan to make it easier in the future versions of DotVVM.

It is also possible to add any validation errors client-side. This can be used to implement validation in JS Commands, or to implement custom validation logic before commands.

The dotvvm.validation.removeErrors("/") function is used to remove existing validation errors. The "/" is the root path of the removal, this argument can be used to clear only certain parts of the view model. Any number of paths can be specified.

The dotvvm.validation.addErrors(errors) adds new errors. The errors argument should be an array of objects with

  • errorMessage - a string displayed to the user
  • propertyPath - an absolute validation path of the invalid property, e.g. /SomeArray/2/InvalidProperty.
myCommand() {
    const model = dotvvm.state.MyForm
    dotvvm.validation.removeErrors("/MyForm")
    const errors = []
    if (model.FirstName.length > model.LastName.length)
        errors.push({ errorMessage: "Persons with longer first name than their last name cannot be registered.", propertyPath: "/MyForm/FirstName"})
    if (errors.length > 0)
    {
        dotvvm.validation.addErrors(errors)
        return
    }
}

Validation events

You can subscribe to the client-side events called dotvvm.validation.events.validationErrorsChanged to be notified if the validation error collection has been changed.

You can also access the collection of all validation errors using dotvvm.validation.allErrors.

See also