Validation extensibility

[djanosik]

The validation framework in DotVMM is not extensible. We can't plug-in custom validation frameworks, client-side validators, error messages, etc. We should change validators to make that possible:

• IPropertyValidator
  • abstraction for ValidationAttribute, etc.
  • .IsValid() returns whether property value is valid
• IPropertyValidationRule
  • replaces ViewModelPropertyValidationRule
  • .Name can be used to localize error messages
  • .ClientRuleName identifies client-side validator to use
    • we will also need to make our scripts extensible
  • .Validator is an implementation of IPropertyValidator
  • .GetClientParameters() returns parameters for the client-side validator
  • .GetErrorMessage() returns error message
  • it should return localized error message when possible
  • we can either use IStringLocalizer or our own IErrorMessageLocalizer
• IValidationMetadataProvider
  • replaces IViewModelValidationMetadataProvider and IValidationRuleTranslator
  • should be registered in DotVVM configuration (collection ValidationMetadataProviders)
  • .GetRulesForProperty() returns implementations of IPropertyValidationRule
• ViewModelValidator
  • collects rules from metadata providers and validates a view model
  • we should handle IValidatableObject somewhere else
• default implementation for DataAnnotations
• sample for FluentValidation

Similar design should solve most of our issues with validation extensibility. But I am still not sure how to handle localization. Client-side rules are currently cached somewhere in DefaultViewModelSerializer so we can't change error messages at runtime. @exyi what do you think?

Do you have something to add?

[exyi]

cc @martindybal - you was experimenting with few improvements few weeks ago, anything to add?