feature: Possibility to change validation rules dymamically

[ili]

Hello, firstly thanks for your great project.

It would be great if validation would support dynamic validation rules change: for example, i do deed to configure some hardware, different options are available for different models, for ex. "Model A" can be connected either via TCP/IP or via RS232, "Model B" supports RS232 only, so for the first TCP/IP settings required, for the second - RS232.

So i do need smth like:

this.WhenAnyValue(_ => _.Model)
.Subscribe(m => 
{
    this.ClearValidationRules(_ => _.IpAddress);
    this.ClearValidationRules(_ => _.Rs232Port);
   //...

  if (m == Model.A)
      this.ValidationRule(_ => _.IpAddress, _ => !string.IsNullOrEmpty(_), "IP address required");

   //...   
}
)

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[glennawatson]

I think the validation rules are stored in a dictionary at the moment so should be possible. I got some other stuff happening at the moment so wouldn't be able to immediately get to it. But reasonable request.

[worldbeater]

Currently, you can achieve the described behavior by doing the following:

// Listen to Model and IpAddress changes and
// require IpAddress property to not be empty 
// only if Model equals to Models.A.
var ipAddressRule = this
    .WhenAnyValue(x => x.IpAddress)
    .CombineLatest(
        this.WhenAnyValue(x => x.Model), 
        (ipAddress, model) => 
            model == Models.A && 
            !string.IsNullOrEmpty(ipAddress) ||
            model == Models.B && ...);

this.ValidationRule(
    x => x.IpAddress, 
    ipAddressRule, 
    "IP address required.");

The code is based on a new feature introduced in #58

[worldbeater]

Now it should be possible to remove validation rules via a call to DisposeWith. E.g. the following code will attach an IValidationComponent to an IValidatableViewModel when we call this.ValidationRule(), and will detach the IValidationComponent when we dispose of the cleanup disposable:

var cleanup = new CompositeDisposable();

// Attach a validation rule to the IValidatableViewModel.
this.ValidationRule(
    viewModel => viewModel.Name,
    name => string.IsNullOrWhiteSpace(name),
    "Name shouldn't be empty.")
    .DisposeWith(cleanup);

// Detach and dispose the validation rule.
cleanup.Dispose();

[github-actions]

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