Wpf prism Notify property changed with validation not set

Question

I just started using a prism with wpf and don't understand why my property is not being updated. I have a binding to a text block with validation and it works until I delete the last character. I looked through the debugger, and the set property is not called, but the validation method is called. Also, I don't understand how updating the can executing method works. It fires returns true when I enter a character in a textbox, but after deletion it does not update. I will be grateful for the answer, here is my code.

My viewmodel (this command in constructor)

SaveCommand = new DelegateCommand(ExecuteSaveCommand, CanExecuteSaveCommand);

 public string ImageTitle
 {
      get => _userImageModel.Title;
      set
      {
          _userImageModel.Title = value;
          RaisePropertyChanged(); 
          SaveCommand.CanExecute();
      }
 }

private bool CanExecuteSaveCommand()
{
        var x = string.IsNullOrWhiteSpace(_userImageModel.Title) == false || 
                                              _userImageModel.Title!=null;
        return x;
}

My validation rule

public class UserImageValidator : ValidationRule
{
    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
        if (value == null)
            return new ValidationResult(false,"value cannot be empty");

        if(!(value is string propertyValue))
            return new ValidationResult(false,"exception");

        if(string.IsNullOrWhiteSpace(propertyValue))
            return new ValidationResult(false,"Required");

        return ValidationResult.ValidResult;
    }
}

My view

 <TextBox
        Grid.Row="0"
        Grid.Column="1"
        MinWidth="200"
        Margin="5"
        VerticalAlignment="Center"
        MinLines="4"
        Validation.ErrorTemplate="{StaticResource ErrorTemplate}">
        <TextBox.Text>
          <Binding Path="ImageTitle" UpdateSourceTrigger="PropertyChanged">
            <Binding.ValidationRules>
              <validateRule:UserImageValidator />
            </Binding.ValidationRules>
          </Binding>
        </TextBox.Text>
      </TextBox>

Answer 1

I don't understand how updating the can executing method works.

You call RaiseCanExecuteChanged on the command instead. The framework calls CanExecute to determine whether a button is enabled or not, for example.

Also, string.IsNullOrWhiteSpace(_userImageModel.Title) == false || _userImageModel.Title!=null string.IsNullOrWhiteSpace(_userImageModel.Title) == false || _userImageModel.Title!=null doesn't make a lot of sense ( != null is true for a white space string), do you mean .string.IsNullOrWhiteSpace( _userImageModel.Title ) ?

Answer 2

check this property, _userImageModel.Title, it sets the property there. Also SaveCommand is not hooked to the textbox.

Answer 3

I found a solution to my problem. As far as we use validation, it is logical that if the value is incorrect, it will not be set. This can be avoided by setting the validation step = Committed Value. However, after that, in the validation class, we will not get the value, but the expression binding (to get the value, just use the solution on this link ValidationRule with ValidationStep="UpdatedValue" is called with BindingExpression instead of updated value