Question

I have a registration form with few fields. It´s a PRISM MVVM application.

XAML of one of the field looks like this (RegisterView.xaml):

<TextBlock>Surname</TextBlock>
<TextBox Validation.ErrorTemplate="{StaticResource validationTemplate}"      HorizontalAlignment="Left" Margin="0" Name="Surname" VerticalAlignment="Top" >
    <TextBox.Text>
        <Binding Path="Surname" UpdateSourceTrigger="LostFocus" >
            <Binding.ValidationRules>
                 <val:Required />
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

As you can see from the code above, I use class Required to validate the field. Function Validate() of class Required then returns ValidationResult object. I also defined some triggers to styles for inputs, so I´m able to show validation result to the user in view.

What I don’t know is how to detect the state of validation of all the inputs in ViewModel. In the ViewModel, I have the SaveUserCanExecute function which should enable/disable registration form submit button on the basic on a validation state of all the inputs.

So is there any simple way how to achieve this?

I could make some workaround for this, but I think is not the proper way.

Now I made a Submit_Click function in a View code behind fired on a Click event of a Submit Button.

In RegisterView.xaml

<Button Content="Register" HorizontalAlignment="Left" Margin="0" Name="Submit" VerticalAlignment="Top" Command="{x:Static inf:Commands.SaveUser}" Click="Submit_Click" />

I also created new public boolean variable “formIsValid” in code behind. When submit button is pressed, then I check whether all inputs has no validation error (with Validation.GetHasError(InputName) function). If so, I set the formIsValid variable to true, otherwise, I set it to false.

In RegisterView.xaml.cs

private void Submit_Click(object sender, RoutedEventArgs e)
{
    if (Validation.GetHasError(Firstname) == false && Validation.GetHasError(Surname) == false)
    {
        registerFormValid = true;
    }
    else
    {
        registerFormValid = false;
    }
}

Then in ViewModel SaveUserCanExecute function looks like this:

private bool SaveUserCanExecute(string parameter)
{
    if (View.registerFormValid == true)
    {
        return true;
    }
    return false;
}

But as I mentioned before, I think it´s not the proper way, and I´m looking for some more clear way.

Answer 1

implement IDataErrorInfo in your ViewModel then you have all information you need in your VM. Your XAML just need the ValidatesOnDataErrors=true

<TextBlock>Surname</TextBlock>
<TextBox Validation.ErrorTemplate="{StaticResource validationTemplate}"      HorizontalAlignment="Left" Margin="0" Name="Surname" VerticalAlignment="Top" >
        <TextBox.Text>
            <Binding Path="Surname" UpdateSourceTrigger="LostFocus" ValidatesOnDataErrors="True">
            </Binding>
        </TextBox.Text>
 </TextBox>

EDIT: check the use of DelegeCommand and then your command CanExecute can simply check then for string.IsNullOrEmpty(this.Error).