How do I properly implement Textbox Validation - wpf

Question

I am new to WPF and creating an application which as few text boxes. Text boxes are bound to some source using MVVM. Now when I click on save button, it should fire validation for all empty text boxes and save event should not be fired. How can I achieve this in WPF.
I have written validater but it is not called. See my code below:
<TextBox Width="250" Grid.Row="0" Grid.Column="1" Margin="10">
<TextBox.Text>
<Binding Path="ContinuousModel.FileName" ValidatesOnDataErrors="True" NotifyOnValidationError="True" Mode="TwoWay" ValidatesOnExceptions="True">
<Binding.ValidationRules>
<validate:RequiredFieldValidatation />
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>
public class RequiredFieldValidatation:ValidationRule
{
public override ValidationResult Validate(object value, CultureInfo cultureInfo)
{
if (value == null || value.ToString() == string.Empty)
return new ValidationResult(false, "Value cannot be empty");
return ValidationResult.ValidResult;
}
}
The above validator is getting called only if some text is written then change focus then come back and remove value and then change focus.
NOTE: Using MVVM and datatemplates, I am loading varios user controls and those controls are bound to viewmodel. Save button is in different user control so I cannot validate all text boxes manually on save button click event.

Answer 1

Add the UpdateSourceTrigger property to your binding;
<Binding Path="ContinuousModel.FileName" UpdateSourceTrigger=PropertyChanged ValidatesOnDataErrors="True" NotifyOnValidationError="True" Mode="TwoWay" ValidatesOnExceptions="True">
<Binding.ValidationRules>
<validate:RequiredFieldValidatation />
</Binding.ValidationRules>
</Binding>