UI Binding Validation in MVVM - wpf

Question

I'm working on converting some code to a more proper MVVM implementation using DataTemplates and am having problems with certain kinds of UI validation.
I've got no problems with validation in the View Models -- IDataErrorInfo is implemented and everything is fine. What I've got a problem with is UI binding errors where they might put letters in a TextBox bound to an int.
Previously, I used :
System.Windows.Controls.Validation.AddErrorHandler(userControl, handler)
... and kept a count of errors added and removed to know whether all the form's data was OK.
But now that I'm doing MVVM I don't have access to the userControl to set up this handler. So I don't really have a hook to get this started.
Is there some sort of global DataTemplateApplied event handler available where I could do something like:
void OnDataTemplateApplied(object data, Control template)
{
if (data is MyViewModelBase)
{
Validation.AddErrorHandler(template, handler);
}
}
Alternatively, maybe I can call AddErrorHandler once in the bootstrapper for the outer Shell window, and then each time the event is fired somehow figure out which ViewModel is powering that particular control?
I know some people like making all VM fields strings and doing lots of type conversion in the VM -- that's not going to be realistic for our system for a variety of reasons.

Answer 1

You might be interested in this answer: https://stackoverflow.com/a/13335971/1094526
The main idea is exactly what you said (subscribe to the error handler). As I understand, the problem is you don't have access to the control from the ViewModel, but it isn't hard to solve
In a project I'm working, I exposed two methods from my ViewModel: AddUIError and RemoveUIError. I create an event handler in my View and there I cast the DataContext to the type of my ViewModel and call AddUIError or RemoveUIError depending on what happened.
I am using DataTemplates to associate a View with a ViewModel, so when the template is applied, the DataContext is automatically set to the ViewModel. If you want, you can store your ViewModel in a private field (in the View) and update the reference each time the DataContext changed (there is a DataContextChanged event)
If this will be done in multiple ViewModels, you can put both methods (AddUIError and RemoveUIError) in a class like ViewModelBase and move the ValidationError event handling to a Behavior and use it in each view.
More info about the behavior part:
The Behavior class is part of the Expression Blend SDK, so you will need it if you want to follow this way.
Behaviors are useful to attach some common functionality to many components without creating derived classes, for example.
First, we need to define the AddUIError and RemoveUIError in a class named ViewModelBase (which is, of course, the base class for all other ViewModels):
class ViewModelBase {
public void AddUIError(...) {/* Details ommitted */ }
public void RemoveUIError(...) {/* Details ommitted */ }
}
Then, create a Behavior by subclassing Behavior. We use FrameworkElement as the template argument so this behavior can be attached to any FrameworkElement (or derived class) instance:
class NotifyDataErrorsBehavior : Behavior<FrameworkElement>
{
// Called when the the Behavior is attached
protected override void OnAttached()
{
base.OnAttached();
// Initialize the handler for the Validation Error Event
_handler = new RoutedEventHandler(OnValidationRaised);
// Add the handler to the event from the element which is attaching this behavior
AssociatedObject.AddHandler(System.Windows.Controls.Validation.ErrorEvent, _handler);
}
protected override void OnDetaching()
{
base.OnDetaching();
// Remove the event handler from the associated object
AssociatedObject.RemoveHandler(System.Windows.Controls.Validation.ErrorEvent, _handler);
}
private RoutedEventHandler _handler = null;
private void OnValidationRaised(object sender, RoutedEventArgs e)
{
var args = (System.Windows.Controls.ValidationErrorEventArgs)e;
ViewModelBase viewModel = AssociatedObject.DataContext as ViewModelBase;
if (viewModel != null)
{
// You can add only Exception validation errors if you want..
if (args.Action == ValidationErrorEventAction.Added)
viewModel.AddUIValidationError(...);
else if (args.Action == ValidationErrorEventAction.Removed)
viewModel.RemoveUIValidationError(...);
else
throw new NotSupportedException("ValidationErrorEventAction has changed");
}
}
}
And finally just use it in XAML:
1. Add a reference to the namespace where NotifyDataErrorsBehavior is located, and also a reference to System.Windows.Interactivity namespace (from Expression Blend SDK):
<UserControl
...
xmlns:behavior="clr-namespace:MyApp.Behaviors"
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
...
>
2. Add the behavior (at the same level as the content of your UserControl:
<i:Interaction.Behaviors>
<behavior:NotifyDataErrorsBehavior/>
</i:Interaction.Behaviors>
Ex:
<UserControl
...
xmlns:behavior="clr-namespace:MyApp.Behaviors"
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
...
>
<i:Interaction.Behaviors>
<behavior:NotifyDataErrorsBehavior/>
</i:Interaction.Behaviors>
<Grid>
...
</Grid>
</UserControl>