WPF access viewmodel from Converter

Question

I have a Label that translates a code inserted by the user to the description. For that I am using a Dictionary inside a converter. This Dictionary is filled in every call to the Converter throw a Service. This is a really ugly thing, and I wanted to have this dictionary in my viewmodel but I have no ideia how to access it from the converter.

Any ideia?

Answer 1

In your view, bind the dictionary as a resource (supplied by the ViewModel). Change the converter to an IMultiValueConverter and use a <MultiBinding> to bind it to both the value that you need to use for lookup, and to the dictionary that you're looking up in.

See http://msdn.microsoft.com/en-us/library/system.windows.data.imultivalueconverter.aspx for more information on converters taking multiple input values, and an example of using one with a <MultiBinding>.

Alternatively, as GazTheDestroyer proposed, put it in your ViewModel.

class MyViewModel : INotifyPropertyChanged
{

   public MyViewModel()
   {
      // Call service to populate _dictionary here...
   }

   private Dictionary<string, string> _dictionary;
   private string _code;

   public string Code
   {
      get { return _code; }
      set
      {
         _code = value;
         Description = _dictionary[_code];
         if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs("Description"));
      }
   }

   public string Description { get; set; }
}

And instead of binding through a converter, bind to the Description property.

UPDATE

To answer your question about updating, you'll need to change the binding in your XAML slightly to change the UpdateSourceTrigger. The default for a Text property is to update when the control loses focus, this will change it to every time the value changes:

<TextBox Text="{Binding Code,UpdateSourceTrigger=PropertyChanged}" />

Further reading is available at http://msdn.microsoft.com/en-us/library/system.windows.data.binding.updatesourcetrigger.aspx

You may have to add some error validation here - every keystroke will alter the value of Description so you may want to include some logic in your ViewModel to only change the value when there is a value to use, or grey out a value that does not correspond to the (half-finished) code. These are usability things though, you can play around to find what is best for your application.

Answer 2

If you put it in the viewmodel you won't need a converter.

Simply expose a Code property in your VM and bind the label to that. Then in the Code setter you use your dictionary to update your Description property.