Any way for a dependency property on a UserControl execute a command on it's view-model on value change?

Question

Suppose there's a UserControl with a dependency property (a collection of some object) - is there anyway for, when the dependency property is set, to automatically execute a command exposed by the view-model without breaking encapsulation and doing this the "MVVM way"?

And yes I know I can always wire-up the property changed static handler, cast the object to an instance of my user control, grab the data-context, cast that to the view-model and call the command manually

EDIT: maybe an example would help. My UserControl has a "ItemsSource" dependency property ..I want it to be settable to a collection of ObjectA. I have an "Items" dependency property which is only gettable and is a collection of ObjectB.

I want behavior such that if I set ItemSource, my collection of ObjectAs will be transformed one-to-one by the view-model into ObjectBs, and the Items dependency property will automatically reflect this.

To do this I want a write-only ItemsSource and a read-only Items property.

score 1 · Accepted Answer · edited May 23 '17 at 12:04

Just to make it clear:

You want a write-only DependencyProperty ItemsSource on the Control
You want a read-only Dependency-Property Items on the control's ViewModel
Items should return a transformed version and reflect changes of ItemsSource

If that is what you want, would that be a solution:

Add an ItemsSource-Property to the ViewModel which is the same type as in the control
Two-way-bind it to the control's DP
In the Set-accessor of the ItemsSource property of the ViewModel, fill the Items collection with transformed versions of the ItemsSource elements
Fire OnPropertyChanged for the Items property

ItemsSource of the Control and ViewModel is readable in this scenario. Write-only DPs are not possible as far as I know:

SO Thread about write-only DPs

However, this approach should work anyway.

Hope this helps, cheers...

score 0 · Answer 2 · answered Nov 08 '12 at 07:12

If you use ObservableCollection you can use it's CollectionChanged event.

ViewModel:

  private ObservableCollection<object> _Objects
  public ObservableCollection<object> Objects
  {
    get { return _Objects; }
    set { _Objects = value; 
           OnPropertyChanged(new PropertyChangedEventArgs("Objects"));
     }
  }

  Objects.CollectionChanged+= new System.Collections.Specialized.NotifyCollectionChangedEventHandler(Objects_CollectionChanged); 

 void Objects_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {

    }

Any way for a dependency property on a UserControl execute a command on it's view-model on value change?

2 Answers2