How to detect if an item in my ObservableCollection has changed

Question

I have a datagrid which is bound to ObservableCollection<Product>. When the grid is updated this automatically updates the Product object in my collection.

What I want to do now is to have some sort of even that is triggered when any object in the collection is updated -or- some sort of binding to the collection that will return true/false depedant on if any Product has been updated.

The overall objective is to have a save button on my main window that is disabled if no changes have been made to my collection and enabled if changes have been made.

I have read into INotifyPropertyChange but I dont see how I can use this to monitor changes on a whole collection.

Additionally, if I implement this interface on my Product class I dont see how my UI can monitor every product in the collection - or can it?

Answer 1

• Implement INotifyPropertyChanged in your Product class with notification for every property.
• Implement INotifyPropertyChanged in your viewmodel.
• Add property IsDirty to your ViewModel (with notification through INotifyPropertyChanged.

• In your viewmodel, subscribe to CollectionChanged

  public YourViewModel()
  {
      ...
      YourCollection.CollectionChanged += YourCollection_CollectionChanged; 
      ...
  }
  
  private void YourCollection_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs args)
  {
      if (args.OldItems != null)
          foreach(var oldItem in args.OldItems)
              oldItem.PropertyChanged -= YourItem_PropertyChanged;
  
      if (args.NewItems != null)
          foreach(var newItem in args.NewItems)
              newItem.PropertyChanged += YourItem_PropertyChanged;
  }
  
  private void Youritem_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs args)
  {
      IsDirty = true;
  }
  

• Now you can bind to IsDirty property of your viewmodel, for example, you can bind Button.IsEnabled property directly to it.

Answer 2

Just use the ObservableCollection. It has an event called CollectionChanged. If you register it, you can do what you want. Example:

ObservableCollection<string> strings = new ObservableCollection<string>();
strings.CollectionChanged += new System.Collections.Specialized.NotifyCollectionChangedEventHandler(changed);
strings.Add("Hello");
strings[0] = "HelloHello";

And:

private void changed(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs args)
{
    //You get notified here two times.
}

Answer 3

The logic needs to go in your Model (Product class). A clean approach would be to expose IsDirty property (backed by field) in your model.

And your ViewModel would have a Command binding with CanSave checking the internal collection, and return true if Any of the item in collection IsDirty=true.

Answer 4

I think subscribing to the PropertyChanged event for each of the objects in your collection and firing this event, for example, in the setter of your objects can work.

However, I think you don't need to do all this to figure out if a cell is changed in your grid. I think you can do something like what they do here instead:

http://social.msdn.microsoft.com/Forums/en/wpf/thread/81131225-90fb-40f9-a311-066952c7bc43