Question

I have a observable collection of viewmodel objects. How can I subscribe to the Property Changed event of each view model in my collection as they are created and track which ones have been changed, so that I can updated them to my database.

Answer 1

I believe that the code below serves as an example of how to solve your problem. In this example MyCollection is a property ViewModel objects. ViewModel implements the INotifyPropertyChanged interface.

private void AddCollectionListener()
    {
        if (MyCollection != null)
        {
            MyCollection.CollectionChanged += 
                new System.Collections.Specialized.NotifyCollectionChangedEventHandler(MyCollection_CollectionChanged);
        }
    }

    void MyCollection_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        // Remove Listeners to each item that has been removed
        foreach (object oldItem in e.OldItems)
        {
            ViewModel viewModel = oldItem as ViewModel;

            if (viewModel != null)
            {
                viewModel.PropertyChanged -= viewModel_PropertyChanged;
            }
        }

        // Add Listeners to each item that has been added
        foreach (object newItem in e.NewItems)
        {
            ViewModel viewModel = newItem as ViewModel;

            if (viewModel != null)
            {
                viewModel.PropertyChanged += new PropertyChangedEventHandler(viewModel_PropertyChanged);
            }
        }
    }

    void viewModel_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        // TODO: Property Changed Logic

        switch (e.PropertyName)
        {
            case "MyPropertyName":
                // TODO: Perform logic necessary when MyPropertyName changes
                break;
            default:
                // TODO: Perform logic for all other property changes.
                break;
        }
    }