Question

I have a observable collection exposed as a property within a view model. The observable collection is loaded with objects from a data access layer (linq2sql).

When a new item is added to the database through another view model what is the best way to update the observable collection? Should I repopulate the observable collection with a query to the database or directly insert the new object into the collection?

Also I am still trying to work out how to get one view model to communicate with another one, but I’ve only been using mvvm for 3 days.

Answer 1

You can use a whole new class to manage notifications from one class to another. For the question regarding whether to load all entities or just add newly added entity, it really depends on the number of possible entities to load each time. If they are going to be very few, you can reload them each time, otherwise just add the newly added object to the collection.

Example:

class ViewModel1
{
    ObservableCollection<Object> entities;

    public ViewModel1()
    {
        EventsManager.ObjectAddedEvent += new EventHandler<ObjectAddedEventArgs>(EventsManager_ObjectAddedEvent);
        entities = new ObservableCollection<Object>();
    }

    void EventsManager_ObjectAddedEvent(object sender, ObjectAddedEventArgs e)
    {
        entities.Add(e.ObjectAdded);
    }
}

class EventsManager
{
    public static event EventHandler<ObjectAddedEventArgs> ObjectAddedEvent;

    public static void RaiseObjectAddedEvent(Object objectAdded)
    {
        EventHandler<ObjectAddedEventArgs> temp = ObjectAddedEvent;
        if (temp != null)
        {
            temp(null, new ObjectAddedEventArgs(objectAdded));
        }
    }
}

class ObjectAddedEventArgs : EventArgs
{
    public Object ObjectAdded { get; protected set; }

    public ObjectAddedEventArgs(Object objectAdded)
    {
        ObjectAdded = objectAdded;
    }
}

class ViewModel2
{
    public void AddObject(Object o)
    {
        EventsManager.RaiseObjectAddedEvent(o);
    }
}