How to track deleted self-tracking entities in ObservableCollection without memory leaks
- by Yannick M.
In our multi-tier business application we have ObservableCollections of Self-Tracking Entities that are returned from service calls.
The idea is we want to be able to get entities, add, update and remove them from the collection client side, and then send these changes to the server side, where they will be persisted to the database.
Self-Tracking Entities, as their name might suggest, track their state themselves.
When a new STE is created, it has the Added state, when you modify a property, it sets the Modified state, it can also have Deleted state but this state is not set when the entity is removed from an ObservableCollection (obviously). If you want this behavior you need to code it yourself.
In my current implementation, when an entity is removed from the ObservableCollection, I keep it in a shadow collection, so that when the ObservableCollection is sent back to the server, I can send the deleted items along, so Entity Framework knows to delete them.
Something along the lines of:
protected IDictionary<int, IList> DeletedCollections = new Dictionary<int, IList>();
protected void SubscribeDeletionHandler<TEntity>(ObservableCollection<TEntity> collection)
{
var deletedEntities = new List<TEntity>();
DeletedCollections[collection.GetHashCode()] = deletedEntities;
collection.CollectionChanged += (o, a) =>
{
if (a.OldItems != null)
{
deletedEntities.AddRange(a.OldItems.Cast<TEntity>());
}
};
}
Now if the user decides to save his changes to the server, I can get the list of removed items, and send them along:
ObservableCollection<Customer> customers = MyServiceProxy.GetCustomers();
customers.RemoveAt(0);
MyServiceProxy.UpdateCustomers(customers);
At this point the UpdateCustomers method will verify my shadow collection if any items were removed, and send them along to the server side.
This approach works fine, until you start to think about the life-cycle these shadow collections. Basically, when the ObservableCollection is garbage collected there is no way of knowing that we need to remove the shadow collection from our dictionary.
I came up with some complicated solution that basically does manual memory management in this case. I keep a WeakReference to the ObservableCollection and every few seconds I check to see if the reference is inactive, in which case I remove the shadow collection.
But this seems like a terrible solution... I hope the collective genius of StackOverflow can shed light on a better solution.
Thanks!
