I have an ObservableCollection of Products in the Model, and I want the ViewModel to listen to any changes in the ObservableCollection of Products.
Im not sure about how to go and implement it. Ive read several tutorials, but most of them are not MVVM specific.
Should i implement the INotifyPropertyChanged in the ViewModel class, how do I specify that i want to listen to the OberserableCollection of Products?
Thanks :)
I know this has already been answered, but there is more to consider.
1) a ViewModel should implement INotifyPropertyChanged, that's one of the things that makes it a ViewModel. Even in the unlikely case that the only property it exposes is the ObservableCollection, it will need to raise property changed when the actual ObservableColelction property is changed (more on this in item 3)
2) do you really want the ViewModel to listen for these changes or the View? Those are two different things. The ViewModel should hold an ObservableCollection which is then bound to the View. What you want is for the View to react to those changes. In that case, Brandon is correct that ObservableCollection does this for you out of the box. So the View has an instance of the ViewModel set as a DataContext and some visual element in your View is bound to the ObservableCollection (like an ItemsSource on a ListBox).
3) the exception is the ObservableCollection property itself in the ViewModel. While ObservableCollection implements INotifyPropertyChanged, that is part of the collection object: when that object reference changes (like it is recreated) in the ViewModel, then the ViewModel still needs to report that the Property for the ObservableCollection has changed.
Just some thoughts.
ObservableCollection already implements INotifyPropertyChanged, so you don't need to.
Related
After spending a whole day trying different suggestions, I'm back at square 1. I'm trying to bind my view, a XAML Window, to one of my ViewModel properties, say, SalesOrders. The ViewModel in turn talks to the Model (an EF Model on top of a database). The question I'm facing is the collection type that I should use to expose my SalesOrders property.
I have tried the following types, none of which does all of what I need.
List<T>
ObservableCollection<T>
BindingList<T>
CollectionViewSource on top of the above
Here's what I need my collection to do:
The view has Previous/Next buttons, so the collection should provide some sort of currency manager.
There's a Save button in the view, which I need to get enabled/disabled immediately based on whether the SalesOrder collection has any changes. Since SalesOrder is already an EF type, all of its fields implement INotifyPropertyChanged.
CollectionViewSource provides me with navigation methods (previous/next) but doesn't listen to PropertyChanged events, so modifying data in the view doesn't turn the Save button on. BindingList can listen to PropertyChanged events, but doesn't provide navigation methods. ObservableCollection lacks both functionalities.
TIA.
Why don't you use ObservableCollection<T> then subscribe to the CollectionChanged event to enable or disable your save button as outlined in the answer of the thread MVVM ObservableCollection Bind TwoWay.
According to MSDN about CollectionView here:
In WPF applications, all collections have an associated default
collection view. Rather than working with the collection directly, the
binding engine always accesses the collection through the associated
view. To get the default view, use the
CollectionViewSource.GetDefaultView method. An internal class based on
CollectionView is the default view for collections that implement only
IEnumerable. ListCollectionView is the default view for collections
that implement IList. BindingListCollectionView is the default view
for collections that implement IBindingListView or IBindingList.
Which means you can use BindingList for SalesOrders and bind it in the View, then to manage the navigation you can access its automatically created CollectionView from the ViewModel with:
myCollectionView = (BindingListCollectionView)CollectionViewSource.GetDefaultView(this.SalesOrders);
I used the List<Person> collection as the ItemsSource for the DataGrid control.
But it did not update the View if i remove the item from the List collection. I was struggling for a long time for the solution.
Then Instead of the List<Person> collection in my ViewModel. I changed this into ObservableCollection<Person> collection. Now it updates the view when ever there is a change in the collection.
I am not sure why it updates only for ObservableCollection<Person> ? Anyone ?
Well its in the name. A simple List doesn't tell the ui to update, in other words "the view can't observe the list". There is no weird magic behind Databinding. WPF and DataBinding needs the Data model to tell them "this is new" or "this is changed", you propably already saw INotifyPropertyChanged, INotifyCollectionChanged is the same but for collections, and the List<> doesn't implement it, ObservableCollection does.
Because List does not implement INotifyCollectionChanged
Because the update of a databinding is not a kind of magic, there are several requirements to make databinding working correctly. If you have a single property to bind on this property must be either a dependency property or its parent class must implement the INotifyPropertyChanged interface to notify the wpf binding system about changes of the property value.
For a collection there is a simelar mechanism: it must implement INotifyPropertyChanged to inform the wpf binding system about removed/moved/added items.
See here for more details: http://msdn.microsoft.com/en-us/library/system.collections.specialized.inotifycollectionchanged.aspx
ObservableCollection<T> fires change events every time you change the items in the collection. List<T> doesn't. That's the reason.
DataBinding is lazy. If you don't tell your view that something has changed, it won't bother updating. Under the hoods, WPF DataBinding registers for change notifications, so that your ViewModel can tell the view when it has changed. It does this with interfaces like INotifyPropertyChanged and INotifyCollectionChanged.
ObservableCollection<T> implements the interface INotifyCollectionChanged. This interface defines the event CollectionChanged, which your View basically attaches its own event handler to. That handler will update the view when the event is raised by the collection.
As i was reading about ObservableCollection i came to know that it implements INotifyPropertyChanged which means whenever the view changes it's underlying collection also gets updated automatically which is exactly what is the purpose of TwoWay databinding. So, what is the difference between ObservableCollection and TwoWay databinding?
Thanks in advance :)
The two "things" TwoWay databinding and ObservableCollection are different things which can collaborate, but are not directly linked. Databinding is a concept and ObservableCollection is an implementation of an interface (INotifyCollectionChanged) which is used in the implementation of the concept of databinding in the .NET framework. In itself, INotifyCollectionChanged is a small part of this implementation.
In fact you can use TwoWay databinding without ObservableCollections and ObservableCollections without databinding.
Databinding is a mecanism allowing you to bind a business object property (Often a ViewModel property) to an UI property. OneWay Databinding provides support for update of UI when the object is changed (which requires implementation of INotifyPropertyChanged and/or INotifyCollectionChanged on the business object). TwoWay Databinding provides twoway support as its name suggest it: Object => UI (like OneWay do) AND UI => Object. UI to Object updates does not requires implementation of INotifyCollectionChanged nor INotifyPropertyChanged.
Observable collection now is interesting because it implements INotifyCollectionChanged, which makes it a good candidate to create databindable collections. But I use often INotifyCollectionChanged without databinding, and I am pretty sure you can find other collections which implement INotifyCollectionChanged (like PagedCollectionView)
Does it answer your question?
If you bind to your collection like this {Binding Path=MyCollection, Mode=TwoWay} then reference to collection will be updated but not its contents. Observable collection allows to track its contents changes, but not reference change (MyCollection.Clear() will trigger UI update, but MyCollection = anotherInstance will not).
The question is a little bit like "what's the difference between a horse and a cart?"
An ObservableCollection can be used for TwoWay databinding (to an ItemsControl).
It is more or less designed to make databinding easy.
Whenever a Control updates an ObservableCollection or the other way around, there is a DataBinding in the middle.
Object needs to implement INotifyPropertyChanged so you are able to bind to properties of this object.
TwoWayDatabinding tells databinding engine it needs to update properties both ways.
You are comparing apples and airplanes. Those are two completly different things.
Also ObservableCollection also implements INotifyCollectionChanged that allows to notify its items changed, not only properties.
Calling OnPropertyChanged for an ObservableCollection only works when there has been some change to the properties of the collection, not the objects it contains (add, remove, clear, etc).
Is there any way to notify the View that there has been a change to an item within the collection?
The objects it contains have to implement INotifyPropertyChanged as well. In your setter you trigger the event, and WPF will pick up on this and read in the new value as long as you are using two-way bindings or read-only bindings.
I have user control to which I bind a viewmodel, this implements the INotifyPropertyChanged, thru the datacontext, this has a property that is a IList that I bind to the itemsdatasource of a grid, then on the code, in another class I add some values to the list, but the UI doesn't reflect this change although in debug I can see that the datagrid has this items added but they don't appear in the UI, can anybody help me, I can't see what is the problem.
ObservableCollection<T> is your friend. It implements INotifyCollectionChanged, saving you the trouble.