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.
Related
I have a wpf application and im using winform host.
In some cases the 'SelectedItem' property is complex type that have a property from type 'List'.
How can i know from code if the collection changed? Or just know of the custom editor opened?
You can use a ObservableCollection<T> instead of a List to have the functionality of indicating changes to the contents of the list.
The ObservableCollection implements the interfaces INotifyCollectionChanged and
INotifyPropertyChanged to archive this behaviour.
More information to ObservableCollection on MSDN:
https://msdn.microsoft.com/de-de/library/ms668604(v=vs.110).aspx
If I understand well you have collection of elements, and each of them have some other collection.
In that way you could also consider using BindingList as your base collection - it implementsIRaiseItemChangedEvents - Indicates whether a class converts property change events to System.ComponentModel.IBindingList.ListChanged
I am starting with a new project based on MVVM approach.I will be loading some data asynchronously from some web service.I am confused about where to implement this INotifyPropertyChanged interface,in a viewmodel,model or both.
Also If I implement INotifyPropertyChanged on a model(let's call it ModelA).Then I create an ObservableCollection<ModelA>,Will not it be redundant as ObservableCollection itself keeps track of any data change made to the list?
ObservableCollection class only updates UI in case of following operations:
Item is added in collection.
Item is deleted from collection.
Collection is clear.
But if any property changes in underlying item i.e. ModelA in your case, it won't be reflected on UI unless your Model class is implementing INotifyPropertyChanged interface and raising PropertyChanged event on property change.
So, as a thumb rule you should implement INPC both at your ViewModel layer and also at Model layer.
Practically you need to implement INotifyPropertyChanged to objects which property will be displayed in UI using data binding. If you bind the UI directly to property of your model then you need to implement it in the model too.
ObservableCollection only notify UI when object added or removed from collection. To get the UI notified when property of one or more objects in that ObservableCollection changed you still need to implement INotifyPropertyChanged in that object class.
The answer is simple. If you need your model to implement INotifyPropertyChanged then it should. Redundant or not if you need to keep track on a single property you will be better of using that event rather than the ObservableCollection.
My class has a public List field. I want to detect when someone changes this collection. However, writing my own Add/Remove wrapper around this collection seems wasteful. Is there a way to get notifications about changes in collection with delegates or something like that?
Use ObservableCollection<T> instead
you may find reference here.
There already exists a collection you've described and it's called ObservableCollection.
It has CollectionChanged event, so just subscribe your event handler there and i'll get called every time item is added or removed from collection..
Use BindingList<T> instead. It offers a ListChanged event. It is also accepted as DataSource in many controls such as a Listbox or a Grid.
As mentioned in this answer the BindingList offers a lot more than ObservableCollection
If you want to change a list in a listbox and changes to that list be reflected to the UI, you can set a BindingList<> as datasource to the Listbox and then simply manipulate the list. The Listbox will handle the ListChanged event and will display changes without having to manually do it by your self.
If you are going to use it in Winforms you should go with BindingList and if you are going to use it on a WPF app then go with ObservableCollection.
I have created a Treeview in WPF MVVM (Project is using MVVM Light).
When I add a data to a DataTemplate of the Treeview, I want to reload the entire treeview. For this I am sending a Message from my data entry viewmodel to my treeview view model.
Though the message is recieved, the tree doesnt get refreshed. I called LoadChildren on each node again, and used RaisePropertyChanged as well but the data didnt get updated.
The TreeView is bound to a hierarchical collection in your ViewModel, and it has a property name, e.g. TreeViewData. Assuming your ViewModel implements INotifyPropertyChanged, simply call the PropertyChanged handler for property TreeViewData. This will completely refresh the TreeView.
My answer is based on the fact you are using Mvvm light, since you did not provide code I will just state the obvious and it might solve your issues.
Is your viewmodel implementing the ViewModelBase interface? (which implements INotifiedPropertyChanged)
I never used TreeViews but assuming it's using a collection as its datatemplate, you have to call RaisePropertyChanged on the collection whenever you add/remove an item and also whenever an item changes. Calling RaisePropertyChanged only in the "set" of the collection won't be enough.
in my Silverlight 4 app, I have a class which contains a list of objects. As I want to bind this list to a listbox and notify the listbox about changes to the list, I use an ObservableCollection<>.
I know want to control the adding and deleting of the items in the ObservableCollection by returning an IEnumerable<> and Add/Remove methods in my class.
But now there is no automatic change notification for the listbox to which the list is bound. How can I achieve the automatic notification of the listbox, when my list is returned as IEnumerable (or any other type that would suit the needs)
Thanks in advance,
Frank
Your class should implement the INotifyCollectionChanged interface, since you're implementing the Add and Remove methods in your class, you can raise the notification events.
There is a list for this: ReadOnlyObservableCollection<>