My view model implements INotifyPropertyChanged for properties that it makes available to my view. It makes available a collection of objects that do not implement INotifyPropertyChanged.
My collection is bound to an ItemControl in my view, with an ItemTemplate used to display each item invidually. The item template is bound to the Name attribute of my collection members.
How can i tell my view to update when properties of my collection members change?
You need to either implement INotifyPropertyChanged for the objects in your collection (recommend approach), or you can manually refresh the binding by something like
myItemsControl.GetBindingExpression(
ItemsControl.ItemsSourceProperty).UpdateTarget();
If you're in the ViewModel, you might be able to raise a PropertyChanged event on your Collection class such the following, although I am not sure if that will update the individual items or not
// My PropertyChanged method is usually called RaisePropertyChanged
RaisePropertyChanged("MyCollection");
You can also do what Mirimon suggeted and set the value to null then back again, although personally I would recommend a different approach if possible.
You must implement INotifyPropertyChanged for collection members. Or you can reset your collection in ViewModel:
public void Reset() {
List<TestData> temp = YourCollection;
YourCollection = null;
YourCollection = temp;
}
Related
I'm new to WPF. I need to Bind UI's ListBox to the source that is deep in Model Layer.
App scheme is on picture below. Desc:
My MainWindowViewModel Class has a Scheduler Property (Scheduler Class in Model layer).
Scheduler Class has a CurrentParser Property (Parser Class in Model layer).
Parser Class has a Result field (ParserResultMetaData Class in Model layer).
ParserResultMetaData Class has a Log field (Log is a List(Of String))
Log can be changed only programmatically from model layer (Parser adds lines during it's work).
So my question is how can I bind my ListBox to this List to match MVVM pattern.
As I get it now, ViewModel must have an ObservableCollection(Of String) witch is a copy of my List(Of String) from Model layer.
Somehow you need to notify the UI when a line is added to the collection. There are multiple ways to achieve this, but if the collection is modified from within the model layer, you need a mechanism for communicating this to other layers in one way or another.
Use an ObservableCollection in your Model layer.
While types like ObservableCollection and INotifyPropertyChanged are widely used in MVVM architectures, they are not specific to them and in some cases it can make sense to use them in the model layer directly. Using an ObservableCollection in your Parser class is one way to provide this notification mechanism. You can then bind the ItemsSource of the ListBox to Scheduler.Parser.Result.Log directly and it will update accordingly.
Create a wrapper property in your ViewModel.
If you don't want to use an ObservableCollection in your model, you can expose the List via a property in your ViewModel, for example:
public IEnumerable<string> ParserLog
{
get { return Scheduler.Parser.Result.Log; }
}
Then you need to manually notify the UI when an item is added, so you're gonna need an event (or something equivalent) which tells your ViewModel that the list changed and it needs to raise the PropertyChanged Event for the ParserLog property. Add code like this in your ViewModel:
this.Scheduler.Parser.ResultUpdated += (s, e) => this.RaisePropertyChanged("ParserLog");
This will tell the ListBox to update the items from the ParserLog property.
I have a View Model that exposes an ObservableCollection. Now I want to add grouping and therefore want to use a View class, like e.g. the ListCollectionView. But that is not a generic type. Also, if I construct a ListCollectionView from an ObservableCollection, then it is not updated when the ObservableCollection is.
Did someone implement an ObservableCollection-backed View-collection somewhere?
If not, is it possible to achieve this dynamism somehow by using XAML?
You don't need a specific implementation ; a CollectionView is just a view of a collection. You can get the default CollectionView for a collection using CollectionViewSource.GetDefaultView:
ObservableCollection<Something> collection = ...
ICollectionView view = CollectionViewSource.GetDefaultView(collection);
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.
I have a combobox bound to a collection, so the user can select one of the items. So far, so good.
The content of the combo box is driven by the item, but also by a value in my viewmodel. Imagine the value in my viewmodel is the language, I have dictionary of descriptions by language in my bound item, and I want to display the correct one.
How should I go about this?
This is a classic example of why the ViewModel exists - you want to have logic which depends on trivial state in the view, as well as the main model.
Imagine you are writing a unit test to run against the ViewModel for this behaviour. You would need the ViewModel to have a property mapped to the selected item. The ViewModel would also have another property which varies according to this selected item as well as the other value in the ViewModel you mentioned.
I think of this as the test-driven approach to ViewModel design - if you can't write a unit test to evaluate it then you haven't got the mix of state and published interfaces right.
So, yes, the ViewModel can solve the problem and if you push all the state down into it you can do the unification within the ViewModel.
Make an observable collection in your viewmodel of type Item. Bind the itemsource of your viewmodel to this observable collection.
public class Item
{
public String description {get;set;}
public String language {get;set;}
public override ToString()
{
return description;
}
}
Selected item would also be bound to a property of type Item as well.
The override of ToString displays the description.
The Selected item propery will have a reference to the selected object property where you can get the language from.
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.