ListView items do not display - wpf

Question

I have a rather simple ListView with GridView style. Cells are binded to some properties of an ObservableCollection elements. I have no idea why thaey are not displaying. I were thinking about implementing INotifyPropertyChanged in a class of element contained by collection, but once an element is constructed and added to a collection it is never changed so it should be working just fine.
C#:
public ObservableCollection<SceneElement> SceneObjects { get; set; }
...
SceneObjects.Add(new Camera());
SceneObjects.Add(new Cuboid(20, 30, 40));
Camera and Cuboid both inherit from SceneObject (SceneObject <- Figure <- Cuboid and SceneObject <- Camera)
XAML:
<ListView Grid.Column="2" Margin="5"
ItemsSource="{Binding }"
DataContext="{Binding SceneObjects}"
ScrollViewer.CanContentScroll="True">
<ListView.Resources>
<GridView x:Key="StyleOne">
<GridViewColumn Header="Type" >
<GridViewColumn.CellTemplate>
<DataTemplate>
<TextBlock Text="1" />
</DataTemplate>
</GridViewColumn.CellTemplate>
</GridViewColumn>
<GridViewColumn Header="Position" >
<GridViewColumn.CellTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<TextBlock Text="X: " />
<TextBlock Text="{Binding Position.X, RelativeSource={RelativeSource Mode=FindAncestor}}" />
<TextBlock Text=" Y: " />
<TextBlock Text="{Binding Position.Y}" />
<TextBlock Text=" Z: " />
<TextBlock Text="{Binding Position.Z}" />
</StackPanel>
</DataTemplate>
</GridViewColumn.CellTemplate>
</GridViewColumn>
</GridView>
</ListView.Resources>
<ListView.Style>
<Style TargetType="ListView">
<Setter Property="View" Value="{StaticResource StyleOne}" />
</Style>
</ListView.Style>
</ListView>
Position property is defined inside an abstract SceneElement class.
Position's X, Y and Z are properties as well.
EDIT
This has solved the issue:
private ObservableCollection<SceneElement> sceneObjects;
public ObservableCollection<SceneElement> SceneObjects
{
get
{
return sceneObjects;
}
set
{
sceneObjects = value;
NotifyPropertyChanged("SceneObjects");
}
}
Can anyone explain why this was necessary? I've used ObservableCollection severeal time so far and it always worked without notifying.

Answer 1

Reason why your application wasn't displaying the information about the collection is because the UI wasn't aware of any changes.
Using ObservableCollection doesn't guarantee the results will be displayed if said collection is not initialized before it is used. As it is a plain property on the ViewModel it needs to notify if said object, in this case ObservableCollection has been changed, but not in the sense items inside of the collection.
This is due to the fact of NOT raising the PropertyChanged event for SceneObjects, which is crucial for WPF apps using Bidning.

Answer 2

at the point where the wpf system attempts to create the binding, your observablecollection was null, as xamimax above said.
instantiate your observablecollection in your constructor, and later you can add the items and they will appear.
for example, the following works because i first instantiated the observable collection and then added items 5 seconds later.
public MainWindow()
{
InitializeComponent();
this.SceneObjects = new ObservableCollection<MyClass>();
Task.Delay(5000).ContinueWith((old) =>
{
this.Dispatcher.Invoke(() =>
{
this.sceneObjects.Add(new MyClass("name"));
this.sceneObjects.Add(new MyClass("name"));
});
});
this.DataContext = this;
}
and here is an example that doesnt work, because i instantiate the observable collection 5 secs after the wpf system tried to bind to it:
public MainWindow()
{
InitializeComponent();
Task.Delay(5000).ContinueWith((old) => { this.SceneObjects = new ObservableCollection<MyClass> { new MyClass("name"), new MyClass("name") }; });
this.DataContext = this;
}
and as you already realized, you can throw propchanged to make wpf system display those items, but i do think you should simply instantiate the observablecollection earlier so that you dont need to throw propchanged later.