Merging(concatenating) items from multiple sources in ItemsControl

[kosmakoff]

I'm looking for a way to have multiple list sources (individual observable collections) to be rendered in ItemsControl, so that all items from all individual sources are shown as if one merged source was provided.

For context: I use Canvas as an Items Panel template, and individual sources are different types of items, like horizontal rulers, vertical rulers, etc, which I would rather have in separate lists as it is more convenient to modify them this way.

I tried to implement my own composite observable collection with signature as follows:

public class CompositeObservableCollection<TItem> :
    IReadOnlyList<TItem>,
    INotifyCollectionChanged,
    INotifyPropertyChanged

Alas, when I bind such source to ItemsSource property of ItemsControl it simply doesn't show anything. Debug shows that individual elements are not even being requested, Count property is not requested either.

Does ItemsSource literally have to be ObservableCollection<> type or am I missing something? Also, if there is any other way to show multiple sources in same list please let me know.

[BAndysc]

https://github.com/AvaloniaUI/Avalonia/blob/release/11.0.9/src/Avalonia.Controls/ItemsSourceView.cs#L267 I am surprised it doesn't throw an exception for you now. Implement IList and it should work

[kosmakoff]

Thanks a lot!!! Btw, is there a way to merge lists visually in any other way? Cause it feels like I am overcomplicating things.

[BAndysc]

Unfortunately I don't know of any other way (but maybe there is one, dunno)

[kosmakoff]

Unfortunately, seems like that's not it. Implementing IList didn't help.

[BAndysc]

You are doing something wrong, this works just fine

public class CustomCollection : IList, INotifyCollectionChanged
{
    private List<object> elements = new();

    public IEnumerator GetEnumerator()
    {
        return ((IEnumerable)elements).GetEnumerator();
    }

    public void CopyTo(Array array, int index)
    {
        ((ICollection)elements).CopyTo(array, index);
    }

    public int Count => elements.Count;

    public bool IsSynchronized => ((ICollection)elements).IsSynchronized;

    public object SyncRoot => ((ICollection)elements).SyncRoot;

    public int Add(object? value)
    {
        var index = ((IList)elements).Add(value);
        CollectionChanged?.Invoke(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, value, index));
        return index;
    }

    public void Clear()
    {
        elements.Clear();
        CollectionChanged?.Invoke(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
    }

    public bool Contains(object? value)
    {
        return ((IList)elements).Contains(value);
    }

    public int IndexOf(object? value)
    {
        return ((IList)elements).IndexOf(value);
    }

    public void Insert(int index, object? value)
    {
        ((IList)elements).Insert(index, value);
        CollectionChanged?.Invoke(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, value, index));
    }

    public void Remove(object? value)
    {
        ((IList)elements).Remove(value);
        CollectionChanged?.Invoke(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Remove, value));
    }

    public void RemoveAt(int index)
    {
        var element = elements[index];
        elements.RemoveAt(index);
        CollectionChanged?.Invoke(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Remove, element, index));
    }

    public bool IsFixedSize => ((IList)elements).IsFixedSize;

    public bool IsReadOnly => ((IList)elements).IsReadOnly;

    public object? this[int index]
    {
        get => ((IList)elements)[index];
        set => ((IList)elements)[index] = value;
    }

    public event NotifyCollectionChangedEventHandler? CollectionChanged;
}

(probably no exception I have mentioned before is also a symptom of your problem). How are you binding the collection? Are you able to show some code snippets?

[kosmakoff]

Silly me. I had complex initialization logic for this collection, therefore it was in constructor. But I put the initialization after InitializeComponent is called, therefore it was simply null. Works now. Many thanks.