Question

When creating ViewModels in WPF it’s sometimes necessary to transform data that is available in an ObservableCollection (the source collection) into a collection of wrapper elements that extend/restrict/project the original elements (the target collection), while the number and order of the elements always mirror the original collection.

Just like the Select extension method, except that it is continuously updated and can therefore be used for WPF bindings.

If an element is added to the source at index x, the Wrapper of the same element is added at the same index x in the target collection. If the element at index y is removed in the source collection, the element at index y is removed in the target collection.

Say there is an ObservableCollection<ClassA>, but what I need to bind to is an ReadOnlyObservableCollection<ClassB> (or equivalent), where ClassB -> ClassA as follows:

class ClassB : INotifyPropertyChanged, IDisposable
{
    public ClassB(ClassA a)
    {
        Wrapped = a;
        (Wrapped as INotifyPropertyChanged).PropertyChanged+=WrappedChanged;
    }
    public ClassA Wrapped { get; private set; }
    public int SomeOtherProperty { get { return SomeFunction(Wrapped); }
    WrappedChanged(object s, NotifyPropertyChangedArgs a) { ... }
    ...
}

I can write my own TemplatedTransformCollectionWrapper, where I can write this:

ObservableCollection<ClassA> source;
TemplatedTransformCollectionWrapper theCollectionThatWillBeUsedInABinding
    = TemplatedTransformCollectionWrapper(source, classA => new ClassB(classA));

TemplatedTransformCollectionWrapper ideally wraps all collections that implement INotifyCollectionChanged and correctly handles all possible add, remove, replace operations of the original, wrapped, collection.

It’s not trivial to write TemplatedTransformCollectionWrapper correctly and it seems to be the kind of thing that someone else has already done, maybe it’s even part of the core framework. But I can’t find it.

Answer 1

I’m posting my workaround – which is a custom class – here. Still hoping for better answers.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.Linq;

namespace ViewLayer
{
    public class TransformObservableCollection<T,Source> : INotifyCollectionChanged, IList, IReadOnlyList<T>, IDisposable
    {
        public TransformObservableCollection(ObservableCollection<Source> wrappedCollection, Func<Source,T> transform)
        {
            m_WrappedCollection = wrappedCollection;
            m_TransformFunc = transform;
            ((INotifyCollectionChanged)m_WrappedCollection).CollectionChanged += TransformObservableCollection_CollectionChanged;
            m_TransformedCollection = new ObservableCollection<T>(m_WrappedCollection.Select(m_TransformFunc));
        }
        public void Dispose()
        {
            if (m_WrappedCollection == null) return;
            ((INotifyCollectionChanged)m_WrappedCollection).CollectionChanged -= TransformObservableCollection_CollectionChanged;
            m_WrappedCollection = null;
        }
        void TransformObservableCollection_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            switch (e.Action)
            {
                case NotifyCollectionChangedAction.Add:
                    if (e.NewItems == null || e.NewItems.Count != 1)
                        break;
                    m_TransformedCollection.Insert(e.NewStartingIndex,m_TransformFunc((Source)e.NewItems[0]));
                    return;
                case NotifyCollectionChangedAction.Move:
                    if (e.NewItems == null || e.NewItems.Count != 1 || e.OldItems == null || e.OldItems.Count != 1)
                        break;
                    m_TransformedCollection.Move(e.OldStartingIndex, e.NewStartingIndex);
                    return;
                case NotifyCollectionChangedAction.Remove:
                    if (e.OldItems == null || e.OldItems.Count != 1)
                        break;
                    m_TransformedCollection.RemoveAt(e.OldStartingIndex);
                    return;
                case NotifyCollectionChangedAction.Replace:
                    if (e.NewItems == null || e.NewItems.Count != 1 || e.OldItems == null || e.OldItems.Count != 1 || e.OldStartingIndex != e.NewStartingIndex)
                        break;
                    m_TransformedCollection[e.OldStartingIndex] = m_TransformFunc((Source)e.NewItems[0]);
                    return;
            } // This  is most likely called on a Clear(), we don't optimize the other cases (yet)
            m_TransformedCollection.Clear();
            foreach (var item in m_WrappedCollection)
                m_TransformedCollection.Add(m_TransformFunc(item));
        }

        #region IList Edit functions that are unsupported because this collection is read only
        public int Add(object value) { throw new InvalidOperationException(); }
        public void Clear() { throw new InvalidOperationException(); }
        public void Insert(int index, object value) { throw new InvalidOperationException(); }
        public void Remove(object value) { throw new InvalidOperationException(); }
        public void RemoveAt(int index) { throw new InvalidOperationException(); }
        #endregion IList Edit functions that are unsupported because this collection is read only

        #region Accessors
        public T this[int index] { get { return m_TransformedCollection[index]; } }
        object IList.this[int index] { get { return m_TransformedCollection[index]; } set { throw new InvalidOperationException(); } }
        public bool Contains(T value) { return m_TransformedCollection.Contains(value); }
        bool IList.Contains(object value) { return ((IList)m_TransformedCollection).Contains(value); }
        public int IndexOf(T value) { return m_TransformedCollection.IndexOf(value); }
        int IList.IndexOf(object value) { return ((IList)m_TransformedCollection).IndexOf(value); }
        public int Count { get { return m_TransformedCollection.Count; } }
        public IEnumerator<T> GetEnumerator() { return m_TransformedCollection.GetEnumerator(); }
        IEnumerator IEnumerable.GetEnumerator() { return ((IEnumerable)m_TransformedCollection).GetEnumerator(); }
        #endregion Accessors

        public bool IsFixedSize { get { return false; } }
        public bool IsReadOnly { get { return true; } }
        public void CopyTo(Array array, int index) { ((IList)m_TransformedCollection).CopyTo(array, index); }
        public void CopyTo(T[] array, int index) { m_TransformedCollection.CopyTo(array, index); }
        public bool IsSynchronized { get { return false; } }
        public object SyncRoot { get { return m_TransformedCollection; } }

        ObservableCollection<T> m_TransformedCollection;
        ObservableCollection<Source> m_WrappedCollection;
        Func<Source, T> m_TransformFunc;

        event NotifyCollectionChangedEventHandler INotifyCollectionChanged.CollectionChanged
        {
            add { ((INotifyCollectionChanged)m_TransformedCollection).CollectionChanged += value; }
            remove { ((INotifyCollectionChanged)m_TransformedCollection).CollectionChanged -= value; }
        }
    }
}