Question

在WPF中创建ViewModel时，有时需要将ObservableCollection（源集合）中可用的数据转换为扩展/限制/投影原始元素（目标集合）的包装元素集合，而元素的数量和顺序始终与原始集合相同。

就像Select扩展方法一样，除了它不断更新，因此可以用于WPF绑定。

如果在索引x处将元素添加到源，则会在目标集合中的相同索引x处添加相同元素的Wrapper。如果在源集合中删除了索引y处的元素，则会在目标集合中删除索引y处的元素。

假设有ObservableCollection<ClassA>，但我需要绑定的是ReadOnlyObservableCollection<ClassB>（或等效的），其中ClassB - ＆gt; ClassA如下：

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) { ... }
    ...
}

我可以写自己的TemplatedTransformCollectionWrapper，我可以写下这个：

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

TemplatedTransformCollectionWrapper理想地包装所有实现INotifyCollectionChanged的集合，并正确处理原始包装集合的所有可能添加，删除，替换操作。

正确编写TemplatedTransformCollectionWrapper并不是一件轻而易举的事情，而这似乎是其他人已经做过的事情，也许它甚至是核心框架的一部分。但我找不到它。

Answer 1

我在这里发布我的解决方法 - 这是一个自定义类。仍然希望有更好的答案。

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; }
        }
    }
}

ObservableCollection元素转换/投影包装器

1 个答案: