diff --git a/src/ObservableCollections/IObservableCollection.cs b/src/ObservableCollections/IObservableCollection.cs
index 474bb8e..efcfae7 100644
--- a/src/ObservableCollections/IObservableCollection.cs
+++ b/src/ObservableCollections/IObservableCollection.cs
@@ -57,6 +57,10 @@ namespace ObservableCollections
     //{
     //}
 
+    public interface ISynchronizedViewList<out TView> : IReadOnlyList<TView>, IDisposable
+    {
+    }
+
     public interface INotifyCollectionChangedSynchronizedView<out TView> : IReadOnlyCollection<TView>, INotifyCollectionChanged, INotifyPropertyChanged, IDisposable
     {
     }
diff --git a/src/ObservableCollections/ISynchronizedViewFilter.cs b/src/ObservableCollections/ISynchronizedViewFilter.cs
index 968b286..a2daec2 100644
--- a/src/ObservableCollections/ISynchronizedViewFilter.cs
+++ b/src/ObservableCollections/ISynchronizedViewFilter.cs
@@ -73,8 +73,8 @@ namespace ObservableCollections
         {
             return filter == SynchronizedViewFilter<T, TView>.Null;
         }
-        
-        
+
+
         internal static void InvokeOnAdd<T, TView>(this ISynchronizedViewFilter<T, TView> filter, (T value, TView view) value, int index)
         {
             filter.InvokeOnAdd(value.value, value.view, index);
@@ -139,5 +139,10 @@ namespace ObservableCollections
                 filter.WhenFalse(value, view);
             }
         }
+
+        internal static bool IsMatch<T, TView>(this ISynchronizedViewFilter<T, TView> filter, (T, TView) value)
+        {
+            return filter.IsMatch(value);
+        }
     }
 }
diff --git a/src/ObservableCollections/Internal/NotifyCollectionChangedSynchronizedView.cs b/src/ObservableCollections/Internal/NotifyCollectionChangedSynchronizedView.cs
index d28aea7..ee25a44 100644
--- a/src/ObservableCollections/Internal/NotifyCollectionChangedSynchronizedView.cs
+++ b/src/ObservableCollections/Internal/NotifyCollectionChangedSynchronizedView.cs
@@ -1,11 +1,117 @@
 using System;
+using System.Buffers;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.ComponentModel;
+using System.Linq;
 
 namespace ObservableCollections.Internal
 {
+    internal class SynchronizedViewList<T, TView> : ISynchronizedViewList<TView>
+    {
+        readonly ISynchronizedView<T, TView> parent;
+        readonly List<TView> listView;
+        readonly Func<T, TView> selector;
+
+        public SynchronizedViewList(ISynchronizedView<T, TView> parent, Func<T, TView> selector)
+        {
+            this.parent = parent;
+            this.selector = selector;
+            parent.RoutingCollectionChanged += Parent_RoutingCollectionChanged; // TODO: -=
+        }
+
+        private void Parent_RoutingCollectionChanged(in NotifyCollectionChangedEventArgs<T> e)
+        {
+            // call in parent.lock.
+            switch (e.Action)
+            {
+                case NotifyCollectionChangedAction.Add:
+                    if (e.IsSingleItem)
+                    {
+                        // parent.CurrentFilter.IsMatch(
+                        
+                        var item = selector(e.NewItem);
+
+                        
+
+
+                        if (e.NewStartingIndex != -1)
+                        {
+                            listView.Insert(e.NewStartingIndex, selector(e.NewItem));
+                        }
+                        else // dict is -1
+                        {
+                            listView.Add(selector(e.NewItem));
+                        }
+                    }
+                    else
+                    {
+                        if (e.NewStartingIndex != -1)
+                        {
+                            var array = ArrayPool<TView>.Shared.Rent(e.NewItems.Length);
+                            try
+                            {
+                                var i = 0;
+                                foreach (var item in e.NewItems)
+                                {
+                                    array[i++] = selector(item);
+                                }
+
+                                listView.InsertRange(e.NewStartingIndex, array.Take(e.NewItems.Length));
+                            }
+                            finally
+                            {
+                                ArrayPool<TView>.Shared.Return(array, true);
+                            }
+                        }
+                        else
+                        {
+                            foreach (var item in e.NewItems)
+                            {
+                                listView.Add(selector(item));
+                            }
+                        }
+                    }
+                    break;
+                case NotifyCollectionChangedAction.Remove:
+                    break;
+                case NotifyCollectionChangedAction.Replace:
+                    break;
+                case NotifyCollectionChangedAction.Move:
+                    break;
+                case NotifyCollectionChangedAction.Reset:
+                    break;
+                default:
+                    break;
+            }
+
+            // throw new NotImplementedException();
+        }
+
+        public TView this[int index] => throw new NotImplementedException();
+
+        public int Count => throw new NotImplementedException();
+
+        public void Dispose()
+        {
+            throw new NotImplementedException();
+        }
+
+        public IEnumerator<TView> GetEnumerator()
+        {
+            throw new NotImplementedException();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            throw new NotImplementedException();
+        }
+    }
+
+
+
+
     internal class NotifyCollectionChangedSynchronizedView<T, TView> :
         INotifyCollectionChangedSynchronizedView<TView>,
         ISynchronizedViewFilter<T, TView>
diff --git a/src/ObservableCollections/ObservableList.Views.cs b/src/ObservableCollections/ObservableList.Views.cs
index 03c56dd..549bbcb 100644
--- a/src/ObservableCollections/ObservableList.Views.cs
+++ b/src/ObservableCollections/ObservableList.Views.cs
@@ -27,7 +27,8 @@ namespace ObservableCollections
             readonly ObservableList<T> source;
             readonly Func<T, TView> selector;
             readonly bool reverse;
-            internal readonly List<(T, TView)> list; // be careful to use
+            readonly List<(T, TView)> list;
+            int filteredCount;
 
             ISynchronizedViewFilter<T, TView> filter;
 
@@ -46,6 +47,7 @@ namespace ObservableCollections
                 lock (source.SyncRoot)
                 {
                     this.list = source.list.Select(x => (x, selector(x))).ToList();
+                    this.filteredCount = list.Count;
                     this.source.CollectionChanged += SourceCollectionChanged;
                 }
             }
@@ -56,7 +58,7 @@ namespace ObservableCollections
                 {
                     lock (SyncRoot)
                     {
-                        return list.Count;
+                        return filteredCount;
                     }
                 }
             }
@@ -160,6 +162,10 @@ namespace ObservableCollections
                                 {
                                     var v = (e.NewItem, selector(e.NewItem));
                                     list.Add(v);
+                                    if (filter.IsMatch(v))
+                                    {
+                                        filteredCount++;
+                                    }
                                     filter.InvokeOnAdd(v, e.NewStartingIndex);
                                 }
                                 else
@@ -169,6 +175,10 @@ namespace ObservableCollections
                                     {
                                         var v = (item, selector(item));
                                         list.Add(v);
+                                        if (filter.IsMatch(v))
+                                        {
+                                            filteredCount++;
+                                        }
                                         filter.InvokeOnAdd(v, i++);
                                     }
                                 }