From 23deea8230908e63519948a169ecb8120784f7bc Mon Sep 17 00:00:00 2001
From: hansmbakker <hansmbakker@hotmail.com>
Date: Tue, 6 Oct 2020 22:32:30 +0200
Subject: [PATCH 1/5] Mutable and subclassable version of ObservableGroup

---
 .../Collections/ObservableGroup.cs            | 21 ++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/Microsoft.Toolkit/Collections/ObservableGroup.cs b/Microsoft.Toolkit/Collections/ObservableGroup.cs
index 304be89e496..542f943d7ef 100644
--- a/Microsoft.Toolkit/Collections/ObservableGroup.cs
+++ b/Microsoft.Toolkit/Collections/ObservableGroup.cs
@@ -4,6 +4,7 @@
 
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.ComponentModel;
 using System.Diagnostics;
 using System.Linq;
 
@@ -16,8 +17,10 @@ namespace Microsoft.Toolkit.Collections
     /// <typeparam name="TKey">The type of the group key.</typeparam>
     /// <typeparam name="TValue">The type of the items in the collection.</typeparam>
     [DebuggerDisplay("Key = {Key}, Count = {Count}")]
-    public sealed class ObservableGroup<TKey, TValue> : ObservableCollection<TValue>, IGrouping<TKey, TValue>, IReadOnlyObservableGroup
+    public class ObservableGroup<TKey, TValue> : ObservableCollection<TValue>, IGrouping<TKey, TValue>, IReadOnlyObservableGroup
     {
+        private TKey _key;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="ObservableGroup{TKey, TValue}"/> class.
         /// </summary>
@@ -49,9 +52,21 @@ public ObservableGroup(TKey key, IEnumerable<TValue> collection)
         }
 
         /// <summary>
-        /// Gets the key of the group.
+        /// Gets or sets the key of the group.
         /// </summary>
-        public TKey Key { get; }
+        public TKey Key
+        {
+            get
+            {
+                return _key;
+            }
+
+            set
+            {
+                _key = value;
+                OnPropertyChanged(new PropertyChangedEventArgs(nameof(Key)));
+            }
+        }
 
         /// <inheritdoc/>
         object IReadOnlyObservableGroup.Key => Key;

From f95595d06b130161220c8c83040fae9c27ae582f Mon Sep 17 00:00:00 2001
From: Hans Bakker <hansmbakker+github@gmail.com>
Date: Wed, 7 Oct 2020 09:48:31 +0200
Subject: [PATCH 2/5] Prevent unnecessary OnPropertyChanged

Co-authored-by: Sergio Pedri <sergio0694@live.com>
---
 Microsoft.Toolkit/Collections/ObservableGroup.cs | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/Microsoft.Toolkit/Collections/ObservableGroup.cs b/Microsoft.Toolkit/Collections/ObservableGroup.cs
index 542f943d7ef..996a90ecc9b 100644
--- a/Microsoft.Toolkit/Collections/ObservableGroup.cs
+++ b/Microsoft.Toolkit/Collections/ObservableGroup.cs
@@ -60,11 +60,14 @@ public TKey Key
             {
                 return _key;
             }
-
             set
             {
-                _key = value;
-                OnPropertyChanged(new PropertyChangedEventArgs(nameof(Key)));
+                if (!EqualityComparer<TKey>.Default.Equals(this.key, value))
+                {
+                    this.key = value;
+                    
+                    OnPropertyChanged(new PropertyChangedEventArgs(nameof(Key)));
+                }
             }
         }
 

From 46d9d343dfef339bc74b55a4e51986f64aebb8bd Mon Sep 17 00:00:00 2001
From: Hans Bakker <hansmbakker+github@gmail.com>
Date: Wed, 7 Oct 2020 09:48:48 +0200
Subject: [PATCH 3/5] Code style suggestion

Co-authored-by: Sergio Pedri <sergio0694@live.com>
---
 Microsoft.Toolkit/Collections/ObservableGroup.cs | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/Microsoft.Toolkit/Collections/ObservableGroup.cs b/Microsoft.Toolkit/Collections/ObservableGroup.cs
index 996a90ecc9b..78a5a8bbcaf 100644
--- a/Microsoft.Toolkit/Collections/ObservableGroup.cs
+++ b/Microsoft.Toolkit/Collections/ObservableGroup.cs
@@ -56,10 +56,7 @@ public ObservableGroup(TKey key, IEnumerable<TValue> collection)
         /// </summary>
         public TKey Key
         {
-            get
-            {
-                return _key;
-            }
+            get => this.key;
             set
             {
                 if (!EqualityComparer<TKey>.Default.Equals(this.key, value))

From eeb1ff98019f4ddee1919295030abd522065c0e7 Mon Sep 17 00:00:00 2001
From: Hans Bakker <hansmbakker+github@gmail.com>
Date: Wed, 7 Oct 2020 09:48:57 +0200
Subject: [PATCH 4/5] Code style suggestion

Co-authored-by: Sergio Pedri <sergio0694@live.com>
---
 Microsoft.Toolkit/Collections/ObservableGroup.cs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Microsoft.Toolkit/Collections/ObservableGroup.cs b/Microsoft.Toolkit/Collections/ObservableGroup.cs
index 78a5a8bbcaf..a247fe6895f 100644
--- a/Microsoft.Toolkit/Collections/ObservableGroup.cs
+++ b/Microsoft.Toolkit/Collections/ObservableGroup.cs
@@ -19,7 +19,7 @@ namespace Microsoft.Toolkit.Collections
     [DebuggerDisplay("Key = {Key}, Count = {Count}")]
     public class ObservableGroup<TKey, TValue> : ObservableCollection<TValue>, IGrouping<TKey, TValue>, IReadOnlyObservableGroup
     {
-        private TKey _key;
+        private TKey key;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ObservableGroup{TKey, TValue}"/> class.

From b601e57d89f665ac0211cf64ebcde504bac16aa7 Mon Sep 17 00:00:00 2001
From: hansmbakker <hansmbakker@hotmail.com>
Date: Wed, 7 Oct 2020 14:34:09 +0200
Subject: [PATCH 5/5] code styling and perf improvement

---
 Microsoft.Toolkit/Collections/ObservableGroup.cs | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/Microsoft.Toolkit/Collections/ObservableGroup.cs b/Microsoft.Toolkit/Collections/ObservableGroup.cs
index a247fe6895f..ce828934917 100644
--- a/Microsoft.Toolkit/Collections/ObservableGroup.cs
+++ b/Microsoft.Toolkit/Collections/ObservableGroup.cs
@@ -19,7 +19,10 @@ namespace Microsoft.Toolkit.Collections
     [DebuggerDisplay("Key = {Key}, Count = {Count}")]
     public class ObservableGroup<TKey, TValue> : ObservableCollection<TValue>, IGrouping<TKey, TValue>, IReadOnlyObservableGroup
     {
-        private TKey key;
+        /// <summary>
+        /// The cached <see cref="PropertyChangedEventArgs"/> for <see cref="Key"/>
+        /// </summary>
+        private static readonly PropertyChangedEventArgs KeyChangedEventArgs = new PropertyChangedEventArgs(nameof(Key));
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ObservableGroup{TKey, TValue}"/> class.
@@ -51,6 +54,8 @@ public ObservableGroup(TKey key, IEnumerable<TValue> collection)
             Key = key;
         }
 
+        private TKey key;
+
         /// <summary>
         /// Gets or sets the key of the group.
         /// </summary>
@@ -62,8 +67,8 @@ public TKey Key
                 if (!EqualityComparer<TKey>.Default.Equals(this.key, value))
                 {
                     this.key = value;
-                    
-                    OnPropertyChanged(new PropertyChangedEventArgs(nameof(Key)));
+
+                    OnPropertyChanged(KeyChangedEventArgs);
                 }
             }
         }