Revamp observable collection APIs

CommunityToolkit.Mvvm/Collections/IReadOnlyObservableGroup.cs

@@ -2,14 +2,16 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Collections;
+using System.Collections.Specialized;
 using System.ComponentModel;
 
 namespace CommunityToolkit.Mvvm.Collections;
 
 /// <summary>
 /// An interface for a grouped collection of items.
 /// </summary>
-public interface IReadOnlyObservableGroup : INotifyPropertyChanged
+public interface IReadOnlyObservableGroup : INotifyPropertyChanged, INotifyCollectionChanged, IEnumerable
 {
     /// <summary>
     /// Gets the key for the current collection.
@@ -20,4 +22,12 @@ public interface IReadOnlyObservableGroup : INotifyPropertyChanged
     /// Gets the number of items currently in the grouped collection.
     /// </summary>
     int Count { get; }
+
+    /// <summary>
+    /// Gets the element at the specified index in the current collection.
+    /// </summary>
+    /// <param name="index">The zero-based index of the element to get.</param>
+    /// <returns>The element at the specified index in the read-only list.</returns>
+    /// <exception cref="System.ArgumentOutOfRangeException">Thrown if the index is out of range.</exception>
+    object? this[int index] { get; }
 }

CommunityToolkit.Mvvm/Collections/IReadOnlyObservableGroup{TKey,TElement}.cs

@@ -0,0 +1,25 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Collections.Generic;
+using System.Linq;
+
+namespace CommunityToolkit.Mvvm.Collections;
+
+/// <summary>
+/// An interface for a grouped collection of items.
+/// </summary>
+/// <typeparam name="TKey">The type of the group key.</typeparam>
+/// <typeparam name="TElement">The type of elements in the group.</typeparam>
+public interface IReadOnlyObservableGroup<out TKey, out TElement> : IReadOnlyObservableGroup<TKey>, IReadOnlyList<TElement>, IGrouping<TKey, TElement>
+    where TKey : notnull
+{
+    /// <summary>
+    /// Gets the element at the specified index in the current collection.
+    /// </summary>
+    /// <param name="index">The zero-based index of the element to get.</param>
+    /// <returns>The element at the specified index in the read-only list.</returns>
+    /// <exception cref="System.ArgumentOutOfRangeException">Thrown if the index is out of range.</exception>
+    new TElement this[int index] { get; }
+}

CommunityToolkit.Mvvm/Collections/IReadOnlyObservableGroup{TKey}.cs

@@ -0,0 +1,18 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace CommunityToolkit.Mvvm.Collections;
+
+/// <summary>
+/// An interface for a grouped collection of items.
+/// </summary>
+/// <typeparam name="TKey">The type of the group key.</typeparam>
+public interface IReadOnlyObservableGroup<out TKey> : IReadOnlyObservableGroup
+    where TKey : notnull
+{
+    /// <summary>
+    /// Gets the key for the current collection.
+    /// </summary>
+    new TKey Key { get; }
+}

CommunityToolkit.Mvvm/Collections/Internals/ObservableGroupHelper.cs

@@ -0,0 +1,18 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.ComponentModel;
+
+namespace CommunityToolkit.Mvvm.Collections.Internals;
+
+/// <summary>
+/// A helper type for the <see cref="ObservableGroup{TKey, TValue}"/> type.
+/// </summary>
+internal static class ObservableGroupHelper
+{
+    /// <summary>
+    /// The cached <see cref="PropertyChangedEventArgs"/> for <see cref="IReadOnlyObservableGroup.Key"/>
+    /// </summary>
+    public static readonly PropertyChangedEventArgs KeyChangedEventArgs = new(nameof(IReadOnlyObservableGroup.Key));
+}

CommunityToolkit.Mvvm/Collections/ObservableGroupedCollection{TKey,TValue}.cs → CommunityToolkit.Mvvm/Collections/ObservableGroupedCollection{TKey,TElement}.cs

@@ -13,9 +13,9 @@ namespace CommunityToolkit.Mvvm.Collections;
 /// <summary>
 /// An observable list of observable groups.
 /// </summary>
-/// <typeparam name="TKey">The type of the group key.</typeparam>
-/// <typeparam name="TValue">The type of the items in the collection.</typeparam>
-public sealed class ObservableGroupedCollection<TKey, TValue> : ObservableCollection<ObservableGroup<TKey, TValue>>
+/// <typeparam name="TKey">The type of the group keys.</typeparam>
+/// <typeparam name="TElement">The type of elements in the collection.</typeparam>
+public sealed class ObservableGroupedCollection<TKey, TElement> : ObservableCollection<ObservableGroup<TKey, TElement>>, ILookup<TKey, TElement>
     where TKey : notnull
 {
     /// <summary>
@@ -29,21 +29,50 @@ public ObservableGroupedCollection()
     /// Initializes a new instance of the <see cref="ObservableGroupedCollection{TKey, TValue}"/> class.
     /// </summary>
     /// <param name="collection">The initial data to add in the grouped collection.</param>
-    public ObservableGroupedCollection(IEnumerable<IGrouping<TKey, TValue>> collection)
-        : base(collection.Select(static c => new ObservableGroup<TKey, TValue>(c)))
+    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="collection"/> is <see langword="null"/>.</exception>
+    public ObservableGroupedCollection(IEnumerable<IGrouping<TKey, TElement>> collection)
+        : base(collection?.Select(static group => new ObservableGroup<TKey, TElement>(group))!)
     {
     }
 
+    /// <inheritdoc/>
+    IEnumerable<TElement> ILookup<TKey, TElement>.this[TKey key]
+    {
+        get
+        {
+            IEnumerable<TElement>? result = null;
+
+            if (key is not null)
+            {
+                result = this.FirstGroupByKeyOrDefault(key);
+            }
+
+            return result ?? Enumerable.Empty<TElement>();
+        }
+    }
+
     /// <summary>
     /// Tries to get the underlying <see cref="List{T}"/> instance, if present.
     /// </summary>
     /// <param name="list">The resulting <see cref="List{T}"/>, if one was in use.</param>
     /// <returns>Whether or not a <see cref="List{T}"/> instance has been found.</returns>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    internal bool TryGetList([NotNullWhen(true)] out List<ObservableGroup<TKey, TValue>>? list)
+    internal bool TryGetList([NotNullWhen(true)] out List<ObservableGroup<TKey, TElement>>? list)
     {
-        list = Items as List<ObservableGroup<TKey, TValue>>;
+        list = Items as List<ObservableGroup<TKey, TElement>>;
 
         return list is not null;
     }
+
+    /// <inheritdoc/>
+    bool ILookup<TKey, TElement>.Contains(TKey key)
+    {
+        return key is not null && this.FirstGroupByKey(key) is not null;
+    }
+
+    /// <inheritdoc/>
+    IEnumerator<IGrouping<TKey, TElement>> IEnumerable<IGrouping<TKey, TElement>>.GetEnumerator()
+    {
+        return GetEnumerator();
+    }
 }

CommunityToolkit.Mvvm/Collections/ObservableGroup{TKey,TValue}.cs → CommunityToolkit.Mvvm/Collections/ObservableGroup{TKey,TElement}.cs

@@ -4,9 +4,11 @@
 
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
-using System.ComponentModel;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
+using System.Runtime.CompilerServices;
+using CommunityToolkit.Mvvm.Collections.Internals;
 
 namespace CommunityToolkit.Mvvm.Collections;
 
@@ -15,30 +17,29 @@ namespace CommunityToolkit.Mvvm.Collections;
 /// It associates a <see cref="Key"/> to an <see cref="ObservableCollection{T}"/>.
 /// </summary>
 /// <typeparam name="TKey">The type of the group key.</typeparam>
-/// <typeparam name="TValue">The type of the items in the collection.</typeparam>
+/// <typeparam name="TElement">The type of elements in the group.</typeparam>
 [DebuggerDisplay("Key = {Key}, Count = {Count}")]
-public class ObservableGroup<TKey, TValue> : ObservableCollection<TValue>, IGrouping<TKey, TValue>, IReadOnlyObservableGroup
+public sealed class ObservableGroup<TKey, TElement> : ObservableCollection<TElement>, IReadOnlyObservableGroup<TKey, TElement>
     where TKey : notnull
 {
-    /// <summary>
-    /// The cached <see cref="PropertyChangedEventArgs"/> for <see cref="Key"/>
-    /// </summary>
-    private static readonly PropertyChangedEventArgs KeyChangedEventArgs = new(nameof(Key));
-
     /// <summary>
     /// Initializes a new instance of the <see cref="ObservableGroup{TKey, TValue}"/> class.
     /// </summary>
     /// <param name="key">The key for the group.</param>
+    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="key"/> is <see langword="null"/>.</exception>
     public ObservableGroup(TKey key)
     {
+        ArgumentNullException.For<TKey>.ThrowIfNull(key);
+
         this.key = key;
     }
 
     /// <summary>
     /// Initializes a new instance of the <see cref="ObservableGroup{TKey, TValue}"/> class.
     /// </summary>
     /// <param name="grouping">The grouping to fill the group.</param>
-    public ObservableGroup(IGrouping<TKey, TValue> grouping)
+    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="grouping"/> is <see langword="null"/>.</exception>
+    public ObservableGroup(IGrouping<TKey, TElement> grouping)
         : base(grouping)
     {
         this.key = grouping.Key;
@@ -49,9 +50,12 @@ public ObservableGroup(IGrouping<TKey, TValue> grouping)
     /// </summary>
     /// <param name="key">The key for the group.</param>
     /// <param name="collection">The initial collection of data to add to the group.</param>
-    public ObservableGroup(TKey key, IEnumerable<TValue> collection)
+    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="key"/> or <paramref name="collection"/> are <see langword="null"/>.</exception>
+    public ObservableGroup(TKey key, IEnumerable<TElement> collection)
         : base(collection)
     {
+        ArgumentNullException.For<TKey>.ThrowIfNull(key);
+
         this.key = key;
     }
 
@@ -60,20 +64,39 @@ public ObservableGroup(TKey key, IEnumerable<TValue> collection)
     /// <summary>
     /// Gets or sets the key of the group.
     /// </summary>
+    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="value"/> is <see langword="null"/>.</exception>
     public TKey Key
     {
         get => this.key;
         set
         {
+            ArgumentNullException.For<TKey>.ThrowIfNull(value);
+
             if (!EqualityComparer<TKey>.Default.Equals(this.key!, value))
             {
                 this.key = value;
 
-                OnPropertyChanged(KeyChangedEventArgs);
+                OnPropertyChanged(ObservableGroupHelper.KeyChangedEventArgs);
             }
         }
     }
 
+    /// <summary>
+    /// Tries to get the underlying <see cref="List{T}"/> instance, if present.
+    /// </summary>
+    /// <param name="list">The resulting <see cref="List{T}"/>, if one was in use.</param>
+    /// <returns>Whether or not a <see cref="List{T}"/> instance has been found.</returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    internal bool TryGetList([NotNullWhen(true)] out List<TElement>? list)
+    {
+        list = Items as List<TElement>;
+
+        return list is not null;
+    }
+
     /// <inheritdoc/>
     object IReadOnlyObservableGroup.Key => Key;
+
+    /// <inheritdoc/>
+    object? IReadOnlyObservableGroup.this[int index] => this[index];
 }