Add support for default choice in selection prompt

Closes #234

src/Spectre.Console/Widgets/Prompt/MultiSelectionPrompt.cs

@@ -23,6 +23,11 @@ namespace Spectre.Console
         /// </summary>
         public List<T> Choices { get; }
 
+        /// <summary>
+        /// Gets the initially selected choices.
+        /// </summary>
+        public HashSet<int> Selected { get; }
+
         /// <summary>
         /// Gets or sets the converter to get the display string for a choice. By default
         /// the corresponding <see cref="TypeConverter"/> is used.
@@ -52,6 +57,7 @@ namespace Spectre.Console
         public MultiSelectionPrompt()
         {
             Choices = new List<T>();
+            Selected = new HashSet<int>();
         }
 
         /// <inheritdoc/>
@@ -73,7 +79,7 @@ namespace Spectre.Console
 
             var converter = Converter ?? TypeConverterHelper.ConvertToString;
 
-            var list = new RenderableMultiSelectionList<T>(console, Title, PageSize, Choices, converter, HighlightStyle);
+            var list = new RenderableMultiSelectionList<T>(console, Title, PageSize, Choices, Selected, converter, HighlightStyle);
             using (new RenderHookScope(console, list))
             {
                 console.Cursor.Hide();

src/Spectre.Console/Widgets/Prompt/MultiSelectionPromptExtensions.cs

@@ -44,6 +44,73 @@ namespace Spectre.Console
             return obj;
         }
 
+        /// <summary>
+        /// Marks an item as selected.
+        /// </summary>
+        /// <typeparam name="T">The prompt result type.</typeparam>
+        /// <param name="obj">The prompt.</param>
+        /// <param name="index">The index of the item to select.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static MultiSelectionPrompt<T> Select<T>(this MultiSelectionPrompt<T> obj, int index)
+        {
+            if (obj is null)
+            {
+                throw new ArgumentNullException(nameof(obj));
+            }
+
+            if (index < 0)
+            {
+                throw new ArgumentException("Index must be greater than zero", nameof(index));
+            }
+
+            obj.Selected.Add(index);
+            return obj;
+        }
+
+        /// <summary>
+        /// Marks multiple items as selected.
+        /// </summary>
+        /// <typeparam name="T">The prompt result type.</typeparam>
+        /// <param name="obj">The prompt.</param>
+        /// <param name="indices">The indices of the items to select.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static MultiSelectionPrompt<T> Select<T>(this MultiSelectionPrompt<T> obj, params int[] indices)
+        {
+            if (obj is null)
+            {
+                throw new ArgumentNullException(nameof(obj));
+            }
+
+            foreach (var index in indices)
+            {
+                Select(obj, index);
+            }
+
+            return obj;
+        }
+
+        /// <summary>
+        /// Marks multiple items as selected.
+        /// </summary>
+        /// <typeparam name="T">The prompt result type.</typeparam>
+        /// <param name="obj">The prompt.</param>
+        /// <param name="indices">The indices of the items to select.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static MultiSelectionPrompt<T> Select<T>(this MultiSelectionPrompt<T> obj, IEnumerable<int> indices)
+        {
+            if (obj is null)
+            {
+                throw new ArgumentNullException(nameof(obj));
+            }
+
+            foreach (var index in indices)
+            {
+                Select(obj, index);
+            }
+
+            return obj;
+        }
+
         /// <summary>
         /// Adds multiple choices.
         /// </summary>

src/Spectre.Console/Widgets/Prompt/Rendering/RenderableMultiSelectionList.cs

@@ -17,14 +17,15 @@ namespace Spectre.Console
 
         public RenderableMultiSelectionList(
             IAnsiConsole console, string? title, int pageSize,
-            List<T> choices, Func<T, string>? converter, Style? highlightStyle)
+            List<T> choices, HashSet<int> selections,
+            Func<T, string>? converter, Style? highlightStyle)
             : base(console, pageSize, choices, converter)
         {
             _console = console ?? throw new ArgumentNullException(nameof(console));
             _title = title;
             _highlightStyle = highlightStyle ?? new Style(foreground: Color.Blue);
 
-            Selections = new HashSet<int>();
+            Selections = new HashSet<int>(selections);
         }
 
         public void Select()
@@ -52,7 +53,8 @@ namespace Spectre.Console
             return pageSize;
         }
 
-        protected override IRenderable Build(int pointerIndex, bool scrollable, IEnumerable<(int Original, int Index, string Item)> choices)
+        protected override IRenderable Build(int pointerIndex, bool scrollable,
+            IEnumerable<(int Original, int Index, string Item)> choices)
         {
             var list = new List<IRenderable>();