Add TabStop/TabIndex (#2795)

* [Core, UWP, Android] support TabStop/TabIndex

* removing linq

* add iOS / MacOS implementation

* add WPF implementation
fixes UWP implementation

* - addressing comments
- improve test
- optimizations
- added previous tab direction in same tab group

* addressing comments

* [Android] support tabstop for pickers

* - moving shared code to a static class
- removed reflection

* ITabStop -> changed to internal, renamed, added in to fastRenderers

* Update ITabStop to be public

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/GitHub1700.cs

@@ -0,0 +1,244 @@
+using System.Collections.Generic;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 1700, "Desktop: TabStop/TabIndex support (for multiple Views)", PlatformAffected.All)]
+	public class GitHub1700 : TestContentPage
+	{
+		IList<View> listViews;
+
+		void IndexDesc()
+		{
+			int index = 100500;
+			foreach (var item in listViews)
+			{
+				if (item is Button but && but.Text.StartsWith("TabIndex"))
+					continue;
+				item.TabIndex = index--;
+			}
+		}
+
+		void IndexNegative()
+		{
+			int index = -100;
+			foreach (var item in listViews)
+			{
+				if (item is Button but && but.Text.StartsWith("TabIndex"))
+					continue;
+				item.TabIndex = index++;
+			}
+		}
+
+		protected override void Init()
+		{
+			var actionGrid = new Grid()
+			{
+				Padding = new Thickness(10),
+				BackgroundColor = Color.Aquamarine
+			};
+			actionGrid.AddChild(new Button()
+			{
+				Text = "Index desc",
+				Command = new Command(() => IndexDesc())
+			}, 0, 0);
+			actionGrid.AddChild(new Button()
+			{
+				Text = "All indexes equal 0",
+				Command = new Command(() => listViews.ForEach(c => c.TabIndex = 0))
+			}, 1, 0);
+			actionGrid.AddChild(new Button()
+			{
+				Text = "Negative indexes",
+				Command = new Command(() => IndexNegative())
+			}, 2, 0);
+			actionGrid.AddChild(new Button()
+			{
+				Text = "TabStops = True",
+				Command = new Command(() => listViews.ForEach(c => c.IsTabStop = true))
+			}, 0, 1);
+			actionGrid.AddChild(new Button()
+			{
+				Text = "TabStops = False",
+				Command = new Command(() => listViews.ForEach(c => c.IsTabStop = false))
+			}, 1, 1);
+			actionGrid.AddChild(new Button()
+			{
+				Text = "TabStops every second",
+				Command = new Command(() =>
+				{
+					for (int i = 0; i < listViews.Count; i++)
+						listViews[i].IsTabStop = i % 2 == 0;
+				})
+			}, 2, 1);
+
+			var pickerStopped = new Picker
+			{
+				Title = $"[+] Picker - Tab stop enable",
+				IsTabStop = true
+			};
+			var pickerNotStopped = new Picker
+			{
+				Title = "[-] Picker - Tab stop disable",
+				IsTabStop = false
+			};
+			for (var i = 1; i < 3; i++) {
+				pickerNotStopped.Items.Add("Sample Option " + i);
+				pickerStopped.Items.Add("Sample Option " + i);
+			}
+
+			var stack = new StackLayout
+			{
+				Children =
+				{
+					actionGrid,
+					pickerStopped,
+					pickerNotStopped,
+					new Button
+					{
+						Text = $"TabIndex 90",
+						IsTabStop = true,
+						TabIndex = 90
+					},
+					new Button
+					{
+						Text = $"TabIndex 100",
+						IsTabStop = true,
+						TabIndex = 100
+					},
+					new Button
+					{
+						Text = $"TabIndex 100",
+						IsTabStop = true,
+						TabIndex = 100
+					},
+					new Button
+					{
+						Text = $"TabIndex 90",
+						IsTabStop = true,
+						TabIndex = 90
+					},
+					new Button
+					{
+						Text = $"[+] Button - TabStop enable",
+						IsTabStop = true
+					},
+					new Button
+					{
+						Text = "Button - Non stop",
+						IsTabStop = false
+					},
+					new DatePicker
+					{
+						IsTabStop = true
+					},
+					new DatePicker
+					{
+						IsTabStop = false
+					},
+					new Editor
+					{
+						Text = $"[+] Editor - Tab stop enable",
+						IsTabStop = true
+					},
+					new Editor
+					{
+						Text = "Editor - Non stop",
+						IsTabStop = false
+					},
+					new Entry
+					{
+						Text = $"[+] Entry - Tab stop enable",
+						IsTabStop = true
+					},
+					new Entry
+					{
+						Text = "Entry - Non stop",
+						IsTabStop = false
+					},
+					new ProgressBar
+					{
+						IsTabStop = true,
+						HeightRequest = 40,
+						Progress = 80
+					},
+					new ProgressBar
+					{
+						IsTabStop = false,
+						HeightRequest = 40,
+						Progress = 40
+					},
+					new SearchBar
+					{
+						Text = $"[+] SearchBar - TabStop enable",
+						IsTabStop = true
+					},
+					new SearchBar
+					{
+						Text = "SearchBar - TabStop disable",
+						IsTabStop = false
+					},
+					new Slider
+					{
+						IsTabStop = true
+					},
+					new Slider
+					{
+						IsTabStop = false
+					},
+					new Stepper
+					{
+						IsTabStop = true
+					},
+					new Stepper
+					{
+						IsTabStop = false
+					},
+					new Switch
+					{
+						IsTabStop = true
+					},
+					new Switch
+					{
+						IsTabStop = false
+					},
+					new TimePicker
+					{
+						IsTabStop = true
+					},
+					new TimePicker
+					{
+						IsTabStop = false
+					},
+				}
+			};
+
+			listViews = stack.Children;
+
+			foreach (var item in listViews)
+			{
+				item.Focused += (_, e) =>
+				{
+					BackgroundColor = e.VisualElement.IsTabStop ? Color.Transparent : Color.OrangeRed;
+					Title = $"{e.VisualElement.TabIndex} - " + (e.VisualElement.IsTabStop ? "[+]" : "WRONG");
+					e.VisualElement.Scale = 0.7;
+				};
+				item.Unfocused += (_, e) =>
+				{
+					BackgroundColor = Color.Transparent;
+					Title = string.Empty;
+					e.VisualElement.Scale = 1;
+				};
+			}
+
+			IndexDesc();
+
+			Content = new ScrollView()
+			{
+				Content = stack
+			};
+		}
+	}
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -248,6 +248,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Effects\AttachedStateEffectList.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)GitHub1648.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)GitHub1702.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)GitHub1700.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)GitHub2598.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue1483.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue1556.cs" />

Xamarin.Forms.Controls/TestCases.cs

@@ -242,19 +242,11 @@ from issueModel in _issues
 				Root.Add(_section);
 			}
 
+			HashSet<string> _exemptNames = new HashSet<string> { "N0", "G342", "G1305", "G1461", "G1653", "G1700" };
+
 			// Legacy reasons, do not add to this list
 			// Going forward, make sure only one Issue attribute exist for a Tracker + Issue number pair
-			bool IsExempt (string name)
-			{
-				if (name == "G1461" || 
-					name == "G342" || 
-					name == "G1305" || 
-					name == "G1653" || 
-					name == "N0")
-					return true;
-				else
-					return false;
-			}
+			bool IsExempt(string name) => _exemptNames.Contains(name);
 		}
 
 		public static NavigationPage GetTestCases ()

Xamarin.Forms.Core/EnumerableExtensions.cs

@@ -59,6 +59,20 @@ public static void ForEach<T>(this IEnumerable<T> enumeration, Action<T> action)
 			}
 		}
 
+		public static IDictionary<TKey, List<TSource>> GroupToDictionary<TSource, TKey>(this IEnumerable<TSource> enumeration, Func<TSource, TKey> func)
+		{
+			var result = new Dictionary<TKey, List<TSource>>();
+			foreach (TSource item in enumeration)
+			{
+				var group = func(item);
+				if (!result.ContainsKey(group))
+					result.Add(group, new List<TSource> { item });
+				else
+					result[group].Add(item);
+			}
+			return result;
+		}
+
 		public static int IndexOf<T>(this IEnumerable<T> enumerable, T item)
 		{
 			if (enumerable == null)

Xamarin.Forms.Core/ProgressBar.cs

@@ -12,6 +12,8 @@ public class ProgressBar : View, IElementConfiguration<ProgressBar>
 
 		public static readonly BindableProperty ProgressProperty = BindableProperty.Create(nameof(Progress), typeof(double), typeof(ProgressBar), 0d, coerceValue: (bo, v) => ((double)v).Clamp(0, 1));
 
+		protected override bool TabStopDefaultValueCreator() => false;
+
 		readonly Lazy<PlatformConfigurationRegistry<ProgressBar>> _platformConfigurationRegistry;
 
 		public ProgressBar()

Xamarin.Forms.Core/TabIndexExtensions.cs

@@ -0,0 +1,84 @@
+using System.Collections.Generic;
+using Xamarin.Forms.Internals;
+
+namespace Xamarin.Forms
+{
+	public static class TabIndexExtensions
+	{
+		public static IDictionary<int, List<VisualElement>> GetTabIndexesOnParentPage(this VisualElement element, out int countChildrensWithTabStopWithoutThis)
+		{
+			countChildrensWithTabStopWithoutThis = 0;
+
+			Element parentPage = element.Parent;
+			while (parentPage != null && !(parentPage is Page))
+				parentPage = parentPage.Parent;
+
+			var descendantsOnPage = parentPage?.VisibleDescendants();
+			if (descendantsOnPage == null)
+				return null;
+
+			var childrensWithTabStop = new List<VisualElement>();
+			foreach (var descendant in descendantsOnPage)
+			{
+				if (descendant is VisualElement visualElement && visualElement.IsTabStop)
+					childrensWithTabStop.Add(visualElement);
+			}
+			if (!childrensWithTabStop.Contains(element))
+				return null;
+
+			countChildrensWithTabStopWithoutThis = childrensWithTabStop.Count - 1;
+			return childrensWithTabStop.GroupToDictionary(c => c.TabIndex);
+		}
+
+		public static VisualElement FindNextElement(this VisualElement element, bool forwardDirection, IDictionary<int, List<VisualElement>> tabIndexes, ref int tabIndex)
+		{
+			var tabGroup = tabIndexes[tabIndex];
+			if (!forwardDirection)
+			{
+				// search prev element in same TabIndex group
+				var prevSubIndex = tabGroup.IndexOf(element) - 1;
+				if (prevSubIndex >= 0 && prevSubIndex < tabGroup.Count)
+				{
+					return tabGroup[prevSubIndex];
+				}
+				else // search prev element in prev TabIndex group
+				{
+					var smallerMax = int.MinValue;
+					var tabIndexesMax = int.MinValue;
+					foreach (var index in tabIndexes.Keys)
+					{
+						if (index < tabIndex && smallerMax < index)
+							smallerMax = index;
+						if (tabIndexesMax < index)
+							tabIndexesMax = index;
+					}
+					tabIndex = smallerMax != int.MinValue ? smallerMax : tabIndexesMax;
+					return tabIndexes[tabIndex][0];
+				}
+			}
+			else // Forward
+			{
+				// search next element in same TabIndex group
+				var nextSubIndex = tabGroup.IndexOf(element) + 1;
+				if (nextSubIndex > 0 && nextSubIndex < tabGroup.Count)
+				{
+					return tabGroup[nextSubIndex];
+				}
+				else // search next element in next TabIndex group
+				{
+					var biggerMin = int.MaxValue;
+					var tabIndexesMin = int.MaxValue;
+					foreach (var index in tabIndexes.Keys)
+					{
+						if (index > tabIndex && biggerMin > index)
+							biggerMin = index;
+						if (tabIndexesMin > index)
+							tabIndexesMin = index;
+					}
+					tabIndex = biggerMin != int.MaxValue ? biggerMin : tabIndexesMin;
+					return tabIndexes[tabIndex][0];
+				}
+			}
+		}
+	}
+}