Fix issue with ColorPicker not working with SwitchPresenter changes

Since optimizing SwitchPresenter, the delayed loading being in another control's template was causing an issue where the cases weren't available to evaluate when SwitchPresenter was ready. Since we no longer listened to all downstream changes in SwitchPresenter this meant we never evaluated once the parent control was loaded. Added a couple of cases to ensure that once we're fully loaded we can evaluate again to ensure we pick a case. This also covers a case where if somehow the entire collection gets swapped out later.

Added some extra checks ahead of the evaluate cases to break out early in case we don't need to update the Visual Tree.

Author: michael-hawker

Microsoft.Toolkit.Uwp.UI.Controls.Primitives/SwitchPresenter/SwitchPresenter.cs

@@ -46,7 +46,7 @@ public CaseCollection SwitchCases
         /// Indicates the <see cref="SwitchCases"/> property.
         /// </summary>
         public static readonly DependencyProperty SwitchCasesProperty =
-            DependencyProperty.Register(nameof(SwitchCases), typeof(CaseCollection), typeof(SwitchPresenter), new PropertyMetadata(null));
+            DependencyProperty.Register(nameof(SwitchCases), typeof(CaseCollection), typeof(SwitchPresenter), new PropertyMetadata(null, OnSwitchCasesPropertyChanged));
 
         /// <summary>
         /// Gets or sets a value indicating the value to compare all cases against. When this value is bound to and changes, the presenter will automatically evaluate cases and select the new appropriate content from the switch.
@@ -81,9 +81,19 @@ public Type TargetType
         private static void OnValuePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
         {
             // When our Switch's expression changes, re-evaluate.
-            var xswitch = (SwitchPresenter)d;
+            if (d is SwitchPresenter xswitch)
+            {
+                xswitch.EvaluateCases();
+            }
+        }
 
-            xswitch.EvaluateCases();
+        private static void OnSwitchCasesPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+        {
+            // If our collection somehow changes, we should re-evaluate.
+            if (d is SwitchPresenter xswitch)
+            {
+                xswitch.EvaluateCases();
+            }
         }
 
         /// <summary>
@@ -92,6 +102,14 @@ private static void OnValuePropertyChanged(DependencyObject d, DependencyPropert
         public SwitchPresenter()
         {
             this.SwitchCases = new CaseCollection();
+
+            Loaded += this.SwitchPresenter_Loaded;
+        }
+
+        private void SwitchPresenter_Loaded(object sender, RoutedEventArgs e)
+        {
+            // In case we're in a template, we may have loaded cases later.
+            EvaluateCases();
         }
 
         /// <inheritdoc/>
@@ -104,8 +122,20 @@ protected override void OnApplyTemplate()
 
         private void EvaluateCases()
         {
-            if (CurrentCase != null &&
-                CurrentCase.Value != null &&
+            if (SwitchCases == null ||
+                SwitchCases.Count == 0)
+            {
+                // If we have no cases, then we can't match anything.
+                if (CurrentCase != null)
+                {
+                    // Only bother clearing our actual content if we had something before.
+                    Content = null;
+                    CurrentCase = null;
+                }
+
+                return;
+            }
+            else if (CurrentCase?.Value != null &&
                 CurrentCase.Value.Equals(Value))
             {
                 // If the current case we're on already matches our current value,
@@ -134,7 +164,7 @@ private void EvaluateCases()
 
             if (newcase == null && xdefault != null)
             {
-                // Inject default if we found one.
+                // Inject default if we found one without matching anything
                 newcase = xdefault;
             }