Merge pull request #878 from skendrot/MasterDetailsSelectedItem

Allow SelectedItem to be set before control is loaded

Author: deltakosh

Microsoft.Toolkit.Uwp.UI.Controls/MasterDetailsView/MasterDetailsView.cs

@@ -71,6 +71,7 @@ protected override void OnApplyTemplate()
             base.OnApplyTemplate();
 
             _detailsPresenter = (ContentPresenter)GetTemplateChild(PartDetailsPresenter);
+            SetDetailsContent();
 
             SetMasterHeaderVisibility();
         }
@@ -86,22 +87,23 @@ protected override void OnApplyTemplate()
         private static void OnSelectedItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
         {
             var view = (MasterDetailsView)d;
-            string noSelectionState = view._stateGroup.CurrentState == view._narrowState
-                ? NoSelectionNarrowState
-                : NoSelectionWideState;
-            VisualStateManager.GoToState(view, view.SelectedItem == null ? noSelectionState : HasSelectionState, true);
+            if (view._stateGroup != null)
+            {
+                view.SetVisualState(view._stateGroup.CurrentState, true);
+            }
 
             view.OnSelectionChanged(new SelectionChangedEventArgs(new List<object> { e.OldValue }, new List<object> { e.NewValue }));
 
             // If there is no selection, do not remove the DetailsPresenter content but let it animate out.
             if (view.SelectedItem != null)
             {
-                view._detailsPresenter.Content = view.MapDetails == null
-                    ? view.SelectedItem
-                    : view.MapDetails(view.SelectedItem);
+                view.SetDetailsContent();
             }
 
-            view.SetBackButtonVisibility(view._stateGroup.CurrentState);
+            if (view._stateGroup != null)
+            {
+                view.SetBackButtonVisibility(view._stateGroup.CurrentState);
+            }
         }
 
         /// <summary>
@@ -140,10 +142,8 @@ private void OnLoaded(object sender, RoutedEventArgs e)
 
             _narrowState = GetTemplateChild(NarrowState) as VisualState;
 
-            string noSelectionState = _stateGroup.CurrentState == _narrowState
-                ? NoSelectionNarrowState
-                : NoSelectionWideState;
-            VisualStateManager.GoToState(this, this.SelectedItem == null ? noSelectionState : HasSelectionState, true);
+            SetVisualState(_stateGroup.CurrentState, true);
+            SetBackButtonVisibility(_stateGroup.CurrentState);
 
             UpdateViewState();
         }
@@ -174,10 +174,7 @@ private void OnVisualStateChanged(object sender, VisualStateChangedEventArgs e)
             SetBackButtonVisibility(e.NewState);
 
             // When adaptive trigger changes state, switch between NoSelectionWide and NoSelectionNarrow.
-            string noSelectionState = e.NewState == _narrowState
-                ? NoSelectionNarrowState
-                : NoSelectionWideState;
-            VisualStateManager.GoToState(this, this.SelectedItem == null ? noSelectionState : HasSelectionState, false);
+            SetVisualState(e.NewState, false);
         }
 
         /// <summary>
@@ -271,5 +268,23 @@ private void UpdateViewState()
                 ViewStateChanged?.Invoke(this, after);
             }
         }
+
+        private void SetVisualState(VisualState state, bool animate)
+        {
+            string noSelectionState = state == _narrowState
+                ? NoSelectionNarrowState
+                : NoSelectionWideState;
+            VisualStateManager.GoToState(this, SelectedItem == null ? noSelectionState : HasSelectionState, animate);
+        }
+
+        private void SetDetailsContent()
+        {
+            if ((SelectedItem != null) && (_detailsPresenter != null))
+            {
+                _detailsPresenter.Content = MapDetails == null
+                    ? SelectedItem
+                    : MapDetails(SelectedItem);
+            }
+        }
     }
 }