Allow CommandBar to expand and show command labels (#594)

* Allow CommandBar to expand and show command labels
Consolidate command bar placement code

* Make title text wrapping consistent between NavigationPage and MasterDetailPage

* Align toolbar/navigation bar behavior with other platforms

1 files changed, 76 insertions, 3 deletions

diff --git a/Xamarin.Forms.Platform.UAP/FormsCommandBar.cs b/Xamarin.Forms.Platform.UAP/FormsCommandBar.cs
index 21e9ca16..41451864 100644
--- a/Xamarin.Forms.Platform.UAP/FormsCommandBar.cs
+++ b/Xamarin.Forms.Platform.UAP/FormsCommandBar.cs
@@ -1,18 +1,30 @@
-using Windows.Foundation.Collections;
+using System;
+using Windows.Foundation.Collections;
 using Windows.UI.Xaml;
 using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Data;
 
 namespace Xamarin.Forms.Platform.UWP
 {
 	public class FormsCommandBar : CommandBar
 	{
-		// TODO Once 10.0.14393.0 is available, enable dynamic overflow: https://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.xaml.controls.commandbar.isdynamicoverflowenabled.aspx 
+		// TODO Once 10.0.14393.0 is available (and we don't have to support lower versions), enable dynamic overflow: https://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.xaml.controls.commandbar.isdynamicoverflowenabled.aspx 
 
+		Windows.UI.Xaml.Controls.Button _moreButton;
+		
 		public FormsCommandBar()
 		{
 			PrimaryCommands.VectorChanged += OnCommandsChanged;
 			SecondaryCommands.VectorChanged += OnCommandsChanged;
 			UpdateVisibility();
+			WatchForContentChanges();
+		}
+
+		protected override void OnApplyTemplate()
+		{
+			base.OnApplyTemplate();
+
+			_moreButton = GetTemplateChild("MoreButton") as Windows.UI.Xaml.Controls.Button;
 		}
 
 		void OnCommandsChanged(IObservableVector<ICommandBarElement> sender, IVectorChangedEventArgs args)
@@ -22,7 +34,68 @@ namespace Xamarin.Forms.Platform.UWP
 
 		void UpdateVisibility()
 		{
-			Visibility = PrimaryCommands.Count + SecondaryCommands.Count > 0 ? Visibility.Visible : Visibility.Collapsed;
+			var visibility = PrimaryCommands.Count + SecondaryCommands.Count > 0 ? Visibility.Visible : Visibility.Collapsed;
+		
+			if (_moreButton != null)
+			{
+				// The "..." button should only be visible if we have commands to display
+				_moreButton.Visibility = visibility;
+
+				// There *is* an OverflowButtonVisibility property that does more or less the same thing, 
+				// but it became available in 10.0.14393.0 and we have to support 10.0.10240
+			}
+
+			// If we have a title (or some other content) inside this command bar
+			// and that content is not collapsed
+			var frameworkElement = Content as FrameworkElement;
+
+			// Temporarily tie the visibility of the toolbar to the visibility of the Title
+			// to be consistent with the old style / other platforms
+			if (frameworkElement != null && frameworkElement.Visibility == Visibility.Collapsed)
+			{
+				Visibility = Visibility.Collapsed;
+				return;
+			}
+
+			if (frameworkElement != null && frameworkElement.Visibility != Visibility.Collapsed)
+			{
+				Visibility = Visibility.Visible;
+			}
+			else
+			{
+				// Otherwise, collapse it if there are no commands
+				Visibility = visibility;
+			}
+		}
+
+		void WatchForContentChanges()
+		{
+			// If the content of the command bar changes while it's collapsed, we need to 
+			// react and update the visibility (e.g., if the bar is placed at the bottom and
+			// has no commands, then is moved to the top and now includes the title)
+
+			// There's no event on CommandBar when the content changes, so we'll bind our own
+			// dependency property to Content and update our visibility when it changes
+			var binding = new Windows.UI.Xaml.Data.Binding
+			{
+				Source = this,
+				Path = new PropertyPath(nameof(Content)),
+				Mode = Windows.UI.Xaml.Data.BindingMode.OneWay
+			};
+
+			BindingOperations.SetBinding(this, s_contentChangeWatcher, binding);
+		}
+
+		static readonly DependencyProperty s_contentChangeWatcher =
+			DependencyProperty.Register(
+				"ContentChangeWatcher",
+				typeof(object),
+				typeof(object),
+				new PropertyMetadata(null, ContentChanged));
+
+		static void ContentChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+		{
+			(d as FormsCommandBar)?.UpdateVisibility();
 		}
 	}
 }
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