Moved code into *.Properties.cs file, wired up IsAlwaysOn flag

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/ViewportBehavior/ViewportBehaviorPage.xaml

@@ -7,7 +7,7 @@
       xmlns:behaviors="using:Microsoft.Toolkit.Uwp.UI.Behaviors"
       mc:Ignorable="d">
     <Page.Resources>
-        <behaviors:ViewportBehavior x:Key="ViewportBehavior" IsAlwaysOn="True"/>
+        <behaviors:ViewportBehavior x:Key="ViewportBehavior" />
     </Page.Resources>
 
     <Grid>

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/ViewportBehavior/ViewportBehaviorXaml.bind

@@ -14,7 +14,7 @@
                         Height="200"
                         Background="Gray">
                     <interactivity:Interaction.Behaviors>
-                        <behaviors:ViewportBehavior x:Name="ViewportBehavior" />
+                        <behaviors:ViewportBehavior x:Name="ViewportBehavior" IsAlwaysOn="@[IsAlwaysOn:Bool:True]" />
                     </interactivity:Interaction.Behaviors>
                     <Image x:Name="EffectElement"
                            Width="100"

Microsoft.Toolkit.Uwp.UI.Behaviors/Viewport/ViewportBehavior.Properties.cs

@@ -1,9 +1,25 @@
+using System;
 using Windows.UI.Xaml;
 
 namespace Microsoft.Toolkit.Uwp.UI.Behaviors
 {
     public partial class ViewportBehavior
     {
+        /// <summary>
+        /// The IsFullyInViewport value of the associated element
+        /// </summary>
+        public static readonly DependencyProperty IsFullyInViewportProperty =
+            DependencyProperty.Register(nameof(IsFullyInViewport), typeof(bool), typeof(ViewportBehavior), new PropertyMetadata(default(bool), OnIsFullyInViewportChanged));
+
+        /// <summary>
+        /// The IsInViewport value of the associated element
+        /// </summary>
+        public static readonly DependencyProperty IsInViewportProperty =
+            DependencyProperty.Register(nameof(IsInViewport), typeof(bool), typeof(ViewportBehavior), new PropertyMetadata(default(bool), OnIsInViewportChanged));
+
+        /// <summary>
+        /// The IsAlwaysOn value of the associated element
+        /// </summary>
         public static readonly DependencyProperty IsAlwaysOnProperty =
             DependencyProperty.Register(nameof(IsAlwaysOn), typeof(bool), typeof(ViewportBehavior), new PropertyMetadata(true));
 
@@ -15,5 +31,69 @@ namespace Microsoft.Toolkit.Uwp.UI.Behaviors
             get { return (bool)GetValue(IsAlwaysOnProperty); }
             set { SetValue(IsAlwaysOnProperty, value); }
         }
+
+        /// <summary>
+        /// Gets a value indicating whether associated element is fully in the ScrollViewer viewport
+        /// </summary>
+        public bool IsFullyInViewport
+        {
+            get { return (bool)GetValue(IsFullyInViewportProperty); }
+            private set { SetValue(IsFullyInViewportProperty, value); }
+        }
+
+        /// <summary>
+        /// Gets a value indicating whether associated element is in the ScrollViewer viewport
+        /// </summary>
+        public bool IsInViewport
+        {
+            get { return (bool)GetValue(IsInViewportProperty); }
+            private set { SetValue(IsInViewportProperty, value); }
+        }
+
+        /// <summary>
+        /// Event tracking when the object is fully within the viewport or not
+        /// </summary>
+        /// <param name="d">ViewportBehavior</param>
+        /// <param name="e">EventArgs</param>
+        private static void OnIsFullyInViewportChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+        {
+            var obj = (ViewportBehavior)d;
+            var value = (bool)e.NewValue;
+
+            if (obj.IsAlwaysOn)
+            {
+                if (value)
+                {
+                    obj.EnteredViewport?.Invoke(obj.AssociatedObject, EventArgs.Empty);
+                }
+                else
+                {
+                    obj.ExitingViewport?.Invoke(obj.AssociatedObject, EventArgs.Empty);
+                }
+            }
+        }
+
+        /// <summary>
+        /// Event tracking the state of the object as it moves in and out of the viewport
+        /// </summary>
+        /// <param name="d">ViewportBehavior</param>
+        /// <param name="e">EventArgs</param>
+        private static void OnIsInViewportChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+        {
+            var obj = (ViewportBehavior)d;
+            var value = (bool)e.NewValue;
+
+            if (obj.IsAlwaysOn)
+            {
+                if (value)
+                {
+                    obj.EnteringViewport?.Invoke(obj.AssociatedObject, EventArgs.Empty);
+                }
+                else
+                {
+                    obj.ExitedViewport?.Invoke(obj.AssociatedObject, EventArgs.Empty);
+                }
+            }
+        }
     }
 }

Microsoft.Toolkit.Uwp.UI.Behaviors/Viewport/ViewportBehavior.cs

@@ -21,24 +21,6 @@ namespace Microsoft.Toolkit.Uwp.UI.Behaviors
         /// </summary>
         private ScrollViewer _hostScrollViewer;
 
-        /// <summary>
-        /// The IsFullyInViewport value of the associated element
-        /// </summary>
-        public static readonly DependencyProperty IsFullyInViewportProperty =
-            DependencyProperty.Register(nameof(IsFullyInViewport), typeof(bool), typeof(ViewportBehavior), new PropertyMetadata(default(bool), OnIsFullyInViewportChanged));
-
-        /// <summary>
-        /// The IsInViewport value of the associated element
-        /// </summary>
-        public static readonly DependencyProperty IsInViewportProperty =
-            DependencyProperty.Register(nameof(IsInViewport), typeof(bool), typeof(ViewportBehavior), new PropertyMetadata(default(bool), OnIsInViewportChanged));
-
-        /// <summary>
-        /// The IsAlwaysOn value of the associated element
-        /// </summary>
-        //public static readonly DependencyProperty IsAlwaysOnProperty =
-        //    DependencyProperty.Register(nameof(IsAlwaysOn), typeof(bool), typeof(ViewportBehavior), new PropertyMetadata(true));
-
         /// <summary>
         /// Associated element fully enter the ScrollViewer viewport event
         /// </summary>
@@ -59,33 +41,6 @@ namespace Microsoft.Toolkit.Uwp.UI.Behaviors
         /// </summary>
         public event EventHandler ExitingViewport;
 
-        ///// <summary>
-        ///// Gets or sets a value indicating whether this behavior will remain attached after the associated element enters the viewport. When false, the behavior will remove itself after entering.
-        ///// </summary>
-        //public bool IsAlwaysOn
-        //{
-        //    get { return (bool)GetValue(IsAlwaysOnProperty); }
-        //    set { SetValue(IsAlwaysOnProperty, value); }
-        //}
-
-        /// <summary>
-        /// Gets a value indicating whether associated element is fully in the ScrollViewer viewport
-        /// </summary>
-        public bool IsFullyInViewport
-        {
-            get { return (bool)GetValue(IsFullyInViewportProperty); }
-            private set { SetValue(IsFullyInViewportProperty, value); }
-        }
-
-        /// <summary>
-        /// Gets a value indicating whether associated element is in the ScrollViewer viewport
-        /// </summary>
-        public bool IsInViewport
-        {
-            get { return (bool)GetValue(IsInViewportProperty); }
-            private set { SetValue(IsInViewportProperty, value); }
-        }
-
         /// <summary>
         /// Called after the behavior is attached to the <see cref="P:Microsoft.Xaml.Interactivity.Behavior.AssociatedObject" />.
         /// </summary>
@@ -120,39 +75,6 @@ namespace Microsoft.Toolkit.Uwp.UI.Behaviors
             _hostScrollViewer = null;
         }
 
-        private static void OnIsFullyInViewportChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
-        {
-            var obj = (ViewportBehavior)d;
-            var value = (bool)e.NewValue;
-            if (value)
-            {
-                obj.EnteredViewport?.Invoke(obj.AssociatedObject, EventArgs.Empty);
-
-                if (!obj.IsAlwaysOn)
-                {
-                    Interaction.GetBehaviors(obj.AssociatedObject).Remove(obj);
-                }
-            }
-            else
-            {
-                obj.ExitingViewport?.Invoke(obj.AssociatedObject, EventArgs.Empty);
-            }
-        }
-
-        private static void OnIsInViewportChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
-        {
-            var obj = (ViewportBehavior)d;
-            var value = (bool)e.NewValue;
-            if (value)
-            {
-                obj.EnteringViewport?.Invoke(obj.AssociatedObject, EventArgs.Empty);
-            }
-            else
-            {
-                obj.ExitedViewport?.Invoke(obj.AssociatedObject, EventArgs.Empty);
-            }
-        }
-
         private void ParentScrollViewer_ViewChanged(object sender, ScrollViewerViewChangedEventArgs e)
         {
             var associatedElementRect = AssociatedObject.TransformToVisual(_hostScrollViewer)