updated drag and drop sample

ch08_HotStuff/DragNDrop/ExchangeNounoursPage.xaml.cs

@@ -24,7 +24,7 @@ public partial class ExchangeNounoursPage : ContentPage
 
 	private void OnDragStarting(object sender, DragStartingEventArgs e)
 	{
-		e.Data.Properties["value"] = (sender as Element)?.Parent.BindingContext;
+		e.Data.Properties["value"] = (sender as Element)?.BindingContext;
 	}
 	
 	private void OnDrop(object sender, DropEventArgs e)

ch08_HotStuff/DragNDrop/InCellsPage.xaml

@@ -66,8 +66,8 @@
                                                 Radius="10" Opacity="0.8"/>
                                     </Border.Shadow>
                                     <Border.GestureRecognizers>
-                                        <DragGestureRecognizer DragStarting="DragGestureRecognizer_DragStarting1"/>
-                                        <DropGestureRecognizer Drop="DropGestureRecognizer_Drop"/>
+                                        <DragGestureRecognizer DragStarting="Cages_DragStarting"/>
+                                        <DropGestureRecognizer Drop="Cages_Drop"/>
                                     </Border.GestureRecognizers>
                                     <Image Source="{Binding MyPrecious.Image}"/>
                                 </Border>

ch08_HotStuff/DragNDrop/InCellsPage.xaml.cs

@@ -23,9 +23,9 @@ public partial class InCellsPage : ContentPage
 		BindingContext = this;
 	}
 
-	private void DropGestureRecognizer_Drop(object sender, DropEventArgs e)
+	private void Cages_Drop(object sender, DropEventArgs e)
 	{
-		if ((sender as Element)?.Parent.BindingContext is Cage receivingElement)
+		if ((sender as Element)?.BindingContext is Cage receivingElement)
 		{
 			if (e.Data.Properties["value"] is Nounours draggedNounours)
 			{
@@ -36,19 +36,12 @@ public partial class InCellsPage : ContentPage
 
 	private void Bag_DragStarting(object sender, DragStartingEventArgs e)
 	{
-		e.Data.Properties["value"] = (sender as Element)?.Parent.BindingContext;
+		e.Data.Properties["value"] = (sender as Element)?.BindingContext;
     }
 
-	private void DropGestureRecognizer_DragLeave(object sender, DragEventArgs e)
+	private void Cages_DragStarting(object sender, DragStartingEventArgs e)
 	{
-		if(e.Data.Properties["value"] is Nounours draggedNounours
-			&& sender is Cage cage)
-			cage.MyPrecious = null;
-    }
-
-	private void DragGestureRecognizer_DragStarting1(object sender, DragStartingEventArgs e)
-	{
-		e.Data.Properties["value"] = ((sender as Element)?.Parent.BindingContext as Cage)?.MyPrecious;
+		e.Data.Properties["value"] = ((sender as Element)?.BindingContext as Cage)?.MyPrecious;
 		if((sender as Element)?.BindingContext is Cage cage)
 			cage.MyPrecious = null;
     }

ch08_HotStuff/DragNDrop/OrganizeNounoursPage.xaml.cs

@@ -22,23 +22,22 @@ public partial class OrganizeNounoursPage : ContentPage
 
 	private void OnDragStarting(object sender, DragStartingEventArgs e)
 	{
-		e.Data.Properties["value"] = (sender as Element)?.Parent.BindingContext;
+		e.Data.Properties["value"] = (sender as Element)?.BindingContext;
 	}
 	
 	private void OnDrop(object sender, DropEventArgs e)
 	{
-		var receivingElement = (sender as Element)?.Parent.BindingContext as Nounours;
-
-		var draggedElement = e.Data.Properties["value"] as Nounours;
-
+		if ((sender as Element)?.BindingContext is not Nounours receivingElement) return;
+		if (e.Data.Properties["value"] as Nounours is not Nounours draggedElement) return;
 
 		int draggedIndex = NounoursCollection.IndexOf(draggedElement);
-		if (NounoursCollection.Contains(draggedElement))
+		if (draggedIndex > -1)
 		{
 			NounoursCollection.RemoveAt(draggedIndex);
 		}
 
 		int receivingIndex = NounoursCollection.IndexOf(receivingElement);
+		if (receivingIndex == -1) throw new ArgumentNullException("There should be a receiving element");
 
 		if (receivingIndex >= draggedIndex)
 		{