a picker behavior

Signed-off-by: Paul Schneider <paul@pschneider.fr>

BookAStar/BookAStar/Behaviors/PickerBehavior.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Xamarin.Forms;
+
+namespace BookAStar.Behaviors
+{
+    public class PickerBehavior : Behavior<Picker>
+    {
+        // Creating BindableProperties with Limited write access: http://iosapi.xamarin.com/index.aspx?link=M%3AXamarin.Forms.BindableObject.SetValue(Xamarin.Forms.BindablePropertyKey%2CSystem.Object) 
+
+        public static readonly BindablePropertyKey SelectedItemPropertyKey = BindableProperty.CreateReadOnly("SelectedItem", typeof(string), typeof(PickerBehavior), default(string));
+
+        public static readonly BindableProperty SelectedItemProperty = SelectedItemPropertyKey.BindableProperty;
+
+        public string SelectedItem
+        {
+            get { return (string)base.GetValue(SelectedItemProperty); }
+            private set { base.SetValue(SelectedItemPropertyKey, value); }
+        }
+
+
+        protected override void OnAttachedTo(Picker bindable)
+        {
+            bindable.SelectedIndexChanged += bindable_SelectedIndexChanged;
+        }
+
+        void bindable_SelectedIndexChanged(object sender, EventArgs e)
+        {
+            var picker = (Picker)sender;
+            if (picker.SelectedIndex <= 0 || picker.SelectedIndex > picker.Items.Count - 1)
+                SelectedItem = null;
+            else
+                SelectedItem=picker.Items[picker.SelectedIndex];
+        }
+
+        protected override void OnDetachingFrom(Picker bindable)
+        {
+            bindable.SelectedIndexChanged -= bindable_SelectedIndexChanged;
+        }
+    }
+}