A star behavior!

BookAStar/BookAStar/Behaviors/StarBehavior.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Xamarin.Forms;
+
+namespace BookAStar.Behaviors
+{
+    public class StarBehavior : Behavior<View>
+    {
+        TapGestureRecognizer tapRecognizer;
+        static List<StarBehavior> defaultBehaviors = new List<StarBehavior>();
+        static Dictionary<string, List<StarBehavior>> starGroups = new Dictionary<string, List<StarBehavior>>();
+        
+        public static readonly BindableProperty GroupNameProperty =
+            BindableProperty.Create("GroupName",
+                                    typeof(string),
+                                    typeof(StarBehavior),
+                                    null,
+                                    propertyChanged: OnGroupNameChanged);
+
+        public string GroupName
+        {
+            set { SetValue(GroupNameProperty, value); }
+            get { return (string)GetValue(GroupNameProperty); }
+        }
+
+        public static readonly BindableProperty RatingProperty =
+           BindableProperty.Create("Rating",
+                                   typeof(int),
+                                   typeof(StarBehavior), default(int),
+                                   BindingMode.TwoWay);
+
+        public int Rating
+        {
+            set { SetValue(RatingProperty, value); }
+            get { return (int)GetValue(RatingProperty); }
+        }
+        
+        static void OnGroupNameChanged(BindableObject bindable, object oldValue, object newValue)
+        {
+            StarBehavior behavior = (StarBehavior)bindable;
+            string oldGroupName = (string)oldValue;
+            string newGroupName = (string)newValue;
+
+            // Remove existing behavior from Group
+            if (String.IsNullOrEmpty(oldGroupName))
+            {
+                defaultBehaviors.Remove(behavior);
+            }
+            else
+            {
+                List<StarBehavior> behaviors = starGroups[oldGroupName];
+                behaviors.Remove(behavior);
+
+                if (behaviors.Count == 0)
+                {
+                    starGroups.Remove(oldGroupName);
+                }
+            }
+            
+            // Add New Behavior to the group
+            if (String.IsNullOrEmpty(newGroupName))
+            {
+                defaultBehaviors.Add(behavior);
+            }
+            else
+            {
+                List<StarBehavior> behaviors = null;
+
+                if (starGroups.ContainsKey(newGroupName))
+                {
+                    behaviors = starGroups[newGroupName];
+                }
+                else
+                {
+                    behaviors = new List<StarBehavior>();
+                    starGroups.Add(newGroupName, behaviors);
+                }
+
+                behaviors.Add(behavior);
+            }
+        }
+
+        public static readonly BindableProperty IsStarredProperty =
+            BindableProperty.Create("IsStarred", 
+                                    typeof(bool), 
+                                    typeof(StarBehavior), 
+                                    false,
+                                    propertyChanged: OnIsStarredChanged);
+
+        public bool IsStarred
+        {
+            set { SetValue(IsStarredProperty, value); }
+            get { return (bool)GetValue(IsStarredProperty); }
+        }
+
+        static void OnIsStarredChanged(BindableObject bindable, object oldValue, object newValue)
+        {
+             StarBehavior behavior = (StarBehavior)bindable;
+
+             if ((bool)newValue)
+             {
+                 string groupName = behavior.GroupName;
+                 List<StarBehavior> behaviors = null;
+
+                 if (string.IsNullOrEmpty(groupName))
+                 {
+                     behaviors = defaultBehaviors;
+                 }
+                 else
+                 {
+                     behaviors = starGroups[groupName];
+                 }
+
+                 bool itemReached = false;
+                 int count = 1, position = 0;
+                 // all positions to left IsStarred = true and all position to the right is false
+                 foreach (var item in behaviors)
+                 {
+                     if (item != behavior && !itemReached)
+                     {
+                         item.IsStarred = true;
+                     }
+                     if (item == behavior)
+                     {
+                         itemReached = true;
+                         item.IsStarred = true;
+                         position = count;
+                     }
+                     if (item != behavior && itemReached)
+                         item.IsStarred = false;
+                     
+                     item.Rating = position;
+                     count++;
+                 }
+                 
+             }
+
+            
+        }
+
+        public StarBehavior()
+        {
+            defaultBehaviors.Add(this);
+        }
+
+        protected override void OnAttachedTo(View view)
+        {
+            tapRecognizer = new TapGestureRecognizer();
+            tapRecognizer.Tapped += OnTapRecognizerTapped;
+            view.GestureRecognizers.Add(tapRecognizer);
+        }
+
+        protected override void OnDetachingFrom(View view)
+        {
+            view.GestureRecognizers.Remove(tapRecognizer);
+            tapRecognizer.Tapped -= OnTapRecognizerTapped;
+        }
+
+        void OnTapRecognizerTapped(object sender, EventArgs args)
+        {
+            // TODO HACK: PropertyChange does not fire, if the value is not changed :-(
+            IsStarred = false;
+            IsStarred = true;
+        }
+    }
+}