Attempt at making PropertyChangedExtensions work..

Stylet/PropertyChangedExtensions.cs

@@ -9,44 +9,148 @@ using System.Threading.Tasks;
 
 namespace Stylet
 {
+    public interface IPropertyChangedBinding
+    {
+        void Unbind();
+    }
+
     public static class PropertyChangedExtensions
     {
-        private static ConditionalWeakTable<INotifyPropertyChanged, List<Listener> eventMapping = new ConditionalWeakTable<INotifyPropertyChanged, List<Listener>();
+        private static ConditionalWeakTable<object, List<WeakPropertyChangedBinding>> eventMapping = new ConditionalWeakTable<object, List<WeakPropertyChangedBinding>>();
         private static object eventMappingLock = new object();
 
-        private class Listener
+        public class WeakPropertyChangedBinding : IPropertyChangedBinding
         {
-            public readonly string PropertyName;
-            public readonly EventHandler<PropertyChangedEventArgs> Handler;
+            private readonly string propertyName;
+            private readonly EventHandler<PropertyChangedEventArgs> handler;
+            private readonly WeakReference binder;
+            private readonly WeakReference<INotifyPropertyChanged> inpc;
 
-            public Listener(string propertyName, EventHandler<PropertyChangedEventArgs> handler)
+            public WeakPropertyChangedBinding(object binder, INotifyPropertyChanged inpc, string propertyName, EventHandler<PropertyChangedEventArgs> handler)
             {
-                this.PropertyName = propertyName;
-                this.Handler = handler;
+                this.binder = new WeakReference(binder);
+                this.inpc = new WeakReference<INotifyPropertyChanged>(inpc);
+                this.propertyName = propertyName;
+                this.handler = handler;
+            }
+
+            public void Unbind()
+            {
+                INotifyPropertyChanged inpc;
+
+                // If the target's still around, unregister ourselves
+                if (this.inpc.TryGetTarget(out inpc))
+                    PropertyChangedEventManager.RemoveHandler(inpc, this.handler, this.propertyName);
+
+                // If the handler's still around (there's a possibility it isn't), remove us from the ConditionalWeakTable
+                var handler = this.handler.Target;
+                if (handler != null)
+                {
+                    lock (eventMappingLock)
+                    {
+                        List<WeakPropertyChangedBinding> listeners;
+                        if (eventMapping.TryGetValue(handler, out listeners))
+                            listeners.Remove(this);
+                    }
+                }
             }
         }
 
-        public static void Bind<TClass, TMember>(this TClass item, Expression<Func<TClass, TMember>> selector, Action<TMember> handler) where TClass : INotifyPropertyChanged
+        public class StrongPropertyChangedBinding : IPropertyChangedBinding
         {
-            var propertyName = selector.NameForProperty();
-            var compiledSelector = selector.Compile();
-            List<Listener> listeners;
-            EventHandler<PropertyChangedEventArgs> ourHandler;
+            private WeakReference<INotifyPropertyChanged> inpc;
+            private PropertyChangedEventHandler handler;
 
-            lock (eventMappingLock)
+            public StrongPropertyChangedBinding(INotifyPropertyChanged inpc, PropertyChangedEventHandler handler)
             {
-                if (!eventMapping.TryGetValue(item, out listeners))
-                {
-                    listeners = new List<Listener>();
-                    eventMapping.Add(item, listeners);
-                }
-
-                ourHandler = (s, e) => handler(compiledSelector(item));
-
-                listeners.Add(new Listener(propertyName, ourHandler));
+                this.inpc = new WeakReference<INotifyPropertyChanged>(inpc);
+                this.handler = handler;
             }
 
-            PropertyChangedEventManager.AddHandler(item, ourHandler, propertyName);
+            public void Unbind()
+            {
+                INotifyPropertyChanged inpc;
+                if (this.inpc.TryGetTarget(out inpc))
+                {
+                    inpc.PropertyChanged -= handler;
+                }
+            }
+        }
+
+        public static IPropertyChangedBinding BindWeak<TMember>(this object binder, Expression<Func<TMember>> targetSelector, Action<TMember> handler)
+        {
+            return BindInternal(binder, targetSelector, handler, true);
+        }
+
+        public static IPropertyChangedBinding Bind<TMember>(this object binder, Expression<Func<TMember>> targetSelector, Action<TMember> handler)
+        {
+            return BindInternal(binder, targetSelector, handler, false);
+        }
+
+        private static IPropertyChangedBinding BindInternal<TMember>(object binder, Expression<Func<TMember>> targetSelector, Action<TMember> handler, bool weak)
+        {
+            var memberSelector = targetSelector.Body as MemberExpression;
+            if (memberSelector == null)
+                throw new ArgumentException("Must be in the form () => someInstance.SomeProperty", "targetSelector");
+
+            var propertyName = memberSelector.Member.Name;
+            var targetExpression = memberSelector.Expression as MemberExpression;
+            if (targetExpression == null)
+                throw new ArgumentException("Must be in the form () => someInstance.SomeProperty", "targetSelector");
+
+            var target = Expression.Lambda<Func<object>>(targetExpression).Compile()();
+            var inpc = target as INotifyPropertyChanged;
+            if (inpc == null)
+                throw new ArgumentException("The someInstance in () => someInstance.SomeProperty must be an INotifyPropertyChanged", "targetSelector");
+
+            var propertyAccess = Expression.Lambda<Func<TMember>>(memberSelector).Compile();
+
+            IPropertyChangedBinding listener;
+
+            if (weak)
+            {
+                EventHandler<PropertyChangedEventArgs> ourHandler = (o, e) =>
+                {
+                    handler(propertyAccess());
+                };
+
+                WeakPropertyChangedBinding weakListener = new WeakPropertyChangedBinding(binder, inpc, propertyName, ourHandler);
+                listener = weakListener;
+
+                // Right, we have target, propertyName, binder.
+                // Now we have to keep the handler we're about to build (which has a reference to the handler we were passed) alive as long as
+                // binder's alive (the handler that was passed to us might refer to a method on a compiler-generated class, which we've got
+                // the only reference to, so we've got to keep that alive).
+                lock (eventMappingLock)
+                {
+                    List<WeakPropertyChangedBinding> listeners;
+                    if (!eventMapping.TryGetValue(binder, out listeners))
+                    {
+                        listeners = new List<WeakPropertyChangedBinding>();
+                        eventMapping.Add(binder, listeners);
+                    }
+
+                    listeners.Add(weakListener);
+                }
+
+                PropertyChangedEventManager.AddHandler(inpc, ourHandler, propertyName);
+            }
+            else
+            {
+                PropertyChangedEventHandler ourHandler = (o, e) =>
+                {
+                    if (e.PropertyName == propertyName || e.PropertyName == String.Empty)
+                    {
+                        handler(propertyAccess());
+                    }
+                };
+
+                inpc.PropertyChanged += ourHandler;
+
+                listener = new StrongPropertyChangedBinding(inpc, ourHandler);
+            }
+
+            return listener;
         }
     }
 }