Don't use observables to detach styles.

Rather than adding a TakeUntil(control.StyleDetach) to each applied
style, keep track of the applied styles in a static list. This
dramatically reduces memory usage.

src/Perspex.Base/Collections/PerspexList.cs

@@ -175,8 +175,6 @@ namespace Perspex.Collections
             set { this[index] = (T)value; }
         }
 
-        public int SubscriberCount => _collectionChanged?.GetInvocationList().Length ?? 0;
-
         /// <summary>
         /// Adds an item to the collection.
         /// </summary>

src/Perspex.Base/Perspex.Base.csproj

@@ -69,6 +69,7 @@
     <Compile Include="Metadata\XmlnsDefinitionAttribute.cs" />
     <Compile Include="PerspexObjectExtensions.cs" />
     <Compile Include="PropertyMetadata.cs" />
+    <Compile Include="Reactive\ObservableEx.cs" />
     <Compile Include="Reactive\WeakPropertyChangedObservable.cs" />
     <Compile Include="StyledProperty.cs" />
     <Compile Include="StyledPropertyBase.cs" />

src/Perspex.Base/Reactive/ObservableEx.cs

@@ -0,0 +1,41 @@
+// Copyright (c) The Perspex Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+using System.Reactive.Disposables;
+
+namespace Perspex.Reactive
+{
+    /// <summary>
+    /// Provides common observable methods not found in standard Rx framework.
+    /// </summary>
+    public static class ObservableEx
+    {
+        /// <summary>
+        /// Returns an observable that fires once with the specified value and never completes.
+        /// </summary>
+        /// <typeparam name="T">The type of the value.</typeparam>
+        /// <param name="value">The value.</param>
+        /// <returns>The observable.</returns>
+        public static IObservable<T> SingleValue<T>(T value)
+        {
+            return new SingleValueImpl<T>(value);
+        }
+
+        private class SingleValueImpl<T> : IObservable<T>
+        {
+            private T _value;
+
+            public SingleValueImpl(T value)
+            {
+                _value = value;
+            }
+
+            public IDisposable Subscribe(IObserver<T> observer)
+            {
+                observer.OnNext(_value);
+                return Disposable.Empty;
+            }
+        }
+    }
+}

src/Perspex.Controls/Control.cs

@@ -96,7 +96,7 @@ namespace Perspex.Controls
         private INameScope _nameScope;
         private Styles _styles;
         private bool _styled;
-        private Subject<Unit> _styleDetach = new Subject<Unit>();
+        private Subject<IStyleable> _styleDetach = new Subject<IStyleable>();
 
         /// <summary>
         /// Initializes static members of the <see cref="Control"/> class.
@@ -308,7 +308,7 @@ namespace Perspex.Controls
         Type IStyleable.StyleKey => GetType();
 
         /// <inheritdoc/>
-        IObservable<Unit> IStyleable.StyleDetach => _styleDetach;
+        IObservable<IStyleable> IStyleable.StyleDetach => _styleDetach;
 
         /// <inheritdoc/>
         IStyleHost IStyleHost.StylingParent => (IStyleHost)InheritanceParent;
@@ -534,7 +534,7 @@ namespace Perspex.Controls
                 }
 
                 _isAttachedToLogicalTree = false;
-                _styleDetach.OnNext(Unit.Default);
+                _styleDetach.OnNext(this);
                 this.TemplatedParent = null;
                 DetachedFromLogicalTree?.Invoke(this, e);
 

src/Perspex.Styling/Styling/ISetter.cs

@@ -16,6 +16,6 @@ namespace Perspex.Styling
         /// <param name="style">The style that is being applied.</param>
         /// <param name="control">The control.</param>
         /// <param name="activator">An optional activator.</param>
-        void Apply(IStyle style, IStyleable control, IObservable<bool> activator);
+        IDisposable Apply(IStyle style, IStyleable control, IObservable<bool> activator);
     }
 }

src/Perspex.Styling/Styling/IStyleable.cs

@@ -3,7 +3,6 @@
 
 using System;
 using Perspex.Collections;
-using System.Reactive;
 
 namespace Perspex.Styling
 {
@@ -13,9 +12,9 @@ namespace Perspex.Styling
     public interface IStyleable : IPerspexObject, INamed
     {
         /// <summary>
-        /// Raised when the control's style should be removed.
+        /// Signalled when the control's style should be removed.
         /// </summary>
-        IObservable<Unit> StyleDetach { get; }
+        IObservable<IStyleable> StyleDetach { get; }
 
         /// <summary>
         /// Gets the list of classes for the control.

src/Perspex.Styling/Styling/Setter.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
+using System.Reactive.Disposables;
 using System.Reactive.Subjects;
 using Perspex.Data;
 using Perspex.Metadata;
@@ -63,7 +64,7 @@ namespace Perspex.Styling
         /// <param name="style">The style that is being applied.</param>
         /// <param name="control">The control.</param>
         /// <param name="activator">An optional activator.</param>
-        public void Apply(IStyle style, IStyleable control, IObservable<bool> activator)
+        public IDisposable Apply(IStyle style, IStyleable control, IObservable<bool> activator)
         {
             Contract.Requires<ArgumentNullException>(control != null);
 
@@ -80,16 +81,12 @@ namespace Perspex.Styling
             {
                 if (activator == null)
                 {
-                    control.SetValue(Property, Value, BindingPriority.Style);
+                    return control.Bind(Property, ObservableEx.SingleValue(Value), BindingPriority.Style);
                 }
                 else
                 {
                     var activated = new ActivatedValue(activator, Value, description);
-                    var instanced = new InstancedBinding(
-                        activated,
-                        BindingMode.OneWay,
-                        BindingPriority.StyleTrigger);
-                    BindingOperations.Apply(control, Property, instanced, null);
+                    return control.Bind(Property, activated, BindingPriority.StyleTrigger);
                 }
             }
             else
@@ -99,9 +96,11 @@ namespace Perspex.Styling
                 if (source != null)
                 {
                     var cloned = Clone(source, style, activator);
-                    BindingOperations.Apply(control, Property, cloned, null);
+                    return BindingOperations.Apply(control, Property, cloned, null);
                 }
             }
+
+            return Disposable.Empty;
         }
 
         private InstancedBinding Clone(InstancedBinding sourceInstance, IStyle style, IObservable<bool> activator)

src/Perspex.Styling/Styling/Style.cs

@@ -13,7 +13,9 @@ namespace Perspex.Styling
     /// </summary>
     public class Style : IStyle
     {
-        private static readonly IObservable<bool> True = Observable.Never<bool>().StartWith(true);
+        private static Dictionary<IStyleable, List<IDisposable>> _applied = 
+            new Dictionary<IStyleable, List<IDisposable>>();
+
         private Dictionary<string, object> _resources;
 
         /// <summary>
@@ -85,20 +87,23 @@ namespace Perspex.Styling
 
                 if (match.ImmediateResult != false)
                 {
-                    var activator = (match.ObservableResult ?? True)
-                        .TakeUntil(control.StyleDetach);
+                    var subs = GetSubscriptions(control);
 
                     foreach (var setter in Setters)
                     {
-                        setter.Apply(this, control, activator);
+                        var sub = setter.Apply(this, control, match.ObservableResult);
+                        subs.Add(sub);
                     }
                 }
             }
             else if (control == container)
             {
+                var subs = GetSubscriptions(control);
+
                 foreach (var setter in Setters)
                 {
-                    setter.Apply(this, control, null);
+                    var sub = setter.Apply(this, control, null);
+                    subs.Add(sub);
                 }
             }
         }
@@ -139,5 +144,36 @@ namespace Perspex.Styling
                 return "Style";
             }
         }
+
+        private static List<IDisposable> GetSubscriptions(IStyleable control)
+        {
+            List<IDisposable> subscriptions;
+
+            if (!_applied.TryGetValue(control, out subscriptions))
+            {
+                subscriptions = new List<IDisposable>(2);
+                subscriptions.Add(control.StyleDetach.Subscribe(ControlDetach));
+                _applied.Add(control, subscriptions);
+            }
+
+            return subscriptions;
+        }
+
+        /// <summary>
+        /// Called when a control's <see cref="IStyleable.StyleDetach"/> is signalled to remove
+        /// all applied styles.
+        /// </summary>
+        /// <param name="control">The control.</param>
+        private static void ControlDetach(IStyleable control)
+        {
+            var subscriptions = _applied[control];
+
+            foreach (var subscription in subscriptions)
+            {
+                subscription.Dispose();
+            }
+
+            _applied.Remove(control);
+        }
     }
 }

tests/Perspex.Base.UnitTests/Perspex.Base.UnitTests.csproj

@@ -69,6 +69,10 @@
       <HintPath>..\..\packages\Rx-PlatformServices.2.2.5\lib\net45\System.Reactive.PlatformServices.dll</HintPath>
       <Private>True</Private>
     </Reference>
+    <Reference Include="System.Reactive.PlatformServices, Version=2.2.5.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
+      <HintPath>..\..\packages\Rx-PlatformServices.2.2.5\lib\net45\System.Reactive.PlatformServices.dll</HintPath>
+      <Private>True</Private>
+    </Reference>
     <Reference Include="xunit.abstractions, Version=2.0.0.0, Culture=neutral, PublicKeyToken=8d05b1bb7a6fdb6c">
       <HintPath>..\..\packages\xunit.abstractions.2.0.0\lib\net35\xunit.abstractions.dll</HintPath>
     </Reference>

tests/Perspex.Markup.Xaml.UnitTests/Converters/PerspexPropertyConverterTest.cs

@@ -97,7 +97,7 @@ namespace Perspex.Markup.Xaml.UnitTests.Converters
                 get { throw new NotImplementedException(); }
             }
 
-            IObservable<Unit> IStyleable.StyleDetach { get; }
+            IObservable<IStyleable> IStyleable.StyleDetach { get; }
         }
 
         private class AttachedOwner

tests/Perspex.Styling.UnitTests/ActivatedValueTests.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
+using Microsoft.Reactive.Testing;
 using Xunit;
 
 namespace Perspex.Styling.UnitTests
@@ -38,5 +39,23 @@ namespace Perspex.Styling.UnitTests
 
             Assert.True(completed);
         }
+
+        [Fact]
+        public void Should_Unsubscribe_From_Activator_When_All_Subscriptions_Disposed()
+        {
+            var scheduler = new TestScheduler();
+            var activator1 = scheduler.CreateColdObservable<bool>();
+            var activator2 = scheduler.CreateColdObservable<bool>();
+            var activator = StyleActivator.And(new[] { activator1, activator2 });
+            var target = new ActivatedValue(activator, 1, string.Empty);
+
+            var subscription = target.Subscribe(_ => { });
+            Assert.Equal(1, activator1.Subscriptions.Count);
+            Assert.Equal(Subscription.Infinite, activator1.Subscriptions[0].Unsubscribe);
+
+            subscription.Dispose();
+            Assert.Equal(1, activator1.Subscriptions.Count);
+            Assert.Equal(0, activator1.Subscriptions[0].Unsubscribe);
+        }
     }
 }

tests/Perspex.Styling.UnitTests/SelectorTests_Child.cs

@@ -105,7 +105,7 @@ namespace Perspex.Styling.UnitTests
 
             public ITemplatedControl TemplatedParent { get; }
 
-            IObservable<Unit> IStyleable.StyleDetach { get; }
+            IObservable<IStyleable> IStyleable.StyleDetach { get; }
 
             IPerspexReadOnlyList<string> IStyleable.Classes => Classes;
 

tests/Perspex.Styling.UnitTests/SelectorTests_Descendent.cs

@@ -138,7 +138,7 @@ namespace Perspex.Styling.UnitTests
 
             IPerspexReadOnlyList<string> IStyleable.Classes => Classes;
 
-            IObservable<Unit> IStyleable.StyleDetach { get; }
+            IObservable<IStyleable> IStyleable.StyleDetach { get; }
 
             public object GetValue(PerspexProperty property)
             {

tests/Perspex.Styling.UnitTests/TestControlBase.cs

@@ -35,7 +35,7 @@ namespace Perspex.Styling.UnitTests
 
         IPerspexReadOnlyList<string> IStyleable.Classes => Classes;
 
-        IObservable<Unit> IStyleable.StyleDetach { get; }
+        IObservable<IStyleable> IStyleable.StyleDetach { get; }
 
         public object GetValue(PerspexProperty property)
         {

tests/Perspex.Styling.UnitTests/TestTemplatedControl.cs

@@ -35,7 +35,7 @@ namespace Perspex.Styling.UnitTests
 
         IPerspexReadOnlyList<string> IStyleable.Classes => Classes;
 
-        IObservable<Unit> IStyleable.StyleDetach { get; }
+        IObservable<IStyleable> IStyleable.StyleDetach { get; }
 
         public object GetValue(PerspexProperty property)
         {