Add Style

Ooui/EventTarget.cs

@@ -85,12 +85,11 @@ namespace Ooui
 
         protected bool SetProperty<T> (ref T backingStore, T newValue, string attributeName, [System.Runtime.CompilerServices.CallerMemberName] string propertyName = "")
         {
-            if (!backingStore.Equals (newValue)) {
-                backingStore = newValue;
-                SendSet (attributeName, newValue);
-                return true;
-            }
-            return false;
+            if (EqualityComparer<T>.Default.Equals (backingStore, newValue))
+                return false;
+            backingStore = newValue;
+            SendSet (attributeName, newValue);
+            return true;
         }
 
         public const char IdPrefix = '\u2999';

Ooui/Style.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using Value = System.Object;
+
+namespace Ooui
+{
+    public class Style : INotifyPropertyChanged
+    {
+        public event PropertyChangedEventHandler PropertyChanged;
+
+        readonly Dictionary<string, Value> properties =
+            new Dictionary<string, Value> ();
+            
+        public Value BackgroundColor {
+            get => GetProperty ("background-color");
+            set => SetProperty ("background-color", value);
+        }
+
+        private Value GetProperty (string propertyName)
+        {
+            lock (properties) {
+                Value p;
+                if (!properties.TryGetValue (propertyName, out p)) {
+                    p = new Value ();
+                    properties[propertyName] = p;
+                }
+                return p;
+            }
+        }
+
+        private void SetProperty (string propertyName, Value value)
+        {
+            lock (properties) {
+                Value old;
+                if (properties.TryGetValue (propertyName, out old)) {
+                    if (EqualityComparer<Value>.Default.Equals (old, value))
+                        return;
+                }
+                properties[propertyName] = value;
+            }
+            PropertyChanged?.Invoke (this, new PropertyChangedEventArgs (propertyName));
+        }
+    }
+}

Tests/StyleTests.cs

@@ -0,0 +1,25 @@
+using System;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+using Ooui;
+
+namespace Tests
+{
+    [TestClass]
+    public class StyleTests
+    {
+        [TestMethod]
+        public void ChangedWhen ()
+        {
+            var s = new Style ();
+            var changeCount = 0;
+            s.PropertyChanged += (_, e) => changeCount++;
+            s.BackgroundColor = "red";
+            Assert.AreEqual (1, changeCount);
+            s.BackgroundColor = "blue";
+            Assert.AreEqual (2, changeCount);
+            s.BackgroundColor = "blue";
+            Assert.AreEqual (2, changeCount);
+        }
+    }
+}