Added coercion to PerspexProperties.

And use it to make sure ScrollViewer's offset is in range.

Perspex.Base.UnitTests/PerspexObjectTests.cs

@@ -4,6 +4,7 @@
 // </copyright>
 // -----------------------------------------------------------------------
 
+using Microsoft.VisualStudio.TestTools.UnitTesting;
 namespace Perspex.Base.UnitTests
 {
     using System;
@@ -29,7 +30,7 @@ namespace Perspex.Base.UnitTests
         {
             string[] names = PerspexObject.GetProperties(typeof(Class1)).Select(x => x.Name).ToArray();
 
-            CollectionAssert.AreEqual(new[] { "Foo", "Baz" }, names);
+            CollectionAssert.AreEqual(new[] { "Foo", "Baz", "Qux" }, names);
         }
 
         [TestMethod]
@@ -37,7 +38,7 @@ namespace Perspex.Base.UnitTests
         {
             string[] names = PerspexObject.GetProperties(typeof(Class2)).Select(x => x.Name).ToArray();
 
-            CollectionAssert.AreEqual(new[] { "Bar", "Foo", "Baz" }, names);
+            CollectionAssert.AreEqual(new[] { "Bar", "Foo", "Baz", "Qux" }, names);
         }
 
         [TestMethod]
@@ -169,6 +170,30 @@ namespace Perspex.Base.UnitTests
             target.SetValue(Class1.FooProperty, 123);
         }
 
+        [TestMethod]
+        public void SetValue_Causes_Coercion()
+        {
+            Class1 target = new Class1();
+
+            target.SetValue(Class1.QuxProperty, 5);
+            Assert.AreEqual(5, target.GetValue(Class1.QuxProperty));
+            target.SetValue(Class1.QuxProperty, -5);
+            Assert.AreEqual(0, target.GetValue(Class1.QuxProperty));
+            target.SetValue(Class1.QuxProperty, 15);
+            Assert.AreEqual(10, target.GetValue(Class1.QuxProperty));
+        }
+
+        [TestMethod]
+        public void CoerceValue_Causes_Recoercion()
+        {
+            Class1 target = new Class1();
+
+            target.SetValue(Class1.QuxProperty, 7);
+            Assert.AreEqual(7, target.GetValue(Class1.QuxProperty));
+            target.MaxQux = 5;
+            target.CoerceValue(Class1.QuxProperty);
+        }
+
         [TestMethod]
         public void GetObservable_Returns_Initial_Value()
         {
@@ -503,6 +528,21 @@ namespace Perspex.Base.UnitTests
 
             public static readonly PerspexProperty<string> BazProperty =
                 PerspexProperty.Register<Class1, string>("Baz", "bazdefault", true);
+
+            public static readonly PerspexProperty<int> QuxProperty =
+                PerspexProperty.Register<Class1, int>("Qux", coerce: Coerce);
+
+            public int MaxQux { get; set; }
+
+            public Class1()
+            {
+                this.MaxQux = 10;
+            }
+
+            private static int Coerce(PerspexObject instance, int value)
+            {
+                return Math.Min(Math.Max(value, 0), ((Class1)instance).MaxQux);
+            }
         }
 
         private class Class2 : Class1

Perspex.Base.UnitTests/PerspexPropertyTests.cs

@@ -20,7 +20,8 @@ namespace Perspex.Base.UnitTests
                 typeof(Class1),
                 "Foo",
                 false,
-                BindingMode.OneWay);
+                BindingMode.OneWay,
+                null);
 
             Assert.AreEqual("test", target.Name);
             Assert.AreEqual(typeof(string), target.PropertyType);
@@ -36,7 +37,8 @@ namespace Perspex.Base.UnitTests
                 typeof(Class1),
                 "Foo",
                 false,
-                BindingMode.OneWay);
+                BindingMode.OneWay,
+                null);
 
             Assert.AreEqual("Foo", target.GetDefaultValue<Class1>());
         }
@@ -49,7 +51,8 @@ namespace Perspex.Base.UnitTests
                 typeof(Class1),
                 "Foo",
                 false,
-                BindingMode.OneWay);
+                BindingMode.OneWay,
+                null);
 
             Assert.AreEqual("Foo", target.GetDefaultValue<Class2>());
         }
@@ -62,7 +65,8 @@ namespace Perspex.Base.UnitTests
                 typeof(Class3),
                 "Foo",
                 false,
-                BindingMode.OneWay);
+                BindingMode.OneWay,
+                null);
 
             Assert.AreEqual("Foo", target.GetDefaultValue<Class2>());
         }
@@ -75,7 +79,8 @@ namespace Perspex.Base.UnitTests
                 typeof(Class1),
                 "Foo",
                 false,
-                BindingMode.OneWay);
+                BindingMode.OneWay,
+                null);
 
             target.OverrideDefaultValue(typeof(Class2), "Bar");
 

Perspex.Base/PerspexObject.cs

@@ -634,9 +634,51 @@ namespace Perspex
             this.GetObservable(property).Subscribe(x => source.SetValue(sourceProperty, x));
         }
 
+        /// <summary>
+        /// Forces the specified property to be re-coerced.
+        /// </summary>
+        /// <param name="property">The property.</param>
+        public void CoerceValue(PerspexProperty property)
+        {
+            PriorityValue value;
+
+            if (this.values.TryGetValue(property, out value))
+            {
+                value.Coerce();
+            }
+        }
+
+        /// <summary>
+        /// Forces re-coercion of properties when a property value changes.
+        /// </summary>
+        /// <param name="property">The property to that affects coercion.</param>
+        /// <param name="affected">The affected properties.</param>
+        protected static void AffectsCoercion(PerspexProperty property, params PerspexProperty[] affected)
+        {
+            property.Changed.Subscribe(e =>
+            {
+                foreach (var p in affected)
+                {
+                    e.Sender.CoerceValue(p);
+                }
+            });
+        }
+
+        /// <summary>
+        /// Creates a <see cref="PriorityValue"/> for a <see cref="PerspexProperty"/>.
+        /// </summary>
+        /// <param name="property">The property.</param>
+        /// <returns>The <see cref="PriorityValue"/>.</returns>
         private PriorityValue CreatePriorityValue(PerspexProperty property)
         {
-            PriorityValue result = new PriorityValue(property.Name, property.PropertyType);
+            Func<object, object> coerce = null;
+
+            if (property.Coerce != null)
+            {
+                coerce = v => property.Coerce(this, v);
+            }
+
+            PriorityValue result = new PriorityValue(property.Name, property.PropertyType, coerce);
 
             result.Changed.Subscribe(x =>
             {

Perspex.Base/PerspexProperty.cs

@@ -39,6 +39,11 @@ namespace Perspex
         /// </summary>
         private Subject<PerspexPropertyChangedEventArgs> changed = new Subject<PerspexPropertyChangedEventArgs>();
 
+        /// <summary>
+        /// The coerce function.
+        /// </summary>
+        private Func<PerspexObject, object, object> coerce;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="PerspexProperty"/> class.
         /// </summary>
@@ -48,13 +53,15 @@ namespace Perspex
         /// <param name="defaultValue">The default value of the property.</param>
         /// <param name="inherits">Whether the property inherits its value.</param>
         /// <param name="defaultBindingMode">The default binding mode for the property.</param>
+        /// <param name="coerce">A coercion function.</param>
         public PerspexProperty(
             string name,
             Type valueType,
             Type ownerType,
             object defaultValue,
             bool inherits,
-            BindingMode defaultBindingMode)
+            BindingMode defaultBindingMode,
+            Func<PerspexObject, object, object> coerce)
         {
             Contract.Requires<NullReferenceException>(name != null);
             Contract.Requires<NullReferenceException>(valueType != null);
@@ -63,9 +70,10 @@ namespace Perspex
             this.Name = name;
             this.PropertyType = valueType;
             this.OwnerType = ownerType;
+            this.defaultValues.Add(ownerType, defaultValue);
             this.Inherits = inherits;
             this.DefaultBindingMode = defaultBindingMode;
-            this.defaultValues.Add(ownerType, defaultValue);
+            this.Coerce = coerce;
         }
 
         /// <summary>
@@ -94,6 +102,11 @@ namespace Perspex
         /// <returns></returns>
         public BindingMode DefaultBindingMode { get; private set; }
 
+        /// <summary>
+        /// Gets the property's coerce function.
+        /// </summary>
+        public Func<PerspexObject, object, object> Coerce { get; private set; }
+
         /// <summary>
         /// Gets an observable that is fired when this property is initialized on a
         /// new <see cref="PerspexObject"/> instance.
@@ -127,12 +140,14 @@ namespace Perspex
         /// <param name="defaultValue">The default value of the property.</param>
         /// <param name="inherits">Whether the property inherits its value.</param>
         /// <param name="defaultBindingMode">The default binding mode for the property.</param>
+        /// <param name="coerce">A coercion function.</param>
         /// <returns>A <see cref="PerspexProperty{TValue}"/></returns>
         public static PerspexProperty<TValue> Register<TOwner, TValue>(
             string name,
             TValue defaultValue = default(TValue),
             bool inherits = false,
-            BindingMode defaultBindingMode = BindingMode.OneWay)
+            BindingMode defaultBindingMode = BindingMode.OneWay,
+            Func<PerspexObject, TValue, TValue> coerce = null)
             where TOwner : PerspexObject
         {
             Contract.Requires<NullReferenceException>(name != null);
@@ -142,7 +157,8 @@ namespace Perspex
                 typeof(TOwner),
                 defaultValue,
                 inherits,
-                defaultBindingMode);
+                defaultBindingMode,
+                coerce);
 
             PerspexObject.Register(typeof(TOwner), result);
 
@@ -159,12 +175,14 @@ namespace Perspex
         /// <param name="defaultValue">The default value of the property.</param>
         /// <param name="inherits">Whether the property inherits its value.</param>
         /// <param name="defaultBindingMode">The default binding mode for the property.</param>
+        /// <param name="coerce">A coercion function.</param>
         /// <returns>A <see cref="PerspexProperty{TValue}"/></returns>
         public static PerspexProperty<TValue> RegisterAttached<TOwner, THost, TValue>(
             string name,
             TValue defaultValue = default(TValue),
             bool inherits = false,
-            BindingMode defaultBindingMode = BindingMode.OneWay)
+            BindingMode defaultBindingMode = BindingMode.OneWay,
+            Func<PerspexObject, TValue, TValue> coerce = null)
             where TOwner : PerspexObject
         {
             Contract.Requires<NullReferenceException>(name != null);
@@ -174,7 +192,8 @@ namespace Perspex
                 typeof(TOwner),
                 defaultValue,
                 inherits,
-                defaultBindingMode);
+                defaultBindingMode,
+                coerce);
 
             PerspexObject.Register(typeof(THost), result);
 
@@ -321,13 +340,22 @@ namespace Perspex
         /// <param name="defaultValue">The default value of the property.</param>
         /// <param name="inherits">Whether the property inherits its value.</param>
         /// <param name="defaultBindingMode">The default binding mode for the property.</param>
+        /// <param name="coerce">A coercion function.</param>
         public PerspexProperty(
             string name,
             Type ownerType,
             TValue defaultValue,
             bool inherits,
-            BindingMode defaultBindingMode)
-            : base(name, typeof(TValue), ownerType, defaultValue, inherits, defaultBindingMode)
+            BindingMode defaultBindingMode,
+            Func<PerspexObject, TValue, TValue> coerce)
+            : base(
+                name, 
+                typeof(TValue), 
+                ownerType, 
+                defaultValue, 
+                inherits, 
+                defaultBindingMode,
+                Convert(coerce))
         {
             Contract.Requires<NullReferenceException>(name != null);
             Contract.Requires<NullReferenceException>(ownerType != null);
@@ -353,5 +381,15 @@ namespace Perspex
         {
             return (TValue)this.GetDefaultValue(typeof(T));
         }
+
+        /// <summary>
+        /// Converts from a typed coercion function to an untyped.
+        /// </summary>
+        /// <param name="f">The typed coercion function.</param>
+        /// <returns>Te untyped coercion function.</returns>
+        private static Func<PerspexObject, object, object> Convert(Func<PerspexObject, TValue, TValue> f)
+        {
+            return f != null ? (o, v) => f(o, (TValue)v) : (Func<PerspexObject, object, object>)null;
+        }
     }
 }

Perspex.Base/PriorityValue.cs

@@ -51,17 +51,21 @@ namespace Perspex
         /// </summary>
         private object value;
 
+        private Func<object, object> coerce;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="PriorityValue"/> class.
         /// </summary>
         /// <param name="name">The name of the property.</param>
         /// <param name="valueType">The value type.</param>
-        public PriorityValue(string name, Type valueType)
+        /// <param name="coerce">An optional coercion function.</param>
+        public PriorityValue(string name, Type valueType, Func<object, object> coerce = null)
         {
             this.name = name;
             this.valueType = valueType;
             this.value = PerspexProperty.UnsetValue;
             this.ValuePriority = int.MaxValue;
+            this.coerce = coerce;
         }
 
         /// <summary>
@@ -238,6 +242,33 @@ namespace Perspex
             return this.bindings;
         }
 
+        /// <summary>
+        /// Causes a re-coercion of the value.
+        /// </summary>
+        public void Coerce()
+        {
+            if (this.coerce != null)
+            {
+                this.SetValue(this.Value, this.ValuePriority);
+            }
+        }
+
+        /// <summary>
+        /// Throws an exception if <paramref name="value"/> is invalid.
+        /// </summary>
+        /// <param name="value">The value.</param>
+        private void VerifyValidValue(object value)
+        {
+            if (!IsValidValue(value, this.valueType))
+            {
+                throw new InvalidOperationException(string.Format(
+                    "Invalid value for Property '{0}': {1} ({2})",
+                    this.name,
+                    value,
+                    value.GetType().FullName));
+            }
+        }
+
         /// <summary>
         /// Called when a binding's value changes.
         /// </summary>
@@ -266,13 +297,12 @@ namespace Perspex
         /// <param name="priority">The priority of the binding which produced the value.</param>
         private void SetValue(object value, int priority)
         {
-            if (!IsValidValue(value, this.valueType))
+            VerifyValidValue(value);
+
+            if (this.coerce != null)
             {
-                throw new InvalidOperationException(string.Format(
-                    "Invalid value for Property '{0}': {1} ({2})",
-                    this.name,
-                    value,
-                    value.GetType().FullName));
+                value = this.coerce(value);
+                VerifyValidValue(value);
             }
 
             object old = this.value;

Perspex.Controls/ScrollViewer.cs

@@ -17,7 +17,7 @@ namespace Perspex.Controls
             PerspexProperty.Register<ScrollViewer, Size>("Extent");
 
         public static readonly PerspexProperty<Vector> OffsetProperty =
-            PerspexProperty.Register<ScrollViewer, Vector>("Offset");
+            PerspexProperty.Register<ScrollViewer, Vector>("Offset", coerce: CoerceOffset);
 
         public static readonly PerspexProperty<Size> ViewportProperty =
             PerspexProperty.Register<ScrollViewer, Size>("Viewport");
@@ -28,6 +28,12 @@ namespace Perspex.Controls
 
         private ScrollBar verticalScrollBar;
 
+        static ScrollViewer()
+        {
+            AffectsCoercion(ExtentProperty, OffsetProperty);
+            AffectsCoercion(ViewportProperty, OffsetProperty);
+        }
+
         public Size Extent
         {
             get { return this.GetValue(ExtentProperty); }
@@ -103,5 +109,28 @@ namespace Perspex.Controls
 
             this.Bind(OffsetProperty, offset);
         }
+
+        private static double Clamp(double value, double min, double max)
+        {
+            return (value < min) ? min : (value > max) ? max : value;
+        }
+
+        private static Vector CoerceOffset(PerspexObject o, Vector value)
+        {
+            ScrollViewer scrollViewer = o as ScrollViewer;
+
+            if (scrollViewer != null)
+            {
+                var extent = scrollViewer.Extent;
+                var viewport = scrollViewer.Viewport;
+                var maxX = Math.Max(extent.Width - viewport.Width, 0);
+                var maxY = Math.Max(extent.Height - viewport.Height, 0);
+                return new Vector(Clamp(value.X, 0, maxX), Clamp(value.Y, 0, maxY));
+            }
+            else
+            {
+                return value;
+            }
+        }
     }
 }