Added FallbackValue to MultiBinding.

Closes #411.

src/Markup/Perspex.Markup.Xaml/Data/MultiBinding.cs

@@ -29,6 +29,11 @@ namespace Perspex.Markup.Xaml.Data
         /// </summary>
         public IMultiValueConverter Converter { get; set; }
 
+        /// <summary>
+        /// Gets or sets the value to use when the binding is unable to produce a value.
+        /// </summary>
+        public object FallbackValue { get; set; }
+
         /// <summary>
         /// Gets or sets the binding mode.
         /// </summary>
@@ -75,8 +80,7 @@ namespace Perspex.Markup.Xaml.Data
             var targetType = targetProperty?.PropertyType ?? typeof(object);
             var result = new BehaviorSubject<object>(PerspexProperty.UnsetValue);
             var children = Bindings.Select(x => x.CreateSubject(target, null));
-            var input = children.CombineLatest().Select(x =>
-                Converter.Convert(x, targetType, null, CultureInfo.CurrentUICulture));
+            var input = children.CombineLatest().Select(x => ConvertValue(x, targetType));
             input.Subscribe(result);
             return result;
         }
@@ -111,5 +115,17 @@ namespace Perspex.Markup.Xaml.Data
                     break;
             }
         }
+
+        private object ConvertValue(IList<object> values, Type targetType)
+        {
+            var converted = Converter.Convert(values, targetType, null, CultureInfo.CurrentUICulture);
+
+            if (converted == PerspexProperty.UnsetValue && FallbackValue != null)
+            {
+                converted = FallbackValue;
+            }
+
+            return converted;
+        }
     }
 }

tests/Perspex.Markup.Xaml.UnitTests/Data/MultiBindingTests.cs

@@ -39,6 +39,28 @@ namespace Perspex.Markup.Xaml.UnitTests.Data
             Assert.Equal("1,2,3", result);
         }
 
+        [Fact]
+        public void Should_Return_FallbackValue_When_Converter_Returns_UnsetValue()
+        {
+            var target = new TextBlock();
+            var source = new { A = 1, B = 2, C = 3 };
+            var binding = new MultiBinding
+            {
+                Converter = new UnsetValueConverter(),
+                Bindings = new[]
+                {
+                    new Binding { Path = "A" },
+                    new Binding { Path = "B" },
+                    new Binding { Path = "C" },
+                },
+                FallbackValue = "fallback",
+            };
+
+            target.Bind(TextBlock.TextProperty, binding);
+
+            Assert.Equal("fallback", target.Text);
+        }
+
         private class ConcatConverter : IMultiValueConverter
         {
             public object Convert(IList<object> values, Type targetType, object parameter, CultureInfo culture)
@@ -46,5 +68,13 @@ namespace Perspex.Markup.Xaml.UnitTests.Data
                 return string.Join(",", values);
             }
         }
+
+        private class UnsetValueConverter : IMultiValueConverter
+        {
+            public object Convert(IList<object> values, Type targetType, object parameter, CultureInfo culture)
+            {
+                return PerspexProperty.UnsetValue;
+            }
+        }
     }
 }