[Core] Share BP across IFontElement implementors (#783)

* [Core] Share BP accross IFontElement implementors

* make sure the converter is used

9 files changed, 193 insertions, 113 deletions

diff --git a/Xamarin.Forms.Core/Button.cs b/Xamarin.Forms.Core/Button.cs
index bcd593ed..7d8d659b 100644
--- a/Xamarin.Forms.Core/Button.cs
+++ b/Xamarin.Forms.Core/Button.cs
@@ -25,13 +25,11 @@ namespace Xamarin.Forms
 
 		public static readonly BindableProperty FontProperty = BindableProperty.Create("Font", typeof(Font), typeof(Button), default(Font), propertyChanged: FontStructPropertyChanged);
 
-		public static readonly BindableProperty FontFamilyProperty = BindableProperty.Create("FontFamily", typeof(string), typeof(Button), default(string), propertyChanged: SpecificFontPropertyChanged);
+		public static readonly BindableProperty FontFamilyProperty = FontElement.FontFamilyProperty;
 
-		public static readonly BindableProperty FontSizeProperty = BindableProperty.Create("FontSize", typeof(double), typeof(Button), -1.0, propertyChanged: SpecificFontPropertyChanged,
-			defaultValueCreator: bindable => Device.GetNamedSize(NamedSize.Default, (Button)bindable));
+		public static readonly BindableProperty FontSizeProperty = FontElement.FontSizeProperty;
 
-		public static readonly BindableProperty FontAttributesProperty = BindableProperty.Create("FontAttributes", typeof(FontAttributes), typeof(Button), FontAttributes.None,
-			propertyChanged: SpecificFontPropertyChanged);
+		public static readonly BindableProperty FontAttributesProperty = FontElement.FontAttributesProperty;
 
 		public static readonly BindableProperty BorderWidthProperty = BindableProperty.Create("BorderWidth", typeof(double), typeof(Button), -1d);
 
@@ -214,19 +212,27 @@ namespace Xamarin.Forms
 			{
 				button.FontFamily = button.Font.FontFamily;
 				if (button.Font.UseNamedSize)
-				{
 					button.FontSize = Device.GetNamedSize(button.Font.NamedSize, button.GetType(), true);
-				}
 				else
-				{
 					button.FontSize = button.Font.FontSize;
-				}
 				button.FontAttributes = button.Font.FontAttributes;
 			}
 
 			button._cancelEvents = false;
 		}
 
+		void IFontElement.OnFontFamilyChanged(string oldValue, string newValue) =>
+			SpecificFontPropertyChanged();
+
+		void IFontElement.OnFontSizeChanged(double oldValue, double newValue) =>
+			SpecificFontPropertyChanged();
+
+		double IFontElement.FontSizeDefaultValueCreator() =>
+			Device.GetNamedSize(NamedSize.Default, (Button)this);
+
+		void IFontElement.OnFontAttributesChanged(FontAttributes oldValue, FontAttributes newValue) =>
+			SpecificFontPropertyChanged();
+
 		void OnCommandChanged()
 		{
 			if (Command != null)
@@ -260,27 +266,21 @@ namespace Xamarin.Forms
 				oldvalue.SourceChanged -= OnSourceChanged;
 		}
 
-		static void SpecificFontPropertyChanged(BindableObject bindable, object oldValue, object newValue)
+		void SpecificFontPropertyChanged()
 		{
-			var button = (Button)bindable;
-
-			if (button._cancelEvents)
+			if (_cancelEvents)
 				return;
 
-			button.InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);
+			InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);
 
-			button._cancelEvents = true;
+			_cancelEvents = true;
 
-			if (button.FontFamily != null)
-			{
-				button.Font = Font.OfSize(button.FontFamily, button.FontSize).WithAttributes(button.FontAttributes);
-			}
+			if (FontFamily != null)
+				Font = Font.OfSize(FontFamily, FontSize).WithAttributes(FontAttributes);
 			else
-			{
-				button.Font = Font.SystemFontOfSize(button.FontSize, button.FontAttributes);
-			}
+				Font = Font.SystemFontOfSize(FontSize, FontAttributes);
 
-			button._cancelEvents = false;
+			_cancelEvents = false;
 		}
 
 		[DebuggerDisplay("Image Position = {Position}, Spacing = {Spacing}")]
diff --git a/Xamarin.Forms.Core/Editor.cs b/Xamarin.Forms.Core/Editor.cs
index 92a6b9df..66c5caa7 100644
--- a/Xamarin.Forms.Core/Editor.cs
+++ b/Xamarin.Forms.Core/Editor.cs
@@ -13,12 +13,11 @@ namespace Xamarin.Forms
 				editor.TextChanged(editor, new TextChangedEventArgs((string)oldValue, (string)newValue));
 		});
 
-		public static readonly BindableProperty FontFamilyProperty = BindableProperty.Create("FontFamily", typeof(string), typeof(Editor), default(string));
+		public static readonly BindableProperty FontFamilyProperty = FontElement.FontFamilyProperty;
 
-		public static readonly BindableProperty FontSizeProperty = BindableProperty.Create("FontSize", typeof(double), typeof(Editor), -1.0,
-			defaultValueCreator: bindable => Device.GetNamedSize(NamedSize.Default, (Editor)bindable));
+		public static readonly BindableProperty FontSizeProperty = FontElement.FontSizeProperty;
 
-		public static readonly BindableProperty FontAttributesProperty = BindableProperty.Create("FontAttributes", typeof(FontAttributes), typeof(Editor), FontAttributes.None);
+		public static readonly BindableProperty FontAttributesProperty = FontElement.FontAttributesProperty;
 
 		public static readonly BindableProperty TextColorProperty = BindableProperty.Create("TextColor", typeof(Color), typeof(Editor), Color.Default);
 		readonly Lazy<PlatformConfigurationRegistry<Editor>> _platformConfigurationRegistry;
@@ -54,6 +53,21 @@ namespace Xamarin.Forms
 			set { SetValue(FontSizeProperty, value); }
 		}
 
+		void IFontElement.OnFontFamilyChanged(string oldValue, string newValue)
+		{
+		}
+
+		void IFontElement.OnFontSizeChanged(double oldValue, double newValue)
+		{
+		}
+
+		double IFontElement.FontSizeDefaultValueCreator() =>
+			Device.GetNamedSize(NamedSize.Default, (Editor)this);
+
+		void IFontElement.OnFontAttributesChanged(FontAttributes oldValue, FontAttributes newValue)
+		{
+		}
+
 		public event EventHandler Completed;
 
 		public event EventHandler<TextChangedEventArgs> TextChanged;
diff --git a/Xamarin.Forms.Core/Entry.cs b/Xamarin.Forms.Core/Entry.cs
index ac16dc52..d499d34e 100644
--- a/Xamarin.Forms.Core/Entry.cs
+++ b/Xamarin.Forms.Core/Entry.cs
@@ -18,12 +18,11 @@ namespace Xamarin.Forms
 
 		public static readonly BindableProperty PlaceholderColorProperty = BindableProperty.Create("PlaceholderColor", typeof(Color), typeof(Entry), Color.Default);
 
-		public static readonly BindableProperty FontFamilyProperty = BindableProperty.Create("FontFamily", typeof(string), typeof(Entry), default(string));
+		public static readonly BindableProperty FontFamilyProperty = FontElement.FontFamilyProperty;
 
-		public static readonly BindableProperty FontSizeProperty = BindableProperty.Create("FontSize", typeof(double), typeof(Entry), -1.0,
-			defaultValueCreator: bindable => Device.GetNamedSize(NamedSize.Default, (Entry)bindable));
+		public static readonly BindableProperty FontSizeProperty = FontElement.FontSizeProperty;
 
-		public static readonly BindableProperty FontAttributesProperty = BindableProperty.Create("FontAttributes", typeof(FontAttributes), typeof(Entry), FontAttributes.None);
+		public static readonly BindableProperty FontAttributesProperty = FontElement.FontAttributesProperty;
 
 		readonly Lazy<PlatformConfigurationRegistry<Entry>> _platformConfigurationRegistry;
 
@@ -87,6 +86,21 @@ namespace Xamarin.Forms
 			set { SetValue(FontSizeProperty, value); }
 		}
 
+		void IFontElement.OnFontFamilyChanged(string oldValue, string newValue)
+		{
+		}
+
+		void IFontElement.OnFontSizeChanged(double oldValue, double newValue)
+		{
+		}
+
+		double IFontElement.FontSizeDefaultValueCreator() =>
+			Device.GetNamedSize(NamedSize.Default, (Entry)this);
+
+		void IFontElement.OnFontAttributesChanged(FontAttributes oldValue, FontAttributes newValue)
+		{
+		}
+
 		public event EventHandler Completed;
 
 		public event EventHandler<TextChangedEventArgs> TextChanged;
diff --git a/Xamarin.Forms.Core/FontElement.cs b/Xamarin.Forms.Core/FontElement.cs
new file mode 100644
index 00000000..66545e07
--- /dev/null
+++ b/Xamarin.Forms.Core/FontElement.cs
@@ -0,0 +1,38 @@
+namespace Xamarin.Forms
+{
+	static class FontElement
+	{
+		public static readonly BindableProperty FontFamilyProperty =
+			BindableProperty.Create("FontFamily", typeof(string), typeof(IFontElement), default(string),
+									propertyChanged: OnFontFamilyChanged);
+
+		public static readonly BindableProperty FontSizeProperty =
+			BindableProperty.Create("FontSize", typeof(double), typeof(IFontElement), -1.0,
+									propertyChanged: OnFontSizeChanged,
+									defaultValueCreator: FontSizeDefaultValueCreator);
+
+		public static readonly BindableProperty FontAttributesProperty =
+			BindableProperty.Create("FontAttributes", typeof(FontAttributes), typeof(IFontElement), FontAttributes.None,
+									propertyChanged: OnFontAttributesChanged);
+
+		static void OnFontFamilyChanged(BindableObject bindable, object oldValue, object newValue)
+		{
+			((IFontElement)bindable).OnFontFamilyChanged((string)oldValue, (string)newValue);
+		}
+
+		static void OnFontSizeChanged(BindableObject bindable, object oldValue, object newValue)
+		{
+			((IFontElement)bindable).OnFontSizeChanged((double)oldValue, (double)newValue);
+		}
+
+		static object FontSizeDefaultValueCreator(BindableObject bindable)
+		{
+			return ((IFontElement)bindable).FontSizeDefaultValueCreator();
+		}
+
+		static void OnFontAttributesChanged(BindableObject bindable, object oldValue, object newValue)
+		{
+			((IFontElement)bindable).OnFontAttributesChanged((FontAttributes)oldValue, (FontAttributes)newValue);
+		}
+	}
+}
\ No newline at end of file
diff --git a/Xamarin.Forms.Core/IFontElement.cs b/Xamarin.Forms.Core/IFontElement.cs
index f1f3a36f..469b09fa 100644
--- a/Xamarin.Forms.Core/IFontElement.cs
+++ b/Xamarin.Forms.Core/IFontElement.cs
@@ -1,11 +1,18 @@
 namespace Xamarin.Forms
 {
-	internal interface IFontElement
+	interface IFontElement
 	{
+		//note to implementor: implement the properties publicly
 		FontAttributes FontAttributes { get; }
-
 		string FontFamily { get; }
 
+		[TypeConverter(typeof(FontSizeConverter))]
 		double FontSize { get; }
+
+		//note to implementor: but implement the methods explicitly
+		void OnFontFamilyChanged(string oldValue, string newValue);
+		void OnFontSizeChanged(double oldValue, double newValue);
+		double FontSizeDefaultValueCreator();
+		void OnFontAttributesChanged(FontAttributes oldValue, FontAttributes newValue);
 	}
 }
\ No newline at end of file
diff --git a/Xamarin.Forms.Core/Label.cs b/Xamarin.Forms.Core/Label.cs
index 0634e080..e6249323 100644
--- a/Xamarin.Forms.Core/Label.cs
+++ b/Xamarin.Forms.Core/Label.cs
@@ -27,13 +27,11 @@ namespace Xamarin.Forms
 
 		public static readonly BindableProperty TextProperty = BindableProperty.Create("Text", typeof(string), typeof(Label), default(string), propertyChanged: OnTextPropertyChanged);
 
-		public static readonly BindableProperty FontFamilyProperty = BindableProperty.Create("FontFamily", typeof(string), typeof(Label), default(string), propertyChanged: OnFontFamilyChanged);
+		public static readonly BindableProperty FontFamilyProperty = FontElement.FontFamilyProperty;
 
-		public static readonly BindableProperty FontSizeProperty = BindableProperty.Create("FontSize", typeof(double), typeof(Label), -1.0, propertyChanged: OnFontSizeChanged,
-			defaultValueCreator: bindable => Device.GetNamedSize(NamedSize.Default, (Label)bindable));
+		public static readonly BindableProperty FontSizeProperty = FontElement.FontSizeProperty;
 
-		public static readonly BindableProperty FontAttributesProperty = BindableProperty.Create("FontAttributes", typeof(FontAttributes), typeof(Label), FontAttributes.None,
-			propertyChanged: OnFontAttributesChanged);
+		public static readonly BindableProperty FontAttributesProperty = FontElement.FontAttributesProperty;
 
 		public static readonly BindableProperty FormattedTextProperty = BindableProperty.Create("FormattedText", typeof(FormattedString), typeof(Label), default(FormattedString),
 			propertyChanging: (bindable, oldvalue, newvalue) =>
@@ -171,93 +169,72 @@ namespace Xamarin.Forms
 			label.InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);
 		}
 
-		static void OnFontAttributesChanged(BindableObject bindable, object oldValue, object newValue)
-		{
-			var label = (Label)bindable;
+		double IFontElement.FontSizeDefaultValueCreator() =>
+			Device.GetNamedSize(NamedSize.Default, (Label)this);
 
-			if (label._cancelEvents)
+		void IFontElement.OnFontAttributesChanged(FontAttributes oldValue, FontAttributes newValue)
+		{
+			if (_cancelEvents)
 				return;
 
-			label._cancelEvents = true;
+			_cancelEvents = true;
 
-			var attributes = (FontAttributes)newValue;
+			var attributes = newValue;
 
-			object[] values = label.GetValues(FontFamilyProperty, FontSizeProperty);
+#pragma warning disable 0618 // retain until Font removed
+			object[] values = GetValues(FontFamilyProperty, FontSizeProperty);
 			var family = (string)values[0];
 			if (family != null)
-			{
-#pragma warning disable 0618 // retain until Font removed
-				label.Font = Font.OfSize(family, (double)values[1]).WithAttributes(attributes);
-#pragma warning restore 0618
-			}
+				Font = Font.OfSize(family, (double)values[1]).WithAttributes(attributes);
 			else
-			{
-#pragma warning disable 0618 // retain until Font removed
-				label.Font = Font.SystemFontOfSize((double)values[1], attributes);
+				Font = Font.SystemFontOfSize((double)values[1], attributes);
 #pragma warning restore 0618
-			}
 
-			label._cancelEvents = false;
+			_cancelEvents = false;
 
-			label.InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);
+			InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);
 		}
 
-		static void OnFontFamilyChanged(BindableObject bindable, object oldValue, object newValue)
+		void IFontElement.OnFontFamilyChanged(string oldValue, string newValue)
 		{
-			var label = (Label)bindable;
-			if (label._cancelEvents)
+			if (_cancelEvents)
 				return;
 
-			label._cancelEvents = true;
-
-			object[] values = label.GetValues(FontSizeProperty, FontAttributesProperty);
+			_cancelEvents = true;
 
-			var family = (string)newValue;
-			if (family != null)
-			{
 #pragma warning disable 0618 // retain until Font removed
-				label.Font = Font.OfSize(family, (double)values[0]).WithAttributes((FontAttributes)values[1]);
-#pragma warning restore 0618
-			}
+			object[] values = GetValues(FontSizeProperty, FontAttributesProperty);
+			var family = newValue;
+			if (family != null)
+				Font = Font.OfSize(family, (double)values[0]).WithAttributes((FontAttributes)values[1]);
 			else
-			{
-#pragma warning disable 0618 // retain until Font removed
-				label.Font = Font.SystemFontOfSize((double)values[0], (FontAttributes)values[1]);
+				Font = Font.SystemFontOfSize((double)values[0], (FontAttributes)values[1]);
 #pragma warning restore 0618
-			}
 
-			label._cancelEvents = false;
-			label.InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);
+			_cancelEvents = false;
+			InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);
 		}
 
-		static void OnFontSizeChanged(BindableObject bindable, object oldValue, object newValue)
+		void IFontElement.OnFontSizeChanged(double oldValue, double newValue)
 		{
-			var label = (Label)bindable;
-			if (label._cancelEvents)
+			if (_cancelEvents)
 				return;
 
-			label._cancelEvents = true;
-
-			object[] values = label.GetValues(FontFamilyProperty, FontAttributesProperty);
+			_cancelEvents = true;
 
-			var size = (double)newValue;
+#pragma warning disable 0618 // retain until Font removed
+			object[] values = GetValues(FontFamilyProperty, FontAttributesProperty);
+			var size = newValue;
 			var family = (string)values[0];
 			if (family != null)
-			{
-#pragma warning disable 0618 // retain until Font removed
-				label.Font = Font.OfSize(family, size).WithAttributes((FontAttributes)values[1]);
-#pragma warning restore 0618
-			}
+				Font = Font.OfSize(family, size).WithAttributes((FontAttributes)values[1]);
 			else
-			{
-#pragma warning disable 0618 // retain until Font removed
-				label.Font = Font.SystemFontOfSize(size, (FontAttributes)values[1]);
+				Font = Font.SystemFontOfSize(size, (FontAttributes)values[1]);
 #pragma warning restore 0618
-			}
 
-			label._cancelEvents = false;
+			_cancelEvents = false;
 
-			label.InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);
+			InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);
 		}
 
 		void OnFormattedTextChanged(object sender, PropertyChangedEventArgs e)
diff --git a/Xamarin.Forms.Core/SearchBar.cs b/Xamarin.Forms.Core/SearchBar.cs
index 6e2a6c63..5e82f84e 100644
--- a/Xamarin.Forms.Core/SearchBar.cs
+++ b/Xamarin.Forms.Core/SearchBar.cs
@@ -24,12 +24,11 @@ namespace Xamarin.Forms
 
 		public static readonly BindableProperty PlaceholderProperty = BindableProperty.Create("Placeholder", typeof(string), typeof(SearchBar), null);
 
-		public static readonly BindableProperty FontFamilyProperty = BindableProperty.Create("FontFamily", typeof(string), typeof(SearchBar), default(string));
+		public static readonly BindableProperty FontFamilyProperty = FontElement.FontFamilyProperty;
 
-		public static readonly BindableProperty FontSizeProperty = BindableProperty.Create("FontSize", typeof(double), typeof(SearchBar), -1.0,
-			defaultValueCreator: bindable => Device.GetNamedSize(NamedSize.Default, (SearchBar)bindable));
+		public static readonly BindableProperty FontSizeProperty = FontElement.FontSizeProperty;
 
-		public static readonly BindableProperty FontAttributesProperty = BindableProperty.Create("FontAttributes", typeof(FontAttributes), typeof(SearchBar), FontAttributes.None);
+		public static readonly BindableProperty FontAttributesProperty = FontElement.FontAttributesProperty;
 
 		public static readonly BindableProperty HorizontalTextAlignmentProperty = BindableProperty.Create("HorizontalTextAlignment", typeof(TextAlignment), typeof(SearchBar), TextAlignment.Start);
 
@@ -111,6 +110,21 @@ namespace Xamarin.Forms
 			set { SetValue(FontSizeProperty, value); }
 		}
 
+		void IFontElement.OnFontFamilyChanged(string oldValue, string newValue)
+		{
+		}
+
+		void IFontElement.OnFontSizeChanged(double oldValue, double newValue)
+		{
+		}
+
+		double IFontElement.FontSizeDefaultValueCreator() =>
+			Device.GetNamedSize(NamedSize.Default, (SearchBar)this);
+
+		void IFontElement.OnFontAttributesChanged(FontAttributes oldValue, FontAttributes newValue)
+		{
+		}
+
 		public event EventHandler SearchButtonPressed;
 
 		public event EventHandler<TextChangedEventArgs> TextChanged;
diff --git a/Xamarin.Forms.Core/Span.cs b/Xamarin.Forms.Core/Span.cs
index 6ffc3f17..a410c100 100644
--- a/Xamarin.Forms.Core/Span.cs
+++ b/Xamarin.Forms.Core/Span.cs
@@ -84,9 +84,11 @@ namespace Xamarin.Forms
 			{
 				if (_fontAttributes == value)
 					return;
+				var oldValue = _fontAttributes;
 				_fontAttributes = value;
 				OnPropertyChanged();
-				UpdateStructFromFontProperties();
+
+				((IFontElement)this).OnFontAttributesChanged(oldValue, value);
 			}
 		}
 
@@ -97,9 +99,10 @@ namespace Xamarin.Forms
 			{
 				if (_fontFamily == value)
 					return;
+				var oldValue = _fontFamily;
 				_fontFamily = value;
 				OnPropertyChanged();
-				UpdateStructFromFontProperties();
+				((IFontElement)this).OnFontFamilyChanged(oldValue, value);
 			}
 		}
 
@@ -111,9 +114,10 @@ namespace Xamarin.Forms
 			{
 				if (_fontSize == value)
 					return;
+				var oldValue = _fontSize;
 				_fontSize = value;
 				OnPropertyChanged();
-				UpdateStructFromFontProperties();
+				((IFontElement)this).OnFontSizeChanged(oldValue, value);
 			}
 		}
 
@@ -149,23 +153,34 @@ namespace Xamarin.Forms
 			_inUpdate = false;
 		}
 
-		void UpdateStructFromFontProperties()
+		void OnSomeFontPropertyChanged()
 		{
-			if (_inUpdate)
-				return;
-			_inUpdate = true;
-
-			if (FontFamily != null)
 			{
-				Font = Font.OfSize(FontFamily, FontSize).WithAttributes(FontAttributes);
-			}
-			else
-			{
-				Font = Font.SystemFontOfSize(FontSize).WithAttributes(FontAttributes);
-			}
+				if (_inUpdate)
+					return;
+				_inUpdate = true;
 
-			_inUpdate = false;
+				if (FontFamily != null)
+					Font = Font.OfSize(FontFamily, FontSize).WithAttributes(FontAttributes);
+				else
+					Font = Font.SystemFontOfSize(FontSize).WithAttributes(FontAttributes);
+
+				_inUpdate = false;
+			}
 		}
-	}
+
 #pragma warning restore
+
+		void IFontElement.OnFontFamilyChanged(string oldValue, string newValue) =>
+			OnSomeFontPropertyChanged();
+
+		void IFontElement.OnFontSizeChanged(double oldValue, double newValue) =>
+			OnSomeFontPropertyChanged();
+
+		double IFontElement.FontSizeDefaultValueCreator() =>
+			Device.GetNamedSize(NamedSize.Default, typeof(Label));
+
+		void IFontElement.OnFontAttributesChanged(FontAttributes oldValue, FontAttributes newValue) =>
+			OnSomeFontPropertyChanged();
+	}
 }
\ No newline at end of file
diff --git a/Xamarin.Forms.Core/Xamarin.Forms.Core.csproj b/Xamarin.Forms.Core/Xamarin.Forms.Core.csproj
index 944309f9..71df2602 100644
--- a/Xamarin.Forms.Core/Xamarin.Forms.Core.csproj
+++ b/Xamarin.Forms.Core/Xamarin.Forms.Core.csproj
@@ -449,6 +449,7 @@
     <Compile Include="XmlnsDefinitionAttribute.cs" />
     <Compile Include="PlatformConfiguration\macOSSpecific\TabbedPage.cs" />
     <Compile Include="PlatformConfiguration\macOSSpecific\TabsStyle.cs" />
+    <Compile Include="FontElement.cs" />
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.CSharp.targets" />
   <ItemGroup>