Throw an exception if FontWeight <= 0

In FormattedText. Fixes #700.

src/Avalonia.SceneGraph/Media/FormattedText.cs

@@ -33,7 +33,16 @@ namespace Avalonia.Media
         {
             Contract.Requires<ArgumentNullException>(text != null);
             Contract.Requires<ArgumentNullException>(fontFamilyName != null);
-            Contract.Requires<ArgumentException>(fontSize > 0);
+
+            if (fontSize <= 0)
+            {
+                throw new ArgumentException("FontSize must be greater than 0");
+            }
+
+            if (fontWeight <= 0)
+            {
+                throw new ArgumentException("FontWeight must be greater than 0");
+            }
 
             Text = text;
             FontFamilyName = fontFamilyName;

tests/Avalonia.SceneGraph.UnitTests/Media/FormattedTextTests.cs

@@ -0,0 +1,28 @@
+using System;
+using Avalonia.Media;
+using Xunit;
+
+namespace Avalonia.SceneGraph.UnitTests.Media
+{
+    public class FormattedTextTests
+    {
+        [Fact]
+        public void Exception_Should_Be_Thrown_If_FontSize_0()
+        {
+            Assert.Throws<ArgumentException>(() => new FormattedText(
+                "foo",
+                "Ariel",
+                0));
+        }
+
+        [Fact]
+        public void Exception_Should_Be_Thrown_If_FontWeight_0()
+        {
+            Assert.Throws<ArgumentException>(() => new FormattedText(
+                "foo",
+                "Ariel",
+                12,
+                fontWeight: 0));
+        }
+    }
+}