[Core] Fix internal children clear logic (#820)

* Fix InternalChildren clear logic

* Improve InternalChildren clear logic

* Add test code to verify InternalChildren clear logic

* Add missing test cases

* Improve time complexity of clear logic

* Fix the test case where InvalidOperationException occurs.

* Correct the name casing of the test case

* Modify a wrong test code

5 files changed, 97 insertions, 1 deletions

diff --git a/Xamarin.Forms.Core.UnitTests/ContentFormUnitTests.cs b/Xamarin.Forms.Core.UnitTests/ContentFormUnitTests.cs
index 5b6f3e60..e52b5dbf 100644
--- a/Xamarin.Forms.Core.UnitTests/ContentFormUnitTests.cs
+++ b/Xamarin.Forms.Core.UnitTests/ContentFormUnitTests.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using NUnit.Framework;
 
@@ -64,5 +65,21 @@ namespace Xamarin.Forms.Core.UnitTests
 
 			Assert.AreEqual (context, toolbarItem.BindingContext);
 		}
+
+        [Test]
+        public void ContentPage_should_have_the_InternalChildren_correctly_when_Content_changed()
+        {
+            var sut = new ContentPage();
+            IList<Element> internalChildren = ((IControlTemplated)sut).InternalChildren;
+            internalChildren.Add(new VisualElement());
+            internalChildren.Add(new VisualElement());
+            internalChildren.Add(new VisualElement());
+
+            var expected = new View();
+            sut.Content = expected;
+
+            Assert.AreEqual(1, internalChildren.Count);
+            Assert.AreSame(expected, internalChildren[0]);
+        }
 	}
 }
diff --git a/Xamarin.Forms.Core.UnitTests/ContentViewUnitTest.cs b/Xamarin.Forms.Core.UnitTests/ContentViewUnitTest.cs
index 73bb531b..5d7dd651 100644
--- a/Xamarin.Forms.Core.UnitTests/ContentViewUnitTest.cs
+++ b/Xamarin.Forms.Core.UnitTests/ContentViewUnitTest.cs
@@ -388,6 +388,21 @@ namespace Xamarin.Forms.Core.UnitTests
 
 			Assert.AreEqual ("Foo", child.BindingContext);
 		}
-	}
 
+        [Test]
+        public void ContentView_should_have_the_InternalChildren_correctly_when_Content_changed()
+        {
+            var sut = new ContentView();
+            IList<Element> internalChildren = ((IControlTemplated)sut).InternalChildren;
+            internalChildren.Add(new VisualElement());
+            internalChildren.Add(new VisualElement());
+            internalChildren.Add(new VisualElement());
+
+            var expected = new View();
+            sut.Content = expected;
+
+            Assert.AreEqual(1, internalChildren.Count);
+            Assert.AreSame(expected, internalChildren[0]);
+        }
+    }
 }
diff --git a/Xamarin.Forms.Core.UnitTests/TemplatedPageUnitTests.cs b/Xamarin.Forms.Core.UnitTests/TemplatedPageUnitTests.cs
new file mode 100644
index 00000000..306abf0d
--- /dev/null
+++ b/Xamarin.Forms.Core.UnitTests/TemplatedPageUnitTests.cs
@@ -0,0 +1,31 @@
+using System.Collections.Generic;
+using NUnit.Framework;
+
+namespace Xamarin.Forms.Core.UnitTests
+{
+    [TestFixture]
+	public class TemplatedPageUnitTests : BaseTestFixture 
+	{
+        [Test]
+        public void TemplatedPage_should_have_the_InternalChildren_correctly_when_ControlTemplate_changed()
+        {
+            var sut = new TemplatedPage();
+            IList<Element> internalChildren = ((IControlTemplated)sut).InternalChildren;
+            internalChildren.Add(new VisualElement());
+            internalChildren.Add(new VisualElement());
+            internalChildren.Add(new VisualElement());
+
+            sut.ControlTemplate = new ControlTemplate(typeof(ExpectedView));
+
+            Assert.AreEqual(1, internalChildren.Count);
+            Assert.IsInstanceOf<ExpectedView>(internalChildren[0]);
+        }
+
+        private class ExpectedView : View
+        {
+            public ExpectedView()
+            {
+            }
+        }
+	}
+}
diff --git a/Xamarin.Forms.Core.UnitTests/TemplatedViewUnitTests.cs b/Xamarin.Forms.Core.UnitTests/TemplatedViewUnitTests.cs
new file mode 100644
index 00000000..6255edaf
--- /dev/null
+++ b/Xamarin.Forms.Core.UnitTests/TemplatedViewUnitTests.cs
@@ -0,0 +1,31 @@
+using System.Collections.Generic;
+using NUnit.Framework;
+
+namespace Xamarin.Forms.Core.UnitTests
+{
+    [TestFixture]
+	public class TemplatedViewUnitTests : BaseTestFixture 
+	{
+        [Test]
+        public void TemplatedView_should_have_the_InternalChildren_correctly_when_ControlTemplate_changed()
+        {
+            var sut = new TemplatedView();
+            IList<Element> internalChildren = ((IControlTemplated)sut).InternalChildren;
+            internalChildren.Add(new VisualElement());
+            internalChildren.Add(new VisualElement());
+            internalChildren.Add(new VisualElement());
+
+            sut.ControlTemplate = new ControlTemplate(typeof(ExpectedView));
+
+            Assert.AreEqual(1, internalChildren.Count);
+            Assert.IsInstanceOf<ExpectedView>(internalChildren[0]);
+        }
+
+        private class ExpectedView : View
+        {
+            public ExpectedView()
+            {
+            }
+        }
+	}
+}
diff --git a/Xamarin.Forms.Core.UnitTests/Xamarin.Forms.Core.UnitTests.csproj b/Xamarin.Forms.Core.UnitTests/Xamarin.Forms.Core.UnitTests.csproj
index 243cbce3..8727b984 100644
--- a/Xamarin.Forms.Core.UnitTests/Xamarin.Forms.Core.UnitTests.csproj
+++ b/Xamarin.Forms.Core.UnitTests/Xamarin.Forms.Core.UnitTests.csproj
@@ -80,6 +80,8 @@
     <Compile Include="ColorUnitTests.cs" />
     <Compile Include="CommandSourceTests.cs" />
     <Compile Include="CommandTests.cs" />
+    <Compile Include="TemplatedViewUnitTests.cs" />
+    <Compile Include="TemplatedPageUnitTests.cs" />
     <Compile Include="ContentFormUnitTests.cs" />
     <Compile Include="ContraintTypeConverterTests.cs" />
     <Compile Include="ControlTemplateTests.cs" />