[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

1 files changed, 31 insertions, 0 deletions

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()
+            {
+            }
+        }
+	}
+}