BindingContext changes are called multiple times on app start (#470)

* inherited binding context fix

* add check for child binding context

* Revert "add check for child binding context"

This reverts commit 71952f39129cfedc2358a32100deb07bd32a3080.

* unit tests

Xamarin.Forms.Core.UnitTests/BindableObjectUnitTests.cs

@@ -110,6 +110,90 @@ namespace Xamarin.Forms.Core.UnitTests
 			Assert.Fail ("The BindingContextChanged event was not fired.");
 		}
 
+		[Test]
+		public void BindingContextChangedOnce()
+		{
+			var count = 0;
+
+			var mock = new MockBindable();
+			mock.BindingContextChanged += (sender, args) => ++count;
+
+			mock.BindingContext = new object();
+			Assert.AreEqual(count, 1);
+		}
+
+		[Test]
+		public void ParentAndChildBindingContextChanged()
+		{
+			int parentCount = 0, childCount = 0;
+
+			var didNotChange = " BindingContext did not change.";
+			var changedMoreThanOnce = " BindingContext changed more than once.";
+			var changedWhenNoChange = " BindingContext was changed when there was no change in context.";
+
+			var parent = new MockBindable();
+			parent.BindingContextChanged += (sender, args) => { ++parentCount; };
+
+			var child = new MockBindable();
+			child.BindingContextChanged += (sender, args) => { ++childCount; };
+
+			child.Parent = parent;
+			Assert.AreEqual(parentCount, 0, "Parent BindingContext was changed while parenting a child.");
+			Assert.AreEqual(childCount, 0, "Child" + changedWhenNoChange);
+
+			child.BindingContext = new object(); // set manually
+			Assert.GreaterOrEqual(childCount, 1, "Child" + didNotChange);
+			Assert.AreEqual(childCount, 1, "Child" + changedMoreThanOnce);
+			Assert.AreEqual(parentCount, 0, "Parent" + changedWhenNoChange);
+
+			parent.BindingContext = new object();
+			Assert.GreaterOrEqual(parentCount, 1, "Parent" + didNotChange);
+			Assert.AreEqual(parentCount, 1, "Parent" + changedMoreThanOnce);
+			Assert.AreEqual(childCount, 1, "Child" + changedWhenNoChange);
+
+			child.BindingContext = new object();
+			Assert.GreaterOrEqual(childCount, 2, "Child" + didNotChange);
+			Assert.AreEqual(childCount, 2, "Child" + changedMoreThanOnce);
+			Assert.AreEqual(parentCount, 1, "Parent" + changedWhenNoChange);
+		}
+
+		[Test]
+		public void ParentSetOnNullChildBindingContext()
+		{
+			var parent = new MockBindable();
+
+			var child = new MockBindable();
+			child.BindingContextChanged += (sender, args) => { Assert.Fail("Child BindingContext was changed when there was no change in context."); };
+
+			child.Parent = parent; // this should not trigger binding context change on child since there is no change
+			parent.BindingContext = new object();
+			parent.BindingContext = new object();
+		}
+
+		[Test]
+		public void ParentSetOnNonNullChildBindingContext()
+		{
+			var count = 0;
+
+			var parent = new MockBindable();
+			parent.BindingContextChanged += (sender, args) => { ++count; };
+
+			var child = new MockBindable();
+			child.BindingContextChanged += (sender, args) => { ++count; };
+
+			child.BindingContext = new object(); // set manually
+			Assert.AreEqual(count, 1);
+
+			child.Parent = parent; // this should not trigger binding context change because child binding was set manually
+			Assert.AreEqual(count, 1);
+
+			parent.BindingContext = new object();
+			Assert.AreEqual(count, 2);
+
+			child.BindingContext = new object();
+			Assert.AreEqual(count, 3);
+		}
+
 		[Test]
 		[Description ("When the BindingContext changes, any bindings should be immediately applied.")]
 		public void BindingContextChangedBindingsApplied()

Xamarin.Forms.Core/BindableObject.cs

@@ -98,6 +98,10 @@ namespace Xamarin.Forms
 				return;
 
 			object oldContext = bindable._inheritedContext;
+
+			if (ReferenceEquals(oldContext, value))
+				return;
+
 			if (bpContext != null && oldContext == null)
 				oldContext = bpContext.Value;
 
@@ -108,9 +112,6 @@ namespace Xamarin.Forms
 			}
 			else
 			{
-				if (ReferenceEquals(oldContext, value))
-					return;
-
 				bindable._inheritedContext = value;
 			}