Update RelativeLayout to make it respond to constraint changes (#425)

* Update RelativeLayout to make it respond to constraint changes

Constraints of a RelativeLayout are bindable properties but the layout
does not update when the constraints are updated.

This change will invalidate the layout whenever  XConstraint,
YConstraint, WidthConstraint or HeightConstraint is changed (either in
code or through a change in the bound property)

* Specified changed handler as named property

* Adding attached property accessors for layout properties

Since the constraint attached properties can now be updated at runtime,
setters are required for those properties. Also, when adding a child
view at runtime using the Add() method with x/y/w/h constraints,
generating the bounds constraints is deferred to the layout phase.

* Unit tests for runtime constraints updates in RelativeLayout

* Rename unit test method

Rename LayoutChangesAtRuntim() to LayoutIsUpdatedWhenConstraintsChange()

* Wrap RelativeLayout update setters in BatchBegin/Commit

* Update documentation of RelativeLayout

Added SetXConstraint(), SetYConstraint(), SetWidthConstraint() and
SetHeightConstraint()

1 files changed, 54 insertions, 8 deletions

diff --git a/Xamarin.Forms.Core/RelativeLayout.cs b/Xamarin.Forms.Core/RelativeLayout.cs
index b3a1b615..2b835013 100644
--- a/Xamarin.Forms.Core/RelativeLayout.cs
+++ b/Xamarin.Forms.Core/RelativeLayout.cs
@@ -8,13 +8,13 @@ namespace Xamarin.Forms
 {
 	public class RelativeLayout : Layout<View>
 	{
-		public static readonly BindableProperty XConstraintProperty = BindableProperty.CreateAttached("XConstraint", typeof(Constraint), typeof(RelativeLayout), null);
+		public static readonly BindableProperty XConstraintProperty = BindableProperty.CreateAttached("XConstraint", typeof(Constraint), typeof(RelativeLayout), null, propertyChanged: ConstraintChanged);
 
-		public static readonly BindableProperty YConstraintProperty = BindableProperty.CreateAttached("YConstraint", typeof(Constraint), typeof(RelativeLayout), null);
+		public static readonly BindableProperty YConstraintProperty = BindableProperty.CreateAttached("YConstraint", typeof(Constraint), typeof(RelativeLayout), null, propertyChanged: ConstraintChanged);
 
-		public static readonly BindableProperty WidthConstraintProperty = BindableProperty.CreateAttached("WidthConstraint", typeof(Constraint), typeof(RelativeLayout), null);
+		public static readonly BindableProperty WidthConstraintProperty = BindableProperty.CreateAttached("WidthConstraint", typeof(Constraint), typeof(RelativeLayout), null, propertyChanged: ConstraintChanged);
 
-		public static readonly BindableProperty HeightConstraintProperty = BindableProperty.CreateAttached("HeightConstraint", typeof(Constraint), typeof(RelativeLayout), null);
+		public static readonly BindableProperty HeightConstraintProperty = BindableProperty.CreateAttached("HeightConstraint", typeof(Constraint), typeof(RelativeLayout), null, propertyChanged: ConstraintChanged);
 
 		public static readonly BindableProperty BoundsConstraintProperty = BindableProperty.CreateAttached("BoundsConstraint", typeof(BoundsConstraint), typeof(RelativeLayout), null);
 
@@ -72,6 +72,25 @@ namespace Xamarin.Forms
 			}
 		}
 
+		static void ConstraintChanged(BindableObject bindable, object oldValue, object newValue)
+		{
+			View view = bindable as View;
+
+			(view?.Parent as RelativeLayout)?.UpdateBoundsConstraint(view);
+		}
+
+		void UpdateBoundsConstraint(View view)
+		{
+			if (GetBoundsConstraint(view) == null)
+				return; // Bounds constraint hasn't been calculated yet, no need to update just yet
+
+			CreateBoundsFromConstraints(view, GetXConstraint(view), GetYConstraint(view), GetWidthConstraint(view), GetHeightConstraint(view));
+
+			_childrenInSolveOrder = null; // New constraints may have impact on solve order
+
+			InvalidateLayout();
+		}
+
 		public static BoundsConstraint GetBoundsConstraint(BindableObject bindable)
 		{
 			return (BoundsConstraint)bindable.GetValue(BoundsConstraintProperty);
@@ -102,6 +121,26 @@ namespace Xamarin.Forms
 			bindable.SetValue(BoundsConstraintProperty, value);
 		}
 
+		public static void SetHeightConstraint(BindableObject bindable, Constraint value)
+		{
+			bindable.SetValue(HeightConstraintProperty, value);
+		}
+
+		public static void SetWidthConstraint(BindableObject bindable, Constraint value)
+		{
+			bindable.SetValue(WidthConstraintProperty, value);
+		}
+
+		public static void SetXConstraint(BindableObject bindable, Constraint value)
+		{
+			bindable.SetValue(XConstraintProperty, value);
+		}
+
+		public static void SetYConstraint(BindableObject bindable, Constraint value)
+		{
+			bindable.SetValue(YConstraintProperty, value);
+		}
+
 		protected override void LayoutChildren(double x, double y, double width, double height)
 		{
 			foreach (View child in ChildrenInSolveOrder)
@@ -248,13 +287,13 @@ namespace Xamarin.Forms
 		static Rectangle SolveView(View view)
 		{
 			BoundsConstraint boundsConstraint = GetBoundsConstraint(view);
-			var result = new Rectangle();
 
 			if (boundsConstraint == null)
 			{
 				throw new Exception("BoundsConstraint should not be null at this point");
 			}
-			result = boundsConstraint.Compute();
+
+			var result = boundsConstraint.Compute();
 
 			return result;
 		}
@@ -280,7 +319,7 @@ namespace Xamarin.Forms
 			public void Add(View view, Expression<Func<Rectangle>> bounds)
 			{
 				if (bounds == null)
-					throw new ArgumentNullException("bounds");
+					throw new ArgumentNullException(nameof(bounds));
 				SetBoundsConstraint(view, BoundsConstraint.FromExpression(bounds));
 
 				base.Add(view);
@@ -308,7 +347,14 @@ namespace Xamarin.Forms
 
 			public void Add(View view, Constraint xConstraint = null, Constraint yConstraint = null, Constraint widthConstraint = null, Constraint heightConstraint = null)
 			{
-				Parent.CreateBoundsFromConstraints(view, xConstraint, yConstraint, widthConstraint, heightConstraint);
+				view.BatchBegin();
+
+				RelativeLayout.SetXConstraint(view, xConstraint);
+				RelativeLayout.SetYConstraint(view, yConstraint);
+				RelativeLayout.SetWidthConstraint(view, widthConstraint);
+				RelativeLayout.SetHeightConstraint(view, heightConstraint);
+
+				view.BatchCommit();
 
 				base.Add(view);
 			}