[WIP][Perf] Reduce BindableObject allocations (event args caching, tree propagation, lazy init)

src/Controls/src/Core/BindableObject.cs

@@ -38,7 +38,7 @@ public BindableObject()
 		}
 
 		internal ushort _triggerCount = 0;
-		internal Dictionary<TriggerBase, SetterSpecificity> _triggerSpecificity = new Dictionary<TriggerBase, SetterSpecificity>();
+		internal Dictionary<TriggerBase, SetterSpecificity> _triggerSpecificity;
 		readonly Dictionary<BindableProperty, BindablePropertyContext> _properties = new Dictionary<BindableProperty, BindablePropertyContext>(4);
 		bool _applying;
 		WeakReference _inheritedContext;
@@ -139,7 +139,7 @@ void ClearValueCore(BindableProperty property, SetterSpecificity specificity)
 			if (changed)
 			{
 				property.PropertyChanging?.Invoke(this, original.Value, newValue);
-				OnPropertyChanging(property.PropertyName);
+				OnPropertyChanging(property.GetPropertyChangingEventArgs());
 			}
 
 			bpcontext.Values.Remove(specificity);
@@ -363,7 +363,10 @@ public static void SetInheritedBindingContext(BindableObject bindable, object va
 			}
 			else
 			{
-				bindable._inheritedContext = new WeakReference(value);
+				if (bindable._inheritedContext is not null)
+					bindable._inheritedContext.Target = value;
+				else
+					bindable._inheritedContext = new WeakReference(value);
 				bindable.ApplyBindings(fromBindingContextChanged: true);
 				bindable.OnBindingContextChanged();
 			}
@@ -407,13 +410,27 @@ protected virtual void OnBindingContextChanged()
 		protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
 			=> PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
 
+		/// <summary>
+		/// Raises the <see cref="PropertyChanged"/> event using a cached PropertyChangedEventArgs.
+		/// </summary>
+		/// <param name="args">The cached PropertyChangedEventArgs to use.</param>
+		internal void OnPropertyChanged(PropertyChangedEventArgs args)
+			=> PropertyChanged?.Invoke(this, args);
+
 		/// <summary>
 		/// Raises the <see cref="PropertyChanging"/> event.
 		/// </summary>
 		/// <param name="propertyName">The name of the property that is changing.</param>
 		protected virtual void OnPropertyChanging([CallerMemberName] string propertyName = null)
 			=> PropertyChanging?.Invoke(this, new PropertyChangingEventArgs(propertyName));
 
+		/// <summary>
+		/// Raises the <see cref="PropertyChanging"/> event using a cached PropertyChangingEventArgs.
+		/// </summary>
+		/// <param name="args">The cached PropertyChangingEventArgs to use.</param>
+		internal void OnPropertyChanging(PropertyChangingEventArgs args)
+			=> PropertyChanging?.Invoke(this, args);
+
 		/// <summary>
 		/// Removes all current bindings from the current context.
 		/// </summary>
@@ -639,7 +656,7 @@ void SetValueActual(BindableProperty property, BindablePropertyContext context,
 			{
 				property.PropertyChanging?.Invoke(this, original, value);
 
-				OnPropertyChanging(property.PropertyName);
+				OnPropertyChanging(property.GetPropertyChangingEventArgs());
 			}
 
 			context.Values[specificity] = value;
@@ -672,18 +689,16 @@ private protected virtual void OnBindablePropertySet(BindableProperty property,
 		{
 			if (willFirePropertyChanged)
 			{
-				OnPropertyChanged(property.PropertyName);
+				OnPropertyChanged(property.GetPropertyChangedEventArgs());
 				property.PropertyChanged?.Invoke(this, original, value);
 			}
 		}
 
 		void ApplyBindings(bool fromBindingContextChanged)
 		{
-			var prop = _properties.Values.ToArray();
-
-			for (int i = 0, propLength = prop.Length; i < propLength; i++)
+			foreach (var kvp in _properties)
 			{
-				BindablePropertyContext context = prop[i];
+				BindablePropertyContext context = kvp.Value;
 				if (ReferenceEquals(context.Property, BindingContextProperty))
 				{
 					// BindingContextProperty Binding is handled separately within SetInheritedBindingContext

src/Controls/src/Core/BindableProperty.cs

@@ -252,6 +252,15 @@ public sealed class BindableProperty
 
 		internal ValidateValueDelegate ValidateValue { get; private set; }
 
+		PropertyChangedEventArgs _cachedPropertyChangedEventArgs;
+		PropertyChangingEventArgs _cachedPropertyChangingEventArgs;
+
+		internal PropertyChangedEventArgs GetPropertyChangedEventArgs()
+			=> _cachedPropertyChangedEventArgs ??= new PropertyChangedEventArgs(PropertyName);
+
+		internal PropertyChangingEventArgs GetPropertyChangingEventArgs()
+			=> _cachedPropertyChangingEventArgs ??= new PropertyChangingEventArgs(PropertyName);
+
 		/// <summary>Creates a new instance of the BindableProperty class.</summary>
 		/// <param name="propertyName">The name of the BindableProperty.</param>
 		/// <param name="returnType">The type of the property.</param>

src/Controls/src/Core/Element/Element.cs

@@ -668,14 +668,19 @@ protected virtual void OnParentSet()
 		}
 
 		HashSet<string> _pendingHandlerUpdatesFromBPSet = new HashSet<string>();
+		BindableProperty _currentPropertyBeingSet;
+
 		private protected override void OnBindablePropertySet(BindableProperty property, object original, object value, bool changed, bool willFirePropertyChanged)
 		{
 			if (willFirePropertyChanged)
 			{
 				_pendingHandlerUpdatesFromBPSet.Add(property.PropertyName);
 			}
 
+			_currentPropertyBeingSet = property;
 			base.OnBindablePropertySet(property, original, value, changed, willFirePropertyChanged);
+			_currentPropertyBeingSet = null;
+
 			_pendingHandlerUpdatesFromBPSet.Remove(property.PropertyName);
 			UpdateHandlerValue(property.PropertyName, changed);
 
@@ -702,7 +707,7 @@ protected override void OnPropertyChanged([CallerMemberName] string propertyName
 
 			if (_effects?.Count > 0)
 			{
-				var args = new PropertyChangedEventArgs(propertyName);
+				var args = _currentPropertyBeingSet?.GetPropertyChangedEventArgs() ?? new PropertyChangedEventArgs(propertyName);
 				foreach (Effect effect in _effects)
 				{
 					effect?.SendOnElementPropertyChanged(args);
@@ -946,14 +951,26 @@ internal INameScope GetNameScope()
 
 		void OnDescendantAdded(Element child)
 		{
-			DescendantAdded?.Invoke(this, new ElementEventArgs(child));
-			RealParent?.OnDescendantAdded(child);
+			var args = new ElementEventArgs(child);
+			OnDescendantAdded(args);
+		}
+
+		void OnDescendantAdded(ElementEventArgs args)
+		{
+			DescendantAdded?.Invoke(this, args);
+			RealParent?.OnDescendantAdded(args);
 		}
 
 		void OnDescendantRemoved(Element child)
 		{
-			DescendantRemoved?.Invoke(this, new ElementEventArgs(child));
-			RealParent?.OnDescendantRemoved(child);
+			var args = new ElementEventArgs(child);
+			OnDescendantRemoved(args);
+		}
+
+		void OnDescendantRemoved(ElementEventArgs args)
+		{
+			DescendantRemoved?.Invoke(this, args);
+			RealParent?.OnDescendantRemoved(args);
 		}
 
 		void OnResourceChanged(BindableProperty property, object value, SetterSpecificity specificity)