Revert "Refactor observer lifecycle (#14828)"

This reverts commit f95f671.

Xamarin.Forms.Platform.iOS/Renderers/EntryRenderer.cs

@@ -40,7 +40,6 @@ public abstract class EntryRendererBase<TControl> : ViewRenderer<Entry, TControl
 
 		bool _disposed;
 		IDisposable _selectedTextRangeObserver;
-		IDisposable _clearButtonSublayerObserver;
 		bool _nativeSelectionIsUpdating;
 
 		bool _cursorPositionChangePending;
@@ -93,7 +92,8 @@ protected override void Dispose(bool disposing)
 					Control.EditingDidEnd -= OnEditingEnded;
 					Control.ShouldChangeCharacters -= ShouldChangeCharacters;
 					_selectedTextRangeObserver?.Dispose();
-					_clearButtonSublayerObserver?.Dispose();
+
+					ClearButton?.Layer?.RemoveObserver(this, new NSString("sublayers"));
 				}
 			}
 
@@ -128,7 +128,7 @@ protected override void OnElementChanged(ElementChangedEventArgs<Entry> e)
 				textField.ShouldChangeCharacters += ShouldChangeCharacters;
 				_selectedTextRangeObserver = textField.AddObserver("selectedTextRange", NSKeyValueObservingOptions.New, UpdateCursorFromControl);
 
-				_clearButtonSublayerObserver = ClearButton?.Layer.AddObserver(new NSString("sublayers"), NSKeyValueObservingOptions.New, UpdateClearButtonSublayer);
+				ClearButton?.Layer.AddObserver(this, new NSString("sublayers"), NSKeyValueObservingOptions.New, IntPtr.Zero);
 			}
 
 			// When we set the control text, it triggers the UpdateCursorFromControl event, which updates CursorPosition and SelectionLength;
@@ -160,6 +160,12 @@ protected override void OnElementChanged(ElementChangedEventArgs<Entry> e)
 				UpdateClearButtonVisibility();
 		}
 
+		public override void ObserveValue(NSString keyPath, NSObject ofObject, NSDictionary change, IntPtr context)
+		{
+			if (keyPath == new NSString("sublayers") && _defaultClearImage == null)
+				UpdateClearButtonVisibility();
+		}
+
 		protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
 		{
 			if (e.PropertyName == Entry.PlaceholderProperty.PropertyName || e.PropertyName == Entry.PlaceholderColorProperty.PropertyName)
@@ -434,15 +440,6 @@ void UpdateCursorFromControl(NSObservedChange obj)
 			}
 		}
 
-		void UpdateClearButtonSublayer(NSObservedChange obj)
-		{
-			if (Control == null || Element == null)
-				return;
-
-			if (_defaultClearImage == null)
-				UpdateClearButtonVisibility();
-		}
-
 		void UpdateCursorSelection()
 		{
 			if (_nativeSelectionIsUpdating || Control == null || Element == null)