[UIKit] Inline the UITraitChangeObservable protocol into the classes …

…that implement it. Fixes #20265. (#21676)

This also required fixing:

* A bug in the static registrar where we'd ignore the generic parameters
in a method
  when looking for methods implementing an interface.

* A bug in the generator where we'd throw
You_Should_Not_Call_base_In_This_Method()
  in some cases where we shouldn't.

Also:

* Enable nullability and fix any resulting issues.
* Clean up some legacy code we don't need anymore.

Ref:
#19410 (comment)

Fixes #20265.

---------

Co-authored-by: GitHub Actions Autoformatter <[email protected]>

src/UIKit/UIPresentationController.cs

@@ -0,0 +1,119 @@
+using System;
+using System.Runtime.InteropServices;
+
+using Foundation;
+using ObjCRuntime;
+
+#nullable enable
+
+namespace UIKit {
+	public partial class UIPresentationController {
+		#region Inlined from the UITraitChangeObservable protocol
+		/// <summary>
+		/// Registers a callback handler that will be executed when one of the specified traits changes.
+		/// </summary>
+		/// <param name="traits">The traits to observe.</param>
+		/// <param name="handler">The callback to execute when any of the specified traits changes.</param>
+		/// <returns>A token that can be used to unregister the callback by calling <see cref="M:UnregisterForTraitChanges" />.</returns>
+		public IUITraitChangeRegistration RegisterForTraitChanges (Type [] traits, Action<IUITraitEnvironment, UITraitCollection> handler)
+		{
+			return IUITraitChangeObservable._RegisterForTraitChanges (this, traits, handler);
+		}
+
+		/// <summary>
+		/// Registers a callback handler that will be executed when one of the specified traits changes.
+		/// </summary>
+		/// <param name="traits">The traits to observe.</param>
+		/// <param name="handler">The callback to execute when any of the specified traits changes.</param>
+		/// <returns>A token that can be used to unregister the callback by calling <see cref="M:UnregisterForTraitChanges" />.</returns>
+		public unsafe IUITraitChangeRegistration RegisterForTraitChanges (Action<IUITraitEnvironment, UITraitCollection> handler, params Type [] traits)
+		{
+			// Add an override with 'params', unfortunately this means reordering the parameters.
+			return IUITraitChangeObservable._RegisterForTraitChanges (this, handler, traits);
+		}
+
+		/// <summary>
+		/// Registers a callback handler that will be executed when the specified trait changes.
+		/// </summary>
+		/// <typeparam name="T">The trait to observe.</typeparam>
+		/// <param name="handler">The callback to execute when any of the specified traits changes.</param>
+		/// <returns>A token that can be used to unregister the callback by calling <see cref="M:UnregisterForTraitChanges" />.</returns>
+		public unsafe IUITraitChangeRegistration RegisterForTraitChanges<T> (Action<IUITraitEnvironment, UITraitCollection> handler)
+			where T : IUITraitDefinition
+		{
+			return IUITraitChangeObservable._RegisterForTraitChanges<T> (this, handler);
+		}
+
+		/// <summary>
+		/// Registers a callback handler that will be executed when any of the specified traits changes.
+		/// </summary>
+		/// <typeparam name="T1">A trait to observe</typeparam>
+		/// <typeparam name="T2">A trait to observe</typeparam>
+		/// <param name="handler">The callback to execute when any of the specified traits changes.</param>
+		/// <returns>A token that can be used to unregister the callback by calling <see cref="M:UnregisterForTraitChanges" />.</returns>
+		public unsafe IUITraitChangeRegistration RegisterForTraitChanges<T1, T2> (Action<IUITraitEnvironment, UITraitCollection> handler)
+			where T1 : IUITraitDefinition
+			where T2 : IUITraitDefinition
+		{
+			return IUITraitChangeObservable._RegisterForTraitChanges<T1, T2> (this, handler);
+		}
+
+		/// <summary>
+		/// Registers a callback handler that will be executed when any of the specified traits changes.
+		/// </summary>
+		/// <typeparam name="T1">A trait to observe</typeparam>
+		/// <typeparam name="T2">A trait to observe</typeparam>
+		/// <typeparam name="T3">A trait to observe</typeparam>
+		/// <param name="handler">The callback to execute when any of the specified traits changes.</param>
+		/// <returns>A token that can be used to unregister the callback by calling <see cref="M:UnregisterForTraitChanges" />.</returns>
+		public unsafe IUITraitChangeRegistration RegisterForTraitChanges<T1, T2, T3> (Action<IUITraitEnvironment, UITraitCollection> handler)
+			where T1 : IUITraitDefinition
+			where T2 : IUITraitDefinition
+			where T3 : IUITraitDefinition
+		{
+			return IUITraitChangeObservable._RegisterForTraitChanges<T1, T2, T3> (this, handler);
+		}
+
+		/// <summary>
+		/// Registers a callback handler that will be executed when any of the specified traits changes.
+		/// </summary>
+		/// <typeparam name="T1">A trait to observe</typeparam>
+		/// <typeparam name="T2">A trait to observe</typeparam>
+		/// <typeparam name="T3">A trait to observe</typeparam>
+		/// <typeparam name="T4">A trait to observe</typeparam>
+		/// <param name="handler">The callback to execute when any of the specified traits changes.</param>
+		/// <returns>A token that can be used to unregister the callback by calling <see cref="M:UnregisterForTraitChanges" />.</returns>
+		public unsafe IUITraitChangeRegistration RegisterForTraitChanges<T1, T2, T3, T4> (Action<IUITraitEnvironment, UITraitCollection> handler)
+			where T1 : IUITraitDefinition
+			where T2 : IUITraitDefinition
+			where T3 : IUITraitDefinition
+			where T4 : IUITraitDefinition
+		{
+			return IUITraitChangeObservable._RegisterForTraitChanges<T1, T2, T3, T4> (this, handler);
+		}
+
+		/// <summary>
+		/// Registers a selector that will be called on the specified object when any of the specified traits changes.
+		/// </summary>
+		/// <param name="traits">The traits to observe.</param>
+		/// <param name="target">The object whose specified selector will be called.</param>
+		/// <param name="action">The selector to call on the specified object.</param>
+		/// <returns>A token that can be used to unregister the callback by calling <see cref="M:UnregisterForTraitChanges" />.</returns>
+		public IUITraitChangeRegistration RegisterForTraitChanges (Type [] traits, NSObject target, Selector action)
+		{
+			return IUITraitChangeObservable._RegisterForTraitChanges (this, traits, target, action);
+		}
+
+		/// <summary>
+		/// Registers a selector that will be called on the current object when any of the specified traits changes.
+		/// </summary>
+		/// <param name="traits">The traits to observe.</param>
+		/// <param name="action">The selector to call on the current object.</param>
+		/// <returns>A token that can be used to unregister the callback by calling <see cref="M:UnregisterForTraitChanges" />.</returns>
+		public IUITraitChangeRegistration RegisterForTraitChanges (Type [] traits, Selector action)
+		{
+			return IUITraitChangeObservable._RegisterForTraitChanges (this, traits, action);
+		}
+		#endregion
+	}
+}