Hook.cs

using System;
using System.Runtime.InteropServices;
using ObjCRuntime;
using UIKit;
using runtime;

namespace hook
{
	public static class Hook
	{
		[DllImport("/usr/lib/libobjc.dylib")]
		private static extern IntPtr class_getInstanceMethod(IntPtr classHandle, IntPtr Selector);

		[DllImport("/usr/lib/libobjc.dylib")]
		private static extern IntPtr method_getImplementation(IntPtr method);

		[DllImport("/usr/lib/libobjc.dylib")]
		private static extern void method_setImplementation(IntPtr method, IntPtr imp);

		[DllImport ("/usr/lib/libobjc.dylib")]
		internal static extern IntPtr object_getClass (IntPtr obj);

		[MonoNativeFunctionWrapper]
		private delegate IntPtr PopDelegate(IntPtr self, IntPtr sel, bool animated);

		private static IntPtr OrigPopImpl;
		static PopDelegate popDelegate;

		public static void Setup ()
		{
			var classHandle = Class.GetHandle("UINavigationController");
			var selector = Selector.GetHandle("popViewControllerAnimated:");
			var method = class_getInstanceMethod(classHandle, selector);
			OrigPopImpl = method_getImplementation (method);

			popDelegate = PopViewControllerAnimated;
			var imp = Marshal.GetFunctionPointerForDelegate (popDelegate);
			method_setImplementation(method, imp);
		}

		[MonoPInvokeCallback(typeof (PopDelegate))]
		private static IntPtr PopViewControllerAnimated (IntPtr self, IntPtr sel, bool animated)
		{
			// When built for simulator this line returns UINavigationController instance
			// When build for device with project iOS build setting 'Enable Device Specific Builds' enabled 
			// line also returns UINavigationController.
			// When build for device with this setting turned off this line returns instance
			// of __NSMallocBlock__ and subsequent call passing parameters on to original
			// selector method crashes
			// This is not specific to the selector chosen for this harness and the marshalling issues
			// are seen in all cases that have been tried. In our production code we are hooking a number
			// of selectors on several classes. The solution works for all but the build configuration
			// outlined above.

			Console.WriteLine ("PopViewControllerAnimated (self: {0} selector: {1} animated: {2})", new Class (object_getClass (self)).Name, Selector.FromHandle (sel).Name, animated);

			var navigationController = ObjCRuntime.Runtime.GetNSObject<UINavigationController>(self);

			Console.WriteLine ("navigationController: {0}", navigationController);

			var del = (PopDelegate) Marshal.GetDelegateForFunctionPointer (OrigPopImpl,typeof(PopDelegate));

			return del (self, sel, animated);
		}
	}
}