swizzle.cs

	public static class DebugConstraint
	{
		delegate IntPtr DescriptionDelegate (IntPtr self, IntPtr sel);
		static DescriptionDelegate newDescImpl = new DescriptionDelegate (Description);

		public static void Swizzle ()
		{
			var constraintClass = Class.GetHandle (typeof(NSLayoutConstraint));
			var method = class_getInstanceMethod (constraintClass, Selector.GetHandle ("description"));
			var originalImpl = class_getMethodImplementation (constraintClass, Selector.GetHandle ("description"));

			// add the original implementation to respond to 'customDescription'
			class_addMethod (constraintClass, Selector.GetHandle ("customDescription"), originalImpl, "@@:");

			// replace the original implementation with our own for the 'descriptor' method.
			var newImpl = System.Runtime.InteropServices.Marshal.GetFunctionPointerForDelegate (newDescImpl);
			method_setImplementation (method, newImpl);
		}

		[ObjCRuntime.MonoPInvokeCallback (typeof (DescriptionDelegate))]
		public static IntPtr Description (IntPtr self, IntPtr sel)
		{
			var originalDescPtr = objc_msgSend (self, Selector.GetHandle ("customDescription"));
			var originalDesc = Runtime.GetNSObject<NSString> (originalDescPtr);
			Console.WriteLine ("Original desc: {0}", originalDesc);
			return new NSString ("my version").Handle;
		}

		[System.Runtime.InteropServices.DllImport ("libobjc.dylib")]
		static extern IntPtr objc_msgSend (IntPtr handle, IntPtr sel);

		[System.Runtime.InteropServices.DllImport ("libobjc.dylib")]
		static extern IntPtr class_getInstanceMethod (IntPtr c, IntPtr sel);

		[System.Runtime.InteropServices.DllImport ("libobjc.dylib")]
		static extern bool class_addMethod (IntPtr cls, IntPtr name, IntPtr imp, string types);

		[System.Runtime.InteropServices.DllImport ("libobjc.dylib")]
		extern static IntPtr class_getMethodImplementation (IntPtr cls, IntPtr sel);

		[System.Runtime.InteropServices.DllImport ("libobjc.dylib")]
		extern static IntPtr method_setImplementation (IntPtr method, IntPtr imp);
	}