ios-samples/KeyValueObserving/AppDelegate.cs

//
// kvo: Shows how to observe property changes on an object
//
using System;
using CoreGraphics;

using Foundation;
using UIKit;

namespace kvo
{
	[Register ("AppDelegate")]
	public partial class AppDelegate : UIApplicationDelegate
	{
		UINavigationController topController;
		UIWindow window;
		UILabel label;

		public override bool FinishedLaunching (UIApplication app, NSDictionary options)
		{
			topController = new UINavigationController ();
			topController.View.BackgroundColor = UIColor.Gray;

			label = new UILabel (ComputeLabelRect ()) {
				Text = "Rotate the phone to observe the `bounds' change",
				LineBreakMode = UILineBreakMode.WordWrap,
				Lines = 20,
				BackgroundColor = UIColor.LightGray
			};

			topController.View.AddSubview (label);

			window = new UIWindow (UIScreen.MainScreen.Bounds) {
				RootViewController = topController
			};

			window.MakeKeyAndVisible ();

			//
			// Observe changes to the main View layer, in this case, we want to
			// observe changes to the "bounds" property (this is the name of the Objective-C
			// property).
			//
			// The changes observed are delivered to "this", which is expected to override
			// the "ObserveValue" method.
			//
			topController.View.Layer.AddObserver (observer: this,
			                                keyPath:  new NSString ("bounds"),
											options:  NSKeyValueObservingOptions.New,
			                                context:  IntPtr.Zero);
			return true;
		}

		//
		// Override the ObserveValue method on the class that you designate as the observer
		//
		public override void ObserveValue (NSString keyPath, NSObject ofObject, NSDictionary change, IntPtr context)
		{
			var str = String.Format ("The {0} property on {1}, the change is: {2}", keyPath, ofObject, change.Description);
			Console.WriteLine (str);
			label.Text = str;

			label.Frame = ComputeLabelRect ();
		}

		CGRect ComputeLabelRect ()
		{
			var rect = topController.View.Bounds;
			rect.Inflate (-20, -80);

			return rect;
		}

		static void Main (string[] args)
		{
			UIApplication.Main (args, null, "AppDelegate");
		}
	}
}