How to detect when app moves to background or foreground in UIViewController class xamarin.ios

Question

I need to enable and disable some timers when app moves to background or foreground that lives in a UIViewController. But it seems I can only access the DidEnterBackground and WillEnterForeground only in AppDelegate class. Is there any way to detect these two events from UIViewController class?

Answer 1

You can't access those event in uiviewcontroller level, you can only get them in App level.

In the AppDelegate, you can send notification when use DidEnterBackground or WillEnterForeground :

public class AppDelegate : UIResponder, IUIApplicationDelegate {

    [Export("window")]
    public UIWindow Window { get; set; }

    [Export ("application:didFinishLaunchingWithOptions:")]
    public bool FinishedLaunching (UIApplication application, NSDictionary launchOptions)
    {
        // Override point for customization after application launch.
        // If not required for your application you can safely delete this method
        return true;
    }

    [Export("applicationDidEnterBackground:")]
    public virtual void DidEnterBackground(UIKit.UIApplication application) {

        NSNotificationCenter.DefaultCenter.PostNotificationName("DidEnterBackground",this);
    }

    [Export("applicationWillEnterForeground:")]
    public virtual void WillEnterForeground(UIKit.UIApplication application) {

        NSNotificationCenter.DefaultCenter.PostNotificationName("WillEnterForeground", this);
    }      
}

Then in the UIViewController which you want to know, AddObserver:

public partial class ViewController : UIViewController
{
    public ViewController (IntPtr handle) : base (handle)
    {
    }

    public override void ViewDidLoad ()
    {
        base.ViewDidLoad ();
        // Perform any additional setup after loading the view, typically from a nib.

        NSString backgroundStr = new NSString("DidEnterBackground");
        NSString foregroundStr = new NSString("WillEnterForeground");

        NSNotificationCenter.DefaultCenter.AddObserver(backgroundStr, enterBackgroundMethod);
        NSNotificationCenter.DefaultCenter.AddObserver(foregroundStr, enterForegroundMethod);

    }

    public void enterBackgroundMethod(NSNotification notification)
    {
        Console.WriteLine("enterBackgroundMethod");
    }

    public void enterForegroundMethod(NSNotification notification)
    {
        Console.WriteLine("enterForegroundMethod");
    }
}