~/Scope for MonoTouch UIAlertViews

09 August 2013

I’m currently working on a MonoTouch app - after a month I’m finally starting to come around to the stack. One issue I’ve been having is a crazy native code crash when I display a UIAlertView. The problem turned out to be objects going out of scope.

The first tip is to use a delegate for the alert view and keep it in the class scope:

public class MyAlertViewDelegate {
  readonly Action _callback;
  public MyAlertViewDelegate(Action callback) {
    _callback = callback;
  }
  public override Clicked(UIAlertView alertview, int buttonIndex) {
    _callback();
  }
}

public class ParentClass {
  MyAlertViewDelegate _myAlertViewDelegate;
  
  void ShowAlert() {
    _myAlertViewDelegate = new MyAlertViewDelegate(() => {});
    var alert = new UIAlertView("title", "message", _alertViewDelegate, "Ok");
    alert.Show();
  }
}

The second tip is to make sure the ParentClass instance doesn’t go out of scope. This is probably by giving it a lifetime scope in the IoC container. For example in TinyIoC:

container.Register<ParentClass>().AsSingleton();