Monotouch/iOS: which place is the best one to unsubscribe the delegate

Question

The best method in iOS to subscribe a event is ViewDidLoad, but when dismiss the view , the ViewDidUnload() is not called(only when the memory warning.)

Which place is the best to unsubscribe the event?

(In the subviewController I subscribe a event that reference the MainViewController, When open the subview twice, I receive two event trigger because the unsubscribe in viewdidunload() is never called.)

How about with subscribe/unsubscribe in ViewWillAppear/ViewWillDisapper?

    public override void ViewWillAppear (bool animated)
    {
        base.ViewWillAppear (animated);

        this.mBL.OrderChanged += HandleOrderChanged;            
    }

    public override void ViewWillDisappear (bool animated)
    {
        base.VieWillDisappear (animated);
        if (this.mBL!=null)
          this.mBL.OrderChanged -= HandleOrderChanged;          
    }

Answer 1

Use ViewDidLoad and ViewDidUnload, those are the appropriate places to subscribe/unsubscribe events from the UI.

Here is a general article on memory management in iOS that I think applies here: http://www.buildingiphoneapps.com/buildingiphoneapps/2010/6/25/memory-management-and-viewdidunload.html

Now, if you're not wanting to have the event run when your View is not visible, do something like this in the event handler:

if (IsViewLoaded && View.Window != null) {
  //code here
}

I've found this is the easiest way to tell if the view is on screen.

Answer 2

I agree, the best time is in ViewWillAppear/ViewWillDisappear, not ViewDidUnload.

ViewDidUnload is called called post iOS 6: https://developer.xamarin.com/api/member/UIKit.UIViewController.ViewDidUnload()/