How to detect Dynamic Font size changes from iOS Settings?

Question

Inside settings->general->text size, after changing the text size, I'd have to exit my own app to have the sizes applied to

 [UIFont preferredFontForTextStyle:..]

Is there a delegate or notification to notify my app to re-apply the new sizes?

Update: I tried the following but interestingly, the font size will apply after I BG and launch the app TWICE.

- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.


[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(fromBg:) name:UIApplicationDidBecomeActiveNotification object:nil];

}


 -(void) fromBg:(NSNotification *)noti{

    self.headline1.font = [UIFont preferredFontForTextStyle:UIFontTextStyleHeadline];
    self.subHeadline.font = [UIFont preferredFontForTextStyle:UIFontTextStyleSubheadline];
    self.body.font = [UIFont preferredFontForTextStyle:UIFontTextStyleBody];
    self.footnote.font = [UIFont preferredFontForTextStyle:UIFontTextStyleFootnote];
    self.caption1.font = [UIFont preferredFontForTextStyle:UIFontTextStyleCaption1];
    self.caption2.font = [UIFont preferredFontForTextStyle:UIFontTextStyleCaption2];
//    [self.view layoutIfNeeded];

}

Answer 1

You listen for the Size Change Notification on UIContentSizeCategory.

Swift 3.0: NSNotification.Name.UIContentSizeCategoryDidChange

Swift 4.0 or later: UIContentSizeCategory.didChangeNotification