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