IPhone : Change Font size of iOS app dynamically in all page from one page

I have an option to change the font size of my application from settings. If i change the size from settings, then it should affect all loaded pages in app.

SO I tried "NSNotificationCenter". I just added Observer in all classes , then I will fire notification from settings.

I have function called "updateUI"

-(void)updateUI{
    // here I will change font size of all UI controlls
}

Observer is

 [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(updateUI) name:"KFontSizeChanged" object:nil];

then I will call like

[[NSNotificationCenter defaultCenter] postNotificationName:@"KFontSizeChanged" object:self];

When Observer observe any new notification , then "updateUI" function will be called. there UI font size will be updated. It works fine , but I want to know Is there any better solution to achieve this

also I have tried following

yourLabel.font = [UIFont preferredFontForTextStyle:UIFontTextStyleBody];

According to this code , I will need iOS7 and above. but I need iOS 5 and above support. also I will need to go to iPhone settings to change size.

Can anyone Please Let me know is there any better solution to achieve this ?

Thanks in advance