How to get subString of a page of a long NSString, font size variable?

Question

I have a very long text, I defined it as NSString, so I need paging, there will be different paging for different fonts, say I'd like to display one page on a multiple line UILabel. So at frist I need to get substring of page by page of the long string. Does anyone know good solution for this? Thanks in advance.

I know there is a NSString UIKIt additions function to test the size of NSString

NSString *longText; [longText sizeWithFont:[UIFont systemFontOfSize:12.0f] constrainedToSize:textSize lineBreakMode:UILineBreakModeWordWrap];

Maybe I need use this to seperate pages?