Custom Fronts in iphone that will work in all other apps

Question

I want to create an app that will provide custom fonts to the user that will work in other apps also like WhatsApp, Viber, Wechat etc.
Some apps on appstore that provide this functionality are as follows:-

KOOL FONT STYLES, Fonts for Whatsapp.

But the problem is that I don't know how to do this and from where I should start?

Answer 1

You can't - at least, not really.

Those applications don't add fonts. They expose existing Unicode characters, and automatically convert text to use these characters (for instance, one "font" might translate A to Ⓐ).