Custom keyboard for iphone and/ andriod.

I want to make a new keyboard layout, which can be used exactly like pre-installed keyboards on iphone. So I want my keyboard layout can be used on all over the iphone system. There are plenty of apps that can install custom keyboard of their own. for example, some apps called emoji, something like that. once I download one of them and open it, the app add new keyboard layout on system. You can see it on the Setting > General > International > Keyboard. And you can use it any apps in iphone. Make my layout on the system.I don't want it to be used on just one app, but on entire system.