It's currently impossible to programmatically compose an SMS. While you can launch the native SMS application and have a single recipient pre-populated, you cannot hand over the message text itself.

This renders many use cases impossible.

One could argue that programmatically sending SMS exposes a security risk. I strongly disagree. I'm not suggesting to allow to SEND an SMS programmatically, but to prepare a complete SMS and have the user manually hit the "Send" button of the native Messages app.