Move to `Compose.on`. Actually the thing we are doing is to redirect the event to the composer, due to `Compose` actually has a listener to `input` changes here https://github.com/fcampo/gaia/blob/b8828e9651639ea8a9f7ffdcb535d1f1c7514664/apps/sms/js/compose.js#L224. As you can see, at the end of this method, we dispatch an event `input` to the composer using the `trigger` function https://github.com/fcampo/gaia/blob/b8828e9651639ea8a9f7ffdcb535d1f1c7514664/apps/sms/js/compose.js#L107 .
So at the end this `Compose.on` is using a `step-in-the-middle` for handling the event.