Use Hangouts on Android? If so, you’re going to want to snag the v7.0 update, which is now rolling out.
Included with this Hangouts update is quick reply support. With this feature, you can reply to incoming messages from their notifications, meaning that you don’t have to jump into the full-on Hangouts app to reply to a message. To perform a quickly reply, you just tap the “REPLY” button on a Hangouts alert, type out your message, and send it along.