No need to interrupt your game to tap out a quick reply to a text.
When you get an incoming message, it appears at the top of your screen as a notification banner. Pull down to reply, and when you’re done, go right back to hunting for animated eggs (or whatever else you were doing).
Swipe up on the notification to get rid of it. Or, just ignore it—it'll disappear on its own in a few seconds.
If you don’t get to a message in time, swipe down from the top of your screen to go to action center and see it—along with your other unread messages. Tap the arrow to see the whole message and reply right from action center.