At long last, iOS is allowing app makers to implement text replies in notifications, a feature Apple used to reserve exclusively for its Messages app.
Twitter was the first App Store app to gain this great time-saver of a feature. On iOS 9.1, WhatsApp for iPhone now supports a handy text field for quickly replying to incoming messages without leaving the app you're currently using.
This feature requires an iPhone with iOS 9.1 or later.