The Cydia Tweak WAQuickReply has been officially released and works with devices that have iOS 8 or higher. WAQuickReply cydia tweak adds a quick reply feature to WhatsApp that works exactly like the feature found on the Messages app. When you receive a message, it is actionable via banner notifications or Lock screen notifications just like a typical iMessage or SMS message is.
WAQuickReply works by using a special backgrounding, which is able to keep WhatsApp backgrounded without causing much drain on battery life. Even after you reboot, you can still receive messages and engage in quick reply without launching the WhatsApp app. After you have installed this tweak, you will be able to configure it to your own personal preferences in the Settings app.
If you are interested, then you can download WAQuickReply iOS 8 from the BigBoss repository in Cydia just $2.99. To install this program requires a jailbroken device (see if and how you can do the jailbreak to your iPhone by clicking here) and you need an Internet connection to download the package. This tweak is compatible with iOS 8.0 and later versions of the mobile operating system from Apple.