Android lets you swipe-away any unwanted app notification from the Notification shade, but once you dismissed a notification, there is no way to undo it. However, the new API introduced with Android Oreo allows third-party apps to do that. For example, the app called ‘Unnotification’ takes the advantage of the new API and allow undo a rejected notification on Oreo. This can be very useful, if you have accidently deleted an important message from the Notification shade.
Of course, Android has a feature to view old and dismissed notifications. It is called notification log and can be accessed via system settings menu. However, some OEMs like Samsung have disabled this feature on their device, so users cannot view rejected or old notification using the notification log feature on these devices. However, app like ‘Unnotification’ can tackle this issue.
Unnotification is a free download on Play store. The app detects swiped-away notifications. When you dismissed a notification, the app displays a new notification with three options: Undo, Ignore, and Notification log. Each option is self-explanatory.
The app also offers few customizations like auto-dismiss timeout, blacklist apps, etc. to meet your requirements.
The app is completely free, but has an in-app purchase option for those who want to donate the app developer.
If you wonder how to use Android’s Notification Log feature to view old notifications without a third-party app, here is how it is. All you need to create a shortcut for the Notification Log. To do that, open Widget > Settings > Notification Log. Just tap on the Notification Log widget to view you old notification.