Apple

I am considering switching to an iPhone but I need a clarify a very specific point. On my old Android phone the icon badges display an unread count. On the new Android phones the OS has changed and the badges are now a notification count. The difference is that the notification count clears when you open the messages app and not when you read the individual message. This means you can now have unread messages on your phone but no reminder to deal with them.

How do the badges work on the iPhone? If I go into messages and leave 2 unread will the number 2 be left on the app icon to remind me or will it vanish like on the latest Android? Thanks.