Per the new Android Material Design checklist:
http://android-developers.blogspot.com/2014/10/material-design-on-android-checklist.html
Where appropriate, upon scrolling down, the app bar can scroll off the screen, leaving more vertical space for content. Upon scrolling back up, the app bar should be shown again.
I realize there are lots of questions/answers to implement this behavior for the ICS-style app bar, but I'm curious if the new Toolbar widget or Lollipop/AppCompat 21 have introduced a more standard way of achieving this effect.
Per Roman Nurik on Google+ (essentially what @tyczj mentioned)
https://plus.google.com/u/0/+RomanNurik/posts/3G8zYvN5oRC
It's not built in but relatively simple to implement. Just observe scrolling and animate the Y translation. Could also use the "quick return" sample +Nick Butcher and I published a few years ago
I believe that example referenced is: https://code.google.com/p/romannurik-code/source/browse/misc/scrolltricks
UPDATE: I also just discovered a great library that does this, along with some other great tricks https://github.com/ksoichiro/Android-ObservableScrollView
来源：https://stackoverflow.com/questions/26616427/material-design-checklist-hiding-app-bar-on-scroll