Question

FloatingActionButton（下文以 fab 代替）是 android support design 组件库中提供的一个视图控件，是 material design 设计中 fab 的官方实现。

此控件的官方介绍如下：

Floating action buttons are used for a promoted action. They are distinguished by a circled icon floating above the UI and have motion behaviors that include morphing, launching, and a transferring anchor point.

关于该控件的设计规范及使用场景请参考文档： http://www.google.com/design/spec/components/buttons-floating-action-button.html#

如果你还不了解 design 组件库，请参考官方博客： http://android-developers.blogspot.hk/2015/05/android-design-support-library.html