Activity 生命周期

onStart(),onStop用来保存状态等值

An activity is a single, focused thing that the user can do. Almost all activities interact with the user, so
the Activity class takes care of creating a window for you in which you can place your UI with

setContentView(View)

.

Activity其实不是显示视图用的，它是一个控制单元，负责与用户进行交互，负责创建Window,然后通过setContentView将UI放置到Window窗口中。