AndroidStudio里的解释是这样子的：
androidx.fragment.app.FragmentStatePagerAdapter public FragmentStatePagerAdapter(@NonNull androidx.fragment.app.FragmentManager fm, int behavior)
Constructor for FragmentStatePagerAdapter. If BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT is passed in, then only the current Fragment is in the Lifecycle.State.RESUMED state, while all other fragments are capped at Lifecycle.State.STARTED. If BEHAVIOR_SET_USER_VISIBLE_HINT is passed, all fragments are in the Lifecycle.State.RESUMED state and there will be callbacks to Fragment.setUserVisibleHint(boolean).
Params:
fm – fragment manager that will interact with this adapter
behavior – determines if only current fragments are in a resumed state
之前了解到Fragment的好处是只会加载当前页面的前后一页，节约了资源
现在看来，又有了优化哈，上面这段的大致意思应该是，如果int behavior设置为BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT ，则只加载当前这一张到某个状态中，叫做懒加载？如果设置为BEHAVIOR_SET_USER_VISIBLE_HINT ，就全部加载进去~
哎，也不知道是不是这样，好像没啥哈