ViewPager2+Fragment+FragmentStateAdapter遇到系统主题更换时Fragment数据丢失

1. 问题描述：

　　在ViewPager 设置壁纸，导致Activity 获取Fragment 数据丢失　

2. 解决方案：　

　　设置 vp.isSaveEnabled = false ，设置不保存，在适配器中销毁item

　　引发问题：vp 重建之后，会丢失之前所在的位置

　　解决方案：Activity onSaveInstanceState 中保存数据，在 onRestoreInstanceState 中恢复状态

3. 代码实现：

在Activity ：    
    vp.isSaveEnabled = false
 
 
    override fun onRestoreInstanceState(savedInstanceState: Bundle) {
        super.onRestoreInstanceState(savedInstanceState)
        mStep = savedInstanceState?.getInt("mStep", mStep)
        if (mStep == STEP2) {
            step(STEP2)
        } else if (mStep == STEP3) {
            step(STEP3)
        }
        LogUtils.i("onRestoreInstanceState mStep=$mStep")
    }
 
    override fun onSaveInstanceState(outState: Bundle) {
        outState.putInt("mStep", mStep)
        super.onSaveInstanceState(outState)
    }
 
适配器：
/**
 * 通用Fragment 适配器
 */
class SimpleFragmentStateAdapter(activity: FragmentManager, val fragments: List<Fragment>) :
    FragmentStatePagerAdapter(activity) {
 
    override fun getCount(): Int {
        return fragments.size
    }
 
    override fun getItem(position: Int): Fragment {
        return fragments[position]
    }
 
    override fun instantiateItem(container: ViewGroup, position: Int): Any {
        val mFragments = javaClass.superclass?.getDeclaredField("mFragments")
        mFragments?.isAccessible = true
        (mFragments?.get(this) as ArrayList<*>?)?.clear()
 
        val mSavedState = javaClass.superclass?.getDeclaredField("mSavedState")
        mSavedState?.isAccessible = true
        (mSavedState?.get(this) as ArrayList<*>?)?.clear()
        return super.instantiateItem(container, position)
    }
 
}