如何在從片段 B 切換到片段 A 時恢復 ViewPager state

Question

在我的場景中，我在片段 A 中有 viewpager，在 viewpage 中有片段 C 和片段 D。現在我移動到片段 B 並返回片段 A 並得到這個崩潰 java.lang.IllegalStateException：指定的孩子已經有一個父母。 您必須首先在孩子的父母上調用 removeView()。

adapter=ViewPagerAdapter(childFragmentManager)
  class ViewPagerAdapter internal constructor(fm: FragmentManager) :  FragmentStatePagerAdapter(fm,BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {

        private val COUNT = 3

        override fun getItem(position: Int): Fragment {
            var fragment: Fragment? = null
            when (position) {
                0 -> fragment = AdditionInfo()
                1 -> fragment = UploadFiles()
                2 -> fragment = UploadDocs()
            }

            return fragment!!
        }

        override fun getCount(): Int {
            return COUNT
        }

        override fun getPageTitle(position: Int): CharSequence? {
            return "Tab " + (position + 1)
        }

        override fun getItemPosition(`object`: Any): Int {
            return POSITION_NONE
        }
    }
     fun viewpagerListener() {
        viewOfLayout!!.upload_info_doc_viewpager?.setAllowedSwipeDirection(SwipeDirection.left)
        viewOfLayout!!.upload_info_doc_viewpager?.offscreenPageLimit = 3
        viewOfLayout!!.upload_info_doc_viewpager?.setSaveFromParentEnabled(true)
        viewOfLayout!!.upload_info_doc_viewpager?.addOnPageChangeListener(object : ViewPager.OnPageChangeListener {

            override fun onPageScrollStateChanged(state: Int) {
            }

            override fun onPageScrolled(position: Int, positionOffset: Float, positionOffsetPixels: Int) {

                currentPage = position

            }

            override fun onPageSelected(position: Int) {

            }


        })
    }

When i get back to Fragment A i do this 
 override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                              savedInstanceState: Bundle?): View? {
 if(viewOfLayout==null){
// my code
viewOfLayout = inflater.inflate(R.layout.layout_file, container, false)
 adapter=ViewPagerAdapter(childFragmentManager)

}
 return viewOfLayout
}
when onCreateView method called and viewOfLayout is already in memory but its called viewpager fragments automatically.```

Answer 1

只保留不顯示的片段A。

mViewPager.setOffscreenPageLimit(number);

number 變量是 viewpager 中的片段數。