问RecyclerView与StaggeredGridLayoutManager在ViewPager中，返回到片段时自动排列项目

EN

class TestFragment : Fragment(R.layout.fragment_test) {

    private val testListAdapter: TestListAdapter by lazy {
        TestListAdapter()
    }

    private var layoutManagerState: Parcelable? = null

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        postListView.apply {
            layoutManager = StaggeredGridLayoutManager(
                2, StaggeredGridLayoutManager.VERTICAL
            ).apply {
                gapStrategy = StaggeredGridLayoutManager.GAP_HANDLING_MOVE_ITEMS_BETWEEN_SPANS
            }
            setHasFixedSize(true)

            adapter = testListAdapter
        }

        testListAdapter.stateRestorationPolicy = RecyclerView.Adapter.StateRestorationPolicy.PREVENT

    }

    override fun onPause() {
        saveLayoutManagerState()
        super.onPause()
    }

    override fun onViewStateRestored(savedInstanceState: Bundle?) {
        super.onViewStateRestored(savedInstanceState)
        restoreLayoutManagerState()
    }

    private fun restoreLayoutManagerState () {
        layoutManagerState?.let { postListView.layoutManager?.onRestoreInstanceState(it) }
    }

    private fun saveLayoutManagerState () {
        layoutManagerState = postListView.layoutManager?.onSaveInstanceState()
    }
}

- If you are using viewModel, you can use [SaveState](https://developer.android.com/topic/libraries/architecture/viewmodel-savedstate). Basically, it can save the data from your fragment, it is like a map data structure, so you can save positions from your list or recycler view. When go back to this fragment, get the position from this saveState object and use `scrollToPosition` method in order to add the real position.
- Recycler view have methods for restore positions. You can see the uses cases for that, because first you need the data and then add the real position, for more details you can visit [this link](https://www.facebook.com/HailAndroidDevs/posts/222285346153647). This configuration for recycler view is useful also when you lose memory and you need to recreate the recycler view with asynchronous data.