如何防止在背面片段导航上再次设置视图模型

Question

我正在使用Android Jetpack Navigation，MVVM和dataBinding，我遇到了一个问题。每当我从详细信息片段导航回来时，我的主要List片段都会刷新（再次调用API，再次绘制recyclerList）。我怎么能阻止这个？

这是Main List Fragment代码：

class PostListFragment : BaseFragment() {

private lateinit var viewModel: PostListViewModel

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {

    val binding = TestFragmentBinding.inflate(inflater, container, false)
    val factory = InjectorUtils.providePostListViewModelFactory()
    viewModel = ViewModelProviders.of(this, factory).get(PostListViewModel::class.java)

    val adapter = MainListAdapter()
    binding.mainList.adapter = adapter

    binding.let {
        it.viewmodel = viewModel
        it.setLifecycleOwner(this@PostListFragment)
    }

    subscribeUi(adapter)
    return binding.root
}

private fun subscribeUi(adapter: MainListAdapter) {

    viewModel.progressVisible.postValue(true)
    addToDisposable(viewModel.getContentRepository().getAllPosts(requireContext())
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe({ result ->
            viewModel.progressVisible.postValue(false)
            adapter.submitList(result)
        }, { error ->
            viewModel.progressVisible.postValue(false)
        }))
}

}

另外我要补充一点，我在MainFragment上使用这个包含viewPager的片段。

我的MainFragment的代码：

class MainFragment : Fragment() {

private lateinit var sharedViewModel: SharedNewPostViewModel
private lateinit var viewModel: MainViewModel

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {

    val binding = MainFragmentBinding.inflate(inflater, container, false)

    val factory = InjectorUtils.provideMainViewModelFactory()
    viewModel = ViewModelProviders.of(this, factory).get(MainViewModel::class.java)

    binding.let {
        it.viewModel = viewModel
        it.setLifecycleOwner(this@MainFragment)
    }

    sharedViewModel = activity?.run {
        ViewModelProviders.of(this).get(SharedNewPostViewModel::class.java)
    } ?: throw Exception("Invalid activity")

    return binding.root
}

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

    val pagerAdapter = MainPagerAdapter(childFragmentManager)

    viewPager.adapter = pagerAdapter
    tabLayout.setupWithViewPager(viewPager)
}

}

Answer 1

需要检查savedInstanceState中的Activity.onCreate()：

if(savedInstanceState == null) {
    ...
}

当它不是null时，不要给Fragment充气 - 后来不会再召唤Fragment.onCreateView()两次（否则很难从那里返回View）。