Android:SaveState、Fragments 和 ViewModel:我做错了什么？

Question

按照 Google 的建议，我有一个顶部有多个 fragment 的单一 Activity 。在一个 fragment 中，我希望放置一个开关，并且当我从其他 fragment 返回时，我希望仍然知道它的状态。示例:我在 fragment 一中，然后我打开开关，导航到 fragment 二或 fragment 三，返回到 fragment 一，我希望在离开时将开关置于打开位置加载该 fragment 。

我曾尝试复制google advocates提供的示例，只是为了看到代码失败而什么都不做。

/////////////////////////////////////////////////////////////////
//Inside the first fragment:
class myFragment : Fragment() {

    companion object {
        fun newInstance() = myFragment()
    }

    private lateinit var viewModel: myViewModel

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.my_fragment, container, false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)

        **viewModel = ViewModelProvider(this, SavedStateVMFactory(this)).get(myViewModel::class.java)

        //Here I was hoping to read the state when I come back.
        switch_on_off.isChecked = viewModel.getSwRoundTimerInit()**

        subscribeToLiveData() //To read liveData

        switch_on_off.setOnCheckedChangeListener { _, isChecked ->
            viewModel.setOnOff(isChecked)
        }
}//End of onActivityCreated
//other code...

/////////////////////////////////////////////////////////////////////////
//On the fragment ViewModel

class myViewModel(private val **mState: SavedStateHandle**) : ViewModel() {

    //SavedStateHandle Keys to save and restore states in the App
    private val swStateKey = "SW_STATE_KEY"

    private var otherSwitch:Boolean //other internal states.

    //Init for the other internal states
    init {      
        otherSwitch = false
    }

    fun getSwRoundTimerInit():Boolean{
        val state = mState[swStateKey] ?: "false"
        return state.toBoolean()
    }

    fun setOnOff(swValue:Boolean){
        mState.set(swStateKey, swValue.toString())
    }

}

这是行不通的。它始终加载默认(关闭)值，就好像 savedState 始终为 null。

Answer 1

改变

//fragment scope
viewModel = ViewModelProvider(thisSavedStateVMFactory(this)).get(myViewModel::class.java) 

到

//activity scope
viewModel = activity?.let { ViewModelProviders.of(it,SavedStateVMFactory(this)).get(myViewModel::class.java) }

https://developer.android.com/topic/libraries/architecture/viewmodel#sharing