当从片段中单击“后退”时，应用程序崩溃/与观察者的导航问题

Question

我有一个片段a，它向网络发送一个搜索查询，如果结果是肯定的，使用Android导航组件导航到片段B，并使用观察者完成。

在导航到片段B之后，我点击屏幕顶部的“<-”箭头，但是它没有返回到片段A，而是再次重新加载片段B。如果使用设备上的本机“后退”按钮，应用程序将崩溃，出现“IllegalArgumentException导航目的地未知”错误。

private val getAssetResult = MutableLiveData<GeneralResponse<Asset>>()
private val updateAssetResult = MutableLiveData<GeneralResponse<Int>>()
private val deleteAssetResult = MutableLiveData<GeneralResponse<Int>>()

init {
    state.value = ViewState(false)
    Log.d(TAG, "State in init: $state")
}

fun getAssetResult(): LiveData<GeneralResponse<Asset>>{

    return getAssetResult
}

fun findAsset(req: GetAssetRequest) {

    scope.launch {
        setProgressIndicator(true)
        val result = repository.getAsset(req)

        getAssetResult.postValue(result)

        setProgressIndicator(false)
    }
}

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

    viewModel = ViewModelProvider(this).get(EditAssetViewModel::class.java)
    setupViewModel()
    initFields()
}


private fun setupViewModel() {

    if (viewModel.getAssetResult().hasObservers()) // <- This is the part that prevents the app from crashing.
        return

    viewModel.getAssetResult().observe(this, Observer {
        if (it == null) return@Observer

        handleSearchResult(it)
    })

    if (viewModel.getState().hasObservers())
        return

    viewModel.getState().observe(this, Observer { handleState(it) })
}

private fun handleSearchResult(response: GeneralResponse<Asset>) {

    if (response.singleValue == null) {
        Toast.makeText(context!!, response.errorMessage, Toast.LENGTH_SHORT).show()
        return
    }

    targetFragment?.let { it ->

        val bundle = bundleOf("asset" to response.singleValue)

        when(it) {
            "UpdateLocation" ->
                Navigation.findNavController(view!!).navigate(R.id.updateLocation, bundle)
            "EditAsset" -> {
                Navigation.findNavController(view!!).navigate(R.id.editAsset, bundle)
            }
        }
    }
}

Answer 1

重写方法onbackpressed()以处理“<-”箭头