class MainActivity : AppCompatActivity() {
    lateinit var viewModel: ListViewModel
    private val countriesAdapter = CountryListAdapter(arrayListOf())
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        viewModel = ViewModelProviders.of(this).get(ListViewModel::class.java)
        viewModel.refresh()
        countriesList.apply {
            layoutManager = LinearLayoutManager(context)
            adapter = countriesAdapter
        }
        observeViewModel()
    }
    ...
}

class ListViewModel: ViewModel() {
    private val countriesService = CountriesService.getCountriesService()
    var job: Job? = null
    private val exceptionHandler = CoroutineExceptionHandler{ coroutineContext, throwable ->
        onError("Exception: ${throwable.localizedMessage}")
    }
    // 生命周期感知型組件 MutableLiveData，可以做到在組件處于激活狀態(tài)的時(shí)候才會(huì)回調(diào)相應(yīng)的方法，從而刷新相應(yīng)的 UI
    val countries = MutableLiveData<List<Country>>()
    val countryLoadError = MutableLiveData<String?>()
    val loading = MutableLiveData<Boolean>()
    fun refresh() {
        fetchCountries()
    }
    private fun fetchCountries() {
        loading.value = true
        // 通過(guò)launch啟動(dòng)一個(gè)攜程回返回一個(gè)Job類型的對(duì)象實(shí)例，我們可以通過(guò)job.start()來(lái)啟動(dòng)攜程（如果launch(start = CoroutineStart.LAZY)
        // 這么設(shè)置的話），可以通過(guò)job.cancel來(lái)取消攜程
        job = CoroutineScope(Dispatchers.IO + exceptionHandler).launch {
            val response : Response<List<Country>> = countriesService.getCountries()
            // after we get the response, we hope that we could switch back to main thread and display on screen.
            withContext(Dispatchers.Main) {
                if (response.isSuccessful){
                    countries.value = response.body()
                    countryLoadError.value = null
                    loading.value = false
                } else
                {
                    onError("Error: ${response.message()}")
                }
            }
        }
    }
    private fun onError(message: String) {
        countryLoadError.value = message
        loading.value = false
    }
    override fun onCleared() {
        super.onCleared()
        job?.cancel()
    }
}

class LoginViewModelFactory(
    private  val repository: RegisterRepository,
    private val application: Application
): ViewModelProvider.Factory{
    @Suppress("Unchecked_cast")
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if(modelClass.isAssignableFrom(LoginViewModel::class.java)) {
            return LoginViewModel(repository, application) as T
        }
        throw IllegalArgumentException("Unknown View Model Class")
    }
}

class LoginViewModel(private val repository: RegisterRepository, application: Application) :
    AndroidViewModel(application), Observable {
    val users = repository.users
    @Bindable
    val inputUsername = MutableLiveData<String>()
    @Bindable
    val inputPassword = MutableLiveData<String>()
    private val viewModelJob = Job()
    private val uiScope = CoroutineScope(Dispatchers.Main + viewModelJob)
...
}

class LoginFragment : Fragment() {
    private lateinit var loginViewModel: LoginViewModel
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val binding: FragmentLoginBinding = DataBindingUtil.inflate(
            inflater,
            R.layout.fragment_login, container, false
        )
        val application = requireNotNull(this.activity).application
        val dao = RegisterDatabase.getInstance(application).registerDatabaseDao
        val repository = RegisterRepository(dao)
        val factory = LoginViewModelFactory(repository, application)
        loginViewModel = ViewModelProvider(this, factory).get(LoginViewModel::class.java)
        binding.myLoginViewModel = loginViewModel
        binding.lifecycleOwner = this
        loginViewModel.navigatetoRegister.observe(this, Observer { hasFinished->
            if (hasFinished == true){
                Log.i("MYTAG","insidi observe")
                displayUsersList()
                loginViewModel.doneNavigatingRegiter()
            }
        })
        ...
    }
}

class LoginViewModelFactory(
    private  val repository: RegisterRepository,
    private val application: Application
): ViewModelProvider.Factory{
    @Suppress("Unchecked_cast")
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if(modelClass.isAssignableFrom(LoginViewModel::class.java)) {
            return LoginViewModel(repository, application) as T
        }
        throw IllegalArgumentException("Unknown View Model Class")
    }
}

val factory = LoginViewModelFactory(repository, application)
loginViewModel = ViewModelProvider(this, factory).get(LoginViewModel::class.java)