无法记录来自 API 调用的任何数据

Question

我对kotlin很陌生，我觉得它让我不知所措。我想问-我如何能显示任何数据从mutableelvedata ?我试着记录它，但它似乎不起作用。我已经把上网权限加到舱单上了。下面是代码:

ApiServices

interface ApiServices {
@GET("/fixer/latest/")
fun getRatesData(
@Query("base") base: String,
@Query("apikey") apikey: String
): Call<CurrencyModel>
companion object {
private const val url = "https://api.apilayer.com/"
var apiServices: ApiServices? = null
fun getInstance(): ApiServices {
if (apiServices == null) {
val retrofit = Retrofit.Builder()
.baseUrl(url)
.addConverterFactory(GsonConverterFactory.create())
.build()
apiServices = retrofit.create(ApiServices::class.java)
}
return apiServices!!
}
}
}

库

class CurrencyRepository constructor(private val apiServices: ApiServices) {
fun getLatestRates() = apiServices.getRatesData("EUR", "API_KEY");
}

ViewModel

class CurrencyViewModel constructor(private val currencyRepository: CurrencyRepository) :
ViewModel() {
val currencyRatesList = MutableLiveData<CurrencyModel>()
val errorMessage = MutableLiveData<String>()
fun getLatestRates() {
val response = currencyRepository.getLatestRates();
response.enqueue(object : retrofit2.Callback<CurrencyModel> {
override fun onResponse(
call: retrofit2.Call<CurrencyModel>,
response: Response<CurrencyModel>
) {
currencyRatesList.postValue(response.body())
}
override fun onFailure(call: retrofit2.Call<CurrencyModel>, t: Throwable) {
errorMessage.postValue(t.message)
}
})
}
}

FactoryViewModel

class CurrencyViewModelFactory constructor(private val repository: CurrencyRepository) :
ViewModelProvider.Factory {
override fun <T : ViewModel?> create(modelClass: Class<T>): T {
return if (modelClass.isAssignableFrom(CurrencyViewModel::class.java)) {
CurrencyViewModel(this.repository) as T
}else{
throw IllegalArgumentException("Couldn't found ViewModel")
}
}
}

MainActivity

class MainActivity : AppCompatActivity() {
private val retrofitService = ApiServices.getInstance()
lateinit var viewModel: CurrencyViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
viewModel = ViewModelProvider(this, CurrencyViewModelFactory(CurrencyRepository(retrofitService)))
.get(CurrencyViewModel::class.java)
viewModel.currencyRatesList.observe(this, Observer {
Log.d(TAG, "onCreate: $it")
})
viewModel.errorMessage.observe(this, Observer {
viewModel.getLatestRates()
})
}
}

Answer 1

您永远不会在onCreate()中调用viewModel.getLatestRates()来获取LiveData的初始值，因此它永远不会发出任何要观察的东西。你唯一调用它的地方是在你的错误监听器中，它不会被调用，直到fetch返回一个错误。

旁注，我建议将函数命名为"fetchLatestRates()"。按照惯例，"get";意味着函数会立即返回它得到的结果，而不是等到LiveData准备好后再传递给它。

还有小费。而不是:

class MainActivity : AppCompatActivity() {
lateinit var viewModel: CurrencyViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
// ...
viewModel = ViewModelProvider(this, CurrencyViewModelFactory(CurrencyRepository(retrofitService)))
.get(CurrencyViewModel::class.java)
//...
}
}

你可以这样做得到相同的结果:

class MainActivity : AppCompatActivity() {
val viewModel: CurrencyViewModel by viewModels(CurrencyViewModelFactory(CurrencyRepository(retrofitService)))
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
// ...
}
}