android - 从 ViewModel 中的服务器获取数据后，LiveData Observer 不会更新 UI

Question

我正在尝试使用 retrofit get 请求从服务器获取数据，在 ViewModel 中，我发起请求，请求的 OnResponse 方法显示数据已成功从服务器检索，但 fragment 中的观察者没有获取更新。我正在分享下面的代码。

改造 API 调用

fun getGenres(application: Context,callback:(List<Genre>)->Unit){
        val baseURL = "https://listen-api.listennotes.com/"
        var genreList: MutableLiveData<List<Genre>> =MutableLiveData()
        val logging = HttpLoggingInterceptor()
//      set your desired log level
        logging.level = HttpLoggingInterceptor.Level.BODY

        val httpClient = OkHttpClient.Builder()
//      add your other interceptors …
//      add logging as last interceptor
        httpClient.addInterceptor(logging)

        val gson = GsonBuilder()
            .setLenient()
            .create()

        val retrofit = Retrofit.Builder()
            .baseUrl(baseURL)
            .addConverterFactory(GsonConverterFactory.create(gson))
            .client(httpClient.build())
            .build()

        val api: ApiFunctions = retrofit.create(ApiFunctions::class.java)



        val call: Call<PodcastGetCategoryResponse> = api.getGenres()

        call.enqueue(object : Callback<PodcastGetCategoryResponse> {
            override fun onFailure(call: Call<PodcastGetCategoryResponse>, t: Throwable) {

                Toast.makeText(application, "failure" + t.stackTrace, Toast.LENGTH_SHORT).show()

            }

            override fun onResponse(
                call: Call<PodcastGetCategoryResponse>,
                response: Response<PodcastGetCategoryResponse>
            ) {


                response.body().apply {
                    callback(this?.genres!!)                    }
                Toast.makeText(application, "success: ${response.body()?.genres?.size}", Toast.LENGTH_SHORT).show()

            }

        })

        return genreList;
    }

这会成功运行并从服务器检索流派列表。

View 模型类

class CategoryViewModel(application: Application) : AndroidViewModel(application) {
private val rootJob = SupervisorJob()

private val coroutineContext: CoroutineContext
    get() = Dispatchers.Main + rootJob

 var listOfCategory: MutableLiveData<List<Genre>> = MutableLiveData()

init {
    getistOfCategory()
}

// this method is responsible for retrieving data from the result of the API call method using a coroutine.

 fun getistOfCategory() {
    CoroutineScope(coroutineContext).launch() {
        ApiFunctions.getGenres(getApplication<Application>(), { l -> listOfCategory.value = l } )
         listOfCategory.postValue(listOfCategory.value)
    }
  }

我认为问题出在这个类上，但我想不通。

** fragment 类**

class MainFragment : Fragment() {
private lateinit var viewModel: CategoryViewModel
private lateinit var binding: FragmentMainBinding

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    viewModel = ViewModelProviders.of(this).get(CategoryViewModel::class.java)
    viewModel.getistOfCategory()

}

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

    binding =DataBindingUtil.inflate(inflater, R.layout.fragment_main, container, false)
    var myView: View = binding.root




    viewModel.listOfCategory.observe(this,
        Observer { list ->
            Toast.makeText(
                context,
                "Data Changed" + list?.size,
                Toast.LENGTH_LONG
            ).show()
        })

    return myView
}

我想要的是，当从服务器检索到新值时，它应该保存在 ViewModel 类的 LiveData 中， fragment 应该成功地观察到它们。

Answer 1

根据您当前的实现，listOfCategory 的值会随着每次更新而不断变化。这意味着当您的列表更新时，观察者不会收到通知，因为他们正在观察另一个 LiveData 对象(在更新发生之前已分配给您的引用)您需要确保仅实例化 liveData 对象一次，并在获得更新时更新其值。

你的代码应该是这样的

fun getGenres(application: Context, callback:(List<Genre>)->Unit){
...
     response.body().apply {
            callback(this?.genres!!)
     }
...
}



fun getistOfCategory(): {
  CoroutineScope(coroutineContext).launch() {
      ApiFunctions.getGenres(getApplication<Application>(), l -> listOfCategory.value = l)
    }

}