LiveData observeForever 不工作

Question

我有一个 WeatherRepository class，它调用 WeatherProvider class 开始获取天气。

成功获取天气后，我只是 post 使用 postValue 函数的天气，但是 WeatherRepository class 的 init 块中实时数据的观察者永远不会被调用.

我很困惑，我错过了什么...

任何见解都将非常有帮助。

这是我的存储库和提供程序代码：

class WeatherRepository @Inject constructor(private var weatherDao: WeatherDao, private var weatherProvider: WeatherProvider) {

    private fun startFetchWeatherService() {
        weatherProvider.startFetchWeatherService()
    }

    init {
        // Control flow always gets to this point
        var weather = weatherProvider.getDownloadedWeather()

        weather.observeForever { // This observer never gets called
            if (it != null) AsyncTask.execute { insertWeather(it) }

        }
        if (isFetchNeeded()) {
            startFetchWeatherService() // Android Studio always execute this line since no data is inserted by observer and fetch is needed
        }
    }
  ....
}


class WeatherProvider(private val context: Context) {
    private val mDownloadedWeather = MutableLiveData<List<Weather>>()
    ...

    fun getDownloadedWeather(): MutableLiveData<List<Weather>> = mDownloadedWeather

    fun getFromInternet() {
        ...
        call.enqueue(object : Callback<WorldWeatherOnline> {
          override fun onFailure(call: Call<WorldWeatherOnline>?, t: Throwable?) {} // TODO show error
          override fun onResponse(call: Call<WorldWeatherOnline>?, response: Response<WorldWeatherOnline>?) {
                if (response != null) {
                    val weather = response.body()?.data
                    if (weather != null) {
                      mDownloadedWeather.postValue(WeatherUtils.extractValues(weather)) // app always gets to this point and WeatherUtils successfully returns the List of weathers full of data
                    }
                }
            }
        })
    }

    fun startFetchWeatherService() {
        val intentToFetch = Intent(context, WeatherSyncIntentService::class.java)
        context.startService(intentToFetch)
    }
 }
    ...

// Dependency injection always works
// Here's my dagger2 module (other modules are very simillar to this one)
@Module
class ApplicationModule(private val weatherApplication: WeatherApplication) {
    @Provides
    internal fun provideWeatherApplication(): WeatherApplication {
        return weatherApplication
    }

    @Provides
    internal fun provideApplication(): Application {
        return weatherApplication
    }

    @Provides
    @Singleton
    internal fun provideWeatherProvider(context: WeatherApplication):   WeatherProvider {
        return WeatherProvider(context)
    }
}

@Singleton
class CustomViewModelFactory constructor(private val weatherRepository: WeatherRepository, private val checklistRepository: ChecklistRepository) : ViewModelProvider.Factory {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        when {
            modelClass.isAssignableFrom(WeatherViewModel::class.java) ->
                return WeatherViewModel(weatherRepository) as T
            modelClass.isAssignableFrom(ChecklistViewModel::class.java) ->
                return ChecklistViewModel(checklistRepository) as T
            else ->
                throw IllegalArgumentException("ViewModel Not Found")
        }
    }
}

class WeatherFragment : Fragment() {
    private lateinit var mWeatherModel: WeatherViewModel
    @Inject
    internal lateinit var viewModelFactory: ViewModelProvider.Factory

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

    mWeatherModel = ViewModelProviders.of(this, viewModelFactory)
            .get(WeatherViewModel::class.java)
...
    }
}

Answer 1

尝试使用

mDownloadedWeather.setValue(WeatherUtils.extractValues(weather))

而不是

mDownloadedWeather.postValue(WeatherUtils.extractValues(weather))

因为postValue() 将任务发布到主线程以设置给定值。因此，如果您在主线程中执行了以下代码：

liveData.postValue("a");
liveData.setValue("b");

值 "b" 将首先设置，稍后主线程将用值 "a" 覆盖它。

如果在主线程执行发布任务之前多次调用此方法，则只会调度最后一个值。

Answer 2

没有必要将您的 postValue 更改为 setValue，因为它是在同一个线程中完成的。这里真正的问题是应该如何设置 Dagger2。

在WeatherFragment.kt中使用

internal lateinit var viewModelFactory: CustomViewModelFactory

而不是

internal lateinit var viewModelFactory: ViewModelProvider.Factory

还需要在你的CustomViewModelFactory.kt的构造函数中添加@Inject 注解。

class CustomViewModelFactory @Inject constructor(

最后，根据您提供的代码，您的 WeatherProvider.kt 根本没有处于 初始化状态 。您可以使用此代码执行此操作：

    init {
        getFromInternet()
    }

LiveData observeForever 不工作

LiveData observeForever not working

android

kotlin

dagger-2

android-livedata