观察者不会在 LiveData 更改时被调用

Question

我有一个狗的列表，当我点击列表中的特定狗时，我可以看到详细信息。但是，出于某种原因，文本视图（dogName、dogPurpose、dogLifespan 等）没有更新。详细信息片段仅显示 XML 布局给出的默认值。可能是什么问题呢？

class DetailFragment : Fragment() {

    private var dogUuid = 0
    private lateinit var viewModel: DetailViewModel

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

        arguments?.let {
            dogUuid = DetailFragmentArgs.fromBundle(it).dogUuid
        }

        viewModel = ViewModelProviders.of(this).get(DetailViewModel::class.java)
        viewModel.fetch(dogUuid)

        observeViewModel()
    }

    private fun observeViewModel(){
        viewModel.dogLiveData.observe(this, Observer{ dog ->
            dog?.let {
                dogName.text = dog.dogBreed
                dogPurpose.text = dog.bredFor
                dogTemperament.text = dog.temperament
                dogLifespan.text = dog.lifeSpan
                context?.let {dogImage.loadImage(dog.imageUrl, getProgressDrawable(it))}
            }
        })
    }
}

这是 DetailViewModel。

class DetailViewModel(application: Application): BaseViewModel(application) {
    val dogLiveData = MutableLiveData<DogBreed>()

    fun fetch(uuid: Int){
        launch {
            val dog = DogDatabase(getApplication()).dogDao().getDog(uuid)
            dogLiveData.value = dog
        }
    }
}

这是我对协程的实现。

abstract class BaseViewModel(application: Application): AndroidViewModel(application), CoroutineScope {

    private val job = Job()

    override val coroutineContext: CoroutineContext
        get() = job + Dispatchers.Main

    override fun onCleared() {
        super.onCleared()
        job.cancel()
    }
}

似乎从未调用过 observeViewModel() 中的 Observer。为什么？

Answer 1

你能确认线路吗

dogLiveData.value = dog

正在用断点调用吗？

试试打电话

viewModel.fetch(dogUuid)

之后

observeViewModel()

Answer 2

在viewModel.fetch(dogUuid)

之前添加observeViewModel()

class DetailFragment : Fragment() {

    private var dogUuid = 0
    private lateinit var viewModel: DetailViewModel

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

        arguments?.let {
            dogUuid = DetailFragmentArgs.fromBundle(it).dogUuid
        }

        viewModel = ViewModelProviders.of(this).get(DetailViewModel::class.java)

 observeViewModel()

 viewModel.fetch(dogUuid)


    }

    private fun observeViewModel(){
        viewModel.dogLiveData.observe(this, Observer{ dog ->
            dog?.let {
                dogName.text = dog.dogBreed
                dogPurpose.text = dog.bredFor
                dogTemperament.text = dog.temperament
                dogLifespan.text = dog.lifeSpan
                context?.let {dogImage.loadImage(dog.imageUrl, getProgressDrawable(it))}
            }
        })
    }
}

Answer 3

问题是我忘记将参数 uuid 传递给 DetailFragment，这导致 dog 对象在 DetailViewModel.fetch(uuid) 中为 null。因此，上面的代码是正确的，问题出在其他地方。

观察者不会在 LiveData 更改时被调用

Observer is not called on LiveData change

android

mvvm

kotlin

android-livedata

android-jetpack