Android Kotlin 观察者未检测到 LiveData 更改

Question

我有一个简单的 Android Kotlin 应用程序，我正在使用它涉及与一个 viewModel 通信的多个片段。 问题是我的第一个片段中的 LiveData 观察器不会在我视图 model 中的列表每次更改时更新。任何人都可以解释我可能哪里出错了吗？

这是我的片段：

class ShoeDetailsFragment : Fragment() {

lateinit var binding: FragmentShoeDetailsBinding
private val viewModel: ShoeViewModel by activityViewModels()

override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                          savedInstanceState: Bundle?): View {
    // Inflate the layout for this fragment
    binding = DataBindingUtil.inflate(inflater, R.layout.fragment_shoe_details, container, false)

    setClickListeners()
    watchForChanges()
    return binding.root
}

private fun watchForChanges(){
    viewModel.shoeList.observe(viewLifecycleOwner) { list ->
        Log.i("Contents of list: ", list.toString())
        binding.viewModelTestEditText.setText(list.toString())
    }
}

private fun createShoeFromInputs(): Shoe {
    val shoeNameString = binding.shoeNameEditText.text.toString()
    val shoeColourString = binding.shoeColourEditText.text.toString()
    val shoeMakerString = binding.shoeMakerEditText.text.toString()
    val shoeSizeString = binding.shoeSizeEditText.text.toString()

    return Shoe(shoeNameString, shoeColourString, shoeMakerString, shoeSizeString)
}



private fun setClickListeners(){
    binding.saveButtonShoeDetails.setOnClickListener{
        saveShoeToList(createShoeFromInputs())
    }

    binding.cancelButtonShoeDetails.setOnClickListener{
        viewModel.removeShoeFromShoeList()
    }
}

private fun saveShoeToList(shoe: Shoe){
    if (validateFields()){
        viewModel.addShoeToShoeList(shoe)
    }
}

private fun validateFields(): Boolean{
    return if (binding.shoeNameEditText.text.isEmpty()
        || binding.shoeColourEditText.text.isEmpty()
        || binding.shoeMakerEditText.text.isEmpty()
        || binding.shoeSizeEditText.text.isEmpty()){
        Toast.makeText(requireContext(), "Please complete all fields", Toast.LENGTH_SHORT).show()
        false
    } else {
        true
    }
}
}

这是我的视图模型：

class ShoeViewModel: ViewModel() {

private val _shoeList = MutableLiveData<MutableList<Shoe>>()
val shoeList: LiveData<MutableList<Shoe>> get () =_shoeList

init {
    _shoeList.value = mutableListOf()
}

fun addShoeToShoeList(shoe: Shoe){
    _shoeList.value!!.add(shoe)
    Log.i("Contents of list in view model: ", _shoeList.value!!.size.toString())
}

fun removeShoeFromShoeList(){
    _shoeList.value!!.removeAt(0)
    Log.i("Contents of list in view model after cancel: ", _shoeList.value!!.size.toString())
}

}

我一遍又一遍地检查了代码，但一定有我遗漏的东西

Answer 1

您没有更改 LiveData 的value 。 它仍然指向 MutableList 的同一个实例。 你修改了那个 MutableList 的内容，但是 LiveData 对你所做的一无所知，所以它不会通知观察者。

我强烈建议您仅将只读列表与 LiveData 一起使用。 您无需改变列表，而是创建一个新列表并将其设置为 LiveData 的新value 。

class ShoeViewModel: ViewModel() {

    private val _shoeList = MutableLiveData<List<Shoe>>()
    val shoeList: LiveData<List<Shoe>> get () =_shoeList
    
    init {
        _shoeList.value = emptyList()
    }
    
    fun addShoeToShoeList(shoe: Shoe){
        _shoeList.value = _shoeList.value.orEmpty() + shoe
        Log.i("Contents of list in view model: ", _shoeList.value.orEmpty().size.toString())
    }
    
    fun removeShoeFromShoeList(){
        _shoeList.value = _shoeList.value.orEmpty().drop(1)
        Log.i("Contents of list in view model after cancel: ", _shoeList.value.orEmpty().size.toString())
    }

}

请注意，可以使用 MutableList，然后在每次改变列表后调用liveData.value = liveData.value以触发它以通知观察者。 我建议您不要这样做的原因是某些视图类（尤其是 RecyclerView 的 ListAdapter）是“智能的”，它们会比较新旧数据以确定它们是否确实需要显示任何更改。 如果新旧数据都是 MutableList 的同一个实例，它不会检测到任何更改，因此 UI 不会更新。

Answer 2

您需要调用 MuableLiveData.setValue() 或 MutableLiveData.postValue() 才能发出事件。

尝试：

fun addShoeToShoeList(shoe: Shoe){
    val currentList = _shoeList.value ?: mutableListOf()
    currentList.add(shoe)
    _shoeList.value = Collections.copy(currentList)
    Log.i("Contents of list in view model: ", _shoeList.value!!.size.toString())
}

fun removeShoeFromShoeList(){
    val currentList = _shoeList.value ?: mutableListOf()
    currentList.removeAt(0)
    _shoeList.value=currentList
    Log.i("Contents of list in view model after cancel: ", _shoeList.value!!.size.toString())
}