如何 Return 来自存储库的 LiveData

Question

我只是看不出如何将 LiveData 从 Repo 链接到 VM，所以我试图将其归结为最简单的示例！：

片段

class LoginFragment : Fragment() {

private lateinit var loginViewModel: LoginViewModel
private var mCurrentName = "Blank!"

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

    binding.apply {
        loginButton.setOnClickListener{
            loginViewModel.changeText()
        }
    }
    return binding.root
}


override fun onActivityCreated(savedInstanceState: Bundle?) {
    super.onActivityCreated(savedInstanceState)
    loginViewModel = ViewModelProviders.of(this).get(LoginViewModel::class.java)

    loginViewModel.getCurrentName().observe(viewLifecycleOwner, Observer {
        mCurrentName = it      // I'm expecting mCurrentName to equal "Button Clicked!" when button clicked..
        makeToast()     // Toast works, but variable remains unchanged..
    })

}

private fun makeToast() {
    Toast.makeText(activity, mCurrentName, Toast.LENGTH_LONG).show()
}

ViewModel

class LoginViewModel : ViewModel() {

private val firestoreRepository : FirestoreRepository = FirestoreRepository()
private var mCurrentName = MutableLiveData<String>()

fun changeText(){
    mCurrentName = firestoreRepository.changeText()
}

存储库

class FirestoreRepository {

    private val mCurrentName = MutableLiveData<String>()

    fun changeText(): MutableLiveData<String> {
            mCurrentName.value = "Button Clicked!!"
            return mCurrentName
    }

我假设我误解了观察者函数的工作原理。

Answer 1

开始观察后，您正在更改 mCurrentName（LiveData 变量本身而不是它的内容）。指导似乎是不要在存储库层中使用 LiveData（例如，使用 Coroutines/Flow）......但是现在你可以有类似的东西（或者可能使用 LiveData 转换之一）

private val mCurrentName = firestoreRepository.currentName()

fun changeText(){
    firestoreRepository.changeText()
}

Answer 2

无需更改 ViewModel 内的 MutableLiveData。尝试将 Repository 发送给 View 的任何内容传递给 View。检查下面

class LoginViewModel : ViewModel() {

    private val firestoreRepository : FirestoreRepository = FirestoreRepository()

    fun getCurrentName(): MutableLiveData<String> {
        return firestoreRepository.getCurrentName()
    }

    fun changeText() {
        firestoreRepository.changeText()
    }
}

还有你的FirestoreRepository

class FirestoreRepository {

    private val mCurrentName = MutableLiveData<String>()

    fun getCurrentName(): MutableLiveData<String> {
        return mCurrentName
    }

    fun changeText() {
        mCurrentName.value = "Button Clicked!!"
    }
}

Answer 3

实际上，如果您在存储库中维护 LiveData，我认为 ViewModel 中也不需要单独的 LiveData。您只需要从 activity 观察一次 LiveData。然后直接对存储库实例进行任何更改。所以如果我必须在你的代码中显示它，它可能看起来像这样。

Activity class：将 makeToast 方法更改为 observeCurrentName()，如下所示：

private fun observeCurrentName() {
    vm.getCurrentName().observe(this, Observer{ 
        //Toast here 
    })
}

您的虚拟机：

class LoginViewModel : ViewModel() {
    ...

    fun getCurrentName(): MutableLiveData<String>{
        return repository.getCurrentName()
    }

    fun setCurrentName(name: String?){
        repository.setCurrentName(name)
    }

    ...
}

您的存储库：

class FirestoreRepository {

    private val mCurrentName = MutableLiveData<String>()

    fun getCurrentName(): MutableLiveData<String>{
        return mCurrentName
    }

    fun setCurrentName(name: String?){
        mCurrentName.value = name //This will trigger observer in Activity
    }
}

如何 Return 来自存储库的 LiveData

How to Return LiveData from Repository

android

repository

mvvm

kotlin

android-livedata