屏幕旋转时的 ViewModel 更新

Question

我创建了一个简单的项目来研究 Kotlin 和 Android 架构

屏幕由RecyclerView和三个EditText组成。

相应的 ViewModel 公开了 7 个 LiveData：

过滤器对应的三个LiveData
通知用户未找到条目的事件
通知用户没有条目存在的事件
SwipeRefreshLayout 的状态
根据过滤器输入显示的连接列表

当用户在过滤器中键入文本时，ViewModel 的 LiveData 会收到有关更改的通知并更新数据。我读过将 MutableLiveData 公开给 Activities/Fragments 是一种不好的做法，但他们必须以某种方式通知 ViewModel 有关更改的信息。当根据用户的输入没有找到任何条目时，将显示 Toast。

问题

当用户输入没有匹配项的过滤器值时，将显示 Toast。如果用户随后旋转设备，Toast 会一次又一次地显示。

我读过这些文章：

https://medium.com/androiddevelopers/livedata-with-snackbar-navigation-and-other-events-the-singleliveevent-case-ac2622673150

https://proandroiddev.com/livedata-with-single-events-2395dea972a8

但我不明白如何将这些应用到我的用例中。我认为问题在于我如何执行更新

private val connections = connectionRepository.allConnections()
private val mediatorConnection = MediatorLiveData<List<Connection>>().also {
    it.value = connections.value
}

private val refreshLiveData = MutableLiveData(RefreshStatus.READY)
private val noMatchesEvent = SingleLiveEvent<Void>()
private val emptyTableEvent = SingleLiveEvent<Void>()

val nameLiveData = MutableLiveData<String>()
val urlLiveData = MutableLiveData<String>()
val actualStatusLiveData = MutableLiveData<String>()

init {
    with(mediatorConnection) {
        addSource(connections) { update() }
        addSource(nameLiveData) { update() }
        addSource(urlLiveData) { update() }
        addSource(actualStatusLiveData) { update() }
    }
}

fun getRefreshLiveData(): LiveData<RefreshStatus> = refreshLiveData
fun getNoMatchesEvent(): LiveData<Void> = noMatchesEvent
fun getEmptyTableEvent(): LiveData<Void> = emptyTableEvent
fun getConnections(): LiveData<List<Connection>> = mediatorConnection

private fun update() {
    if (connections.value.isNullOrEmpty()) {
        emptyTableEvent.call()
    } else {
        mediatorConnection.value = connections.value?.filter { connection -> getPredicate().test(connection) }

        if (mediatorConnection.value.isNullOrEmpty()) {
            noMatchesEvent.call()
        }
    }
}

update() 由于对 mediatorConnection 的新订阅和 MediatorLiveData.onActive() 被调用而在屏幕旋转时被触发。这是有意的行为

显示toast的代码

package com.github.aoreshin.shtatus.fragments

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.EditText
import android.widget.Toast
import androidx.core.widget.addTextChangedListener
import androidx.fragment.app.Fragment
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProvider
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView
import androidx.swiperefreshlayout.widget.SwipeRefreshLayout
import com.github.aoreshin.shtatus.R
import com.github.aoreshin.shtatus.ShatusApplication
import com.github.aoreshin.shtatus.viewmodels.ConnectionListViewModel
import javax.inject.Inject

class ConnectionListFragment : Fragment() {
    @Inject
    lateinit var viewModelFactory: ViewModelProvider.Factory

    private lateinit var refreshLayout: SwipeRefreshLayout

    private lateinit var nameEt: EditText
    private lateinit var urlEt: EditText
    private lateinit var statusCodeEt: EditText

    private lateinit var viewModel: ConnectionListViewModel

    private lateinit var recyclerView: RecyclerView
    private lateinit var viewAdapter: ConnectionListAdapter

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val view = inflater.inflate(R.layout.fragment_connection_list, container, false)

        val application = (requireActivity().application as ShatusApplication)
        application.appComponent.inject(this)

        val viewModelProvider = ViewModelProvider(this, viewModelFactory)
        viewModel = viewModelProvider.get(ConnectionListViewModel::class.java)

        bindViews(view)
        setupObservers()
        setupListeners()
        addFilterValues()
        setupRecyclerView()
        return view
    }

    private fun setupObservers() {
        with(viewModel) {
            getConnections().observe(viewLifecycleOwner, Observer { viewAdapter.submitList(it) })

            getRefreshLiveData().observe(viewLifecycleOwner, Observer { status ->
                when (status) {
                    ConnectionListViewModel.RefreshStatus.LOADING -> refreshLayout.isRefreshing = true
                    ConnectionListViewModel.RefreshStatus.READY -> refreshLayout.isRefreshing = false
                    else -> throwException(status.toString())
                }
            })

            getNoMatchesEvent().observe(viewLifecycleOwner, Observer { showToast(R.string.status_no_matches) })
            getEmptyTableEvent().observe(viewLifecycleOwner, Observer { showToast(R.string.status_no_connections) })
        }
    }

    private fun setupRecyclerView() {
        viewAdapter = ConnectionListAdapter(parentFragmentManager, ConnectionItemCallback())
        recyclerView.apply {
            layoutManager = LinearLayoutManager(context)
            adapter = viewAdapter
        }
    }

    private fun addFilterValues() {
        with(viewModel) {
            nameEt.setText(nameLiveData.value)
            urlEt.setText(urlLiveData.value)
            statusCodeEt.setText(actualStatusLiveData.value)
        }
    }

    private fun bindViews(view: View) {
        with(view) {
            recyclerView = findViewById(R.id.recycler_view)
            refreshLayout = findViewById(R.id.refresher)
            nameEt = findViewById(R.id.nameEt)
            urlEt = findViewById(R.id.urlEt)
            statusCodeEt = findViewById(R.id.statusCodeEt)
        }
    }

    private fun setupListeners() {
        with(viewModel) {
            refreshLayout.setOnRefreshListener { send() }
            nameEt.addTextChangedListener { nameLiveData.value = it.toString() }
            urlEt.addTextChangedListener { urlLiveData.value = it.toString() }
            statusCodeEt.addTextChangedListener { actualStatusLiveData.value = it.toString() }
        }
    }

    private fun throwException(status: String) {
        throw IllegalStateException(getString(R.string.error_no_such_status) + status)
    }

    private fun showToast(resourceId: Int) {
        Toast.makeText(context, getString(resourceId), Toast.LENGTH_SHORT).show()
    }

    override fun onDestroyView() {
        super.onDestroyView()
        with(viewModel) {
            getNoMatchesEvent().removeObservers(viewLifecycleOwner)
            getRefreshLiveData().removeObservers(viewLifecycleOwner)
            getEmptyTableEvent().removeObservers(viewLifecycleOwner)
            getConnections().removeObservers(viewLifecycleOwner)
        }
    }
}

我该如何解决这个问题？

Answer 1

经过一番摸索之后，我决定使用内部 ViewModel 状态，这样 Activity/Fragment 中的逻辑将保持在最低限度。

所以现在我的 ViewModel 看起来像这样：

package com.github.aoreshin.shtatus.viewmodels

import android.util.Log
import androidx.lifecycle.LiveData
import androidx.lifecycle.MediatorLiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import com.github.aoreshin.shtatus.events.SingleLiveEvent
import com.github.aoreshin.shtatus.room.Connection
import io.reactivex.FlowableSubscriber
import io.reactivex.Single
import io.reactivex.android.schedulers.AndroidSchedulers
import io.reactivex.schedulers.Schedulers
import io.reactivex.subscribers.DisposableSubscriber
import okhttp3.ResponseBody
import retrofit2.Response
import java.util.function.Predicate
import javax.inject.Inject
import javax.inject.Singleton

@Singleton
class ConnectionListViewModel @Inject constructor(
    private val connectionRepository: ConnectionRepository
) : ViewModel() {
    private var tableStatus = TableStatus.OK

    private val connections = connectionRepository.allConnections()
    private val mediatorConnection = MediatorLiveData<List<Connection>>()

    private val stopRefreshingEvent = SingleLiveEvent<Void>()
    private val noMatchesEvent = SingleLiveEvent<Void>()
    private val emptyTableEvent = SingleLiveEvent<Void>()

    private val nameLiveData = MutableLiveData<String>()
    private val urlLiveData = MutableLiveData<String>()
    private val statusLiveData = MutableLiveData<String>()

    init {
        with(mediatorConnection) {
            addSource(connections) { update() }
            addSource(nameLiveData) { update() }
            addSource(urlLiveData) { update() }
            addSource(statusLiveData) { update() }
        }
    }

    fun getStopRefreshingEvent(): LiveData<Void> = stopRefreshingEvent
    fun getNoMatchesEvent(): LiveData<Void> = noMatchesEvent
    fun getEmptyTableEvent(): LiveData<Void> = emptyTableEvent
    fun getConnections(): LiveData<List<Connection>> = mediatorConnection
    fun getName(): String? = nameLiveData.value
    fun getUrl(): String? = urlLiveData.value
    fun getStatus(): String? = statusLiveData.value
    fun setName(name: String) { nameLiveData.value = name }
    fun setUrl(url: String) { urlLiveData.value = url }
    fun setStatus(status: String) { statusLiveData.value = status }

    private fun update() {
        if (connections.value != null) {
            if (connections.value.isNullOrEmpty()) {
                if (tableStatus != TableStatus.EMPTY) {
                    emptyTableEvent.call()
                    tableStatus = TableStatus.EMPTY
                }
            } else {
                mediatorConnection.value = connections.value?.filter { connection -> getPredicate().test(connection) }

                if (mediatorConnection.value.isNullOrEmpty()) {
                    if (tableStatus != TableStatus.NO_MATCHES) {
                        noMatchesEvent.call()
                        tableStatus = TableStatus.NO_MATCHES
                    }
                } else {
                    tableStatus = TableStatus.OK
                }
            }
        }
    }

    fun send() {
        if (!connections.value.isNullOrEmpty()) {
            val singles = connections.value?.map { connection ->
                val id = connection.id
                val description = connection.description
                val url = connection.url
                var message = ""

                connectionRepository.sendRequest(url)
                    .doOnSuccess { message = it.code().toString() }
                    .doOnError { message = it.message!! }
                    .doFinally {
                        val result = Connection(id, description, url, message)
                        connectionRepository.insert(result)
                    }
            }

            Single.mergeDelayError(singles)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .doFinally { stopRefreshingEvent.call() }
                .subscribe(getSubscriber())
        } else {
            stopRefreshingEvent.call()
        }
    }

    private fun getSubscriber() : FlowableSubscriber<Response<ResponseBody>> {
        return object: DisposableSubscriber<Response<ResponseBody>>() {
            override fun onComplete() { Log.d(TAG, "All requests sent") }
            override fun onNext(t: Response<ResponseBody>?) { Log.d(TAG, "Request is done") }
            override fun onError(t: Throwable?) { Log.d(TAG, t!!.message!!) }
        }
    }

    private fun getPredicate(): Predicate<Connection> {
        return Predicate { connection ->
            connection.description.contains(nameLiveData.value.toString(), ignoreCase = true)
                    && connection.url.contains(urlLiveData.value.toString(), ignoreCase = true)
                    && connection.actualStatusCode.contains(
                statusLiveData.value.toString(),
                ignoreCase = true
            )
        }
    }

    private enum class TableStatus {
        NO_MATCHES,
        EMPTY,
        OK
    }

    companion object {
        private const val TAG = "ConnectionListViewModel"
    }
}

相应的 Fragment 如下所示：

package com.github.aoreshin.shtatus.fragments

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.EditText
import android.widget.Toast
import androidx.core.widget.addTextChangedListener
import androidx.fragment.app.Fragment
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProvider
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView
import androidx.swiperefreshlayout.widget.SwipeRefreshLayout
import com.github.aoreshin.shtatus.R
import com.github.aoreshin.shtatus.ShatusApplication
import com.github.aoreshin.shtatus.viewmodels.ConnectionListViewModel
import javax.inject.Inject

class ConnectionListFragment : Fragment() {
    @Inject
    lateinit var viewModelFactory: ViewModelProvider.Factory

    private lateinit var refreshLayout: SwipeRefreshLayout

    private lateinit var nameEt: EditText
    private lateinit var urlEt: EditText
    private lateinit var statusCodeEt: EditText

    private lateinit var viewModel: ConnectionListViewModel

    private lateinit var recyclerView: RecyclerView
    private lateinit var viewAdapter: ConnectionListAdapter

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val view = inflater.inflate(R.layout.fragment_connection_list, container, false)

        val application = (requireActivity().application as ShatusApplication)
        application.appComponent.inject(this)

        val viewModelProvider = ViewModelProvider(this, viewModelFactory)
        viewModel = viewModelProvider.get(ConnectionListViewModel::class.java)

        bindViews(view)
        setupObservers()
        setupListeners()
        addFilterValues()
        setupRecyclerView()
        return view
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        if (savedInstanceState != null) {
            refreshLayout.isRefreshing = savedInstanceState.getBoolean(REFRESHING, false)
        }
    }

    private fun setupObservers() {
        with(viewModel) {
            getConnections().observe(viewLifecycleOwner, Observer { viewAdapter.submitList(it) })
            getStopRefreshingEvent().observe(viewLifecycleOwner, Observer { refreshLayout.isRefreshing = false })
            getNoMatchesEvent().observe(viewLifecycleOwner, Observer { showToast(R.string.status_no_matches) })
            getEmptyTableEvent().observe(viewLifecycleOwner, Observer { showToast(R.string.status_no_connections) })
        }
    }

    private fun setupRecyclerView() {
        viewAdapter = ConnectionListAdapter(parentFragmentManager, ConnectionItemCallback())
        recyclerView.apply {
            layoutManager = LinearLayoutManager(context)
            adapter = viewAdapter
        }
    }

    private fun addFilterValues() {
        with(viewModel) {
            nameEt.setText(getName())
            urlEt.setText(getUrl())
            statusCodeEt.setText(getStatus())
        }
    }

    private fun bindViews(view: View) {
        with(view) {
            recyclerView = findViewById(R.id.recycler_view)
            refreshLayout = findViewById(R.id.refresher)
            nameEt = findViewById(R.id.nameEt)
            urlEt = findViewById(R.id.urlEt)
            statusCodeEt = findViewById(R.id.statusCodeEt)
        }
    }

    private fun setupListeners() {
        with(viewModel) {
            refreshLayout.setOnRefreshListener { send() }
            nameEt.addTextChangedListener { setName(it.toString()) }
            urlEt.addTextChangedListener { setUrl(it.toString()) }
            statusCodeEt.addTextChangedListener { setStatus(it.toString()) }
        }
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        outState.putBoolean(REFRESHING, refreshLayout.isRefreshing)
    }

    private fun showToast(resourceId: Int) {
        Toast.makeText(context, getString(resourceId), Toast.LENGTH_SHORT).show()
    }

    override fun onDestroyView() {
        super.onDestroyView()
        with(viewModel) {
            getNoMatchesEvent().removeObservers(viewLifecycleOwner)
            getEmptyTableEvent().removeObservers(viewLifecycleOwner)
            getStopRefreshingEvent().removeObservers(viewLifecycleOwner)
            getConnections().removeObservers(viewLifecycleOwner)
        }
    }

    companion object {
        private const val REFRESHING = "isRefreshing"
    }
}

优点

没有额外的依赖项
广泛使用 SingleLiveEvent
实施起来非常简单

缺点

即使在这种简单的情况下，条件逻辑也会很快失控，肯定需要重构。不确定这种方法是否适用于现实生活中的复杂场景。

如果有更简洁明了的方法来解决这个问题，我会很高兴听到它们！

Answer 2

在你的解决方案中；您引入了 TableStatus，它的作用类似于标志，不需要。

如果您真的在寻找好的Android架构；相反你可以做

if (viewLifecycleOwner.lifecycle.currentState == Lifecycle.State.RESUMED) {
            showToast(R.string.status_no_connections)

和

if (viewLifecycleOwner.lifecycle.currentState == Lifecycle.State.RESUMED) {
            showToast(R.string.status_no_matches)

注意：
viewLifecycleOwner.lifecycle.currentState == Lifecycle.State.RESUMED 不是补丁 Google 在支持库中也实现了此修复。

并从中删除@Singleton（为什么需要它是单例）

@Singleton
class ConnectionListViewModel @Inject constructor(

PS:
从我的头顶看起来像；你可能也不需要 SingleLiveEvent 你的情况。
（如果你愿意，我很乐意就此进行更多讨论，我也刚刚开始使用 Kotlin + Clear & Scalable Android 架构）

屏幕旋转时的 ViewModel 更新

ViewModel updates on screen rotation

android

kotlin

android-recyclerview

android-livedata

android-architecture-components