基于 kotlin版本的 Android的MVI架构

从双向绑定到单向数据流

何为MVI？

MVI即Model-View-Intent，它受Cycle.js前端框架的启发，提倡一种单向数据流的设计思想，非常适合数据驱动型的UI展示项目：

Model : 与其他MVVM中的Model不同的是，MVI的Model主要指UI状态（State）。

当前界面展示的内容无非就是UI状态的一个快照：例如数据加载过程、控件位置等都是一种UI状态

View : 与其他MVX中的View一致，可能是一个Activity、Fragment或者任意UI承载单元。

MVI中的View通过订阅Intent的变化实现界面刷新（不是Activity的Intent、后面介绍）

Intent: 此Intent不是Activity的Intent，用户的任何操作都被包装成Intent后发送给Model进行数据请求

单向数据流

用户操作以Intent的形式通知Model => Model基于Intent更新State => View接收到State变化刷新UI。

数据永远在一个环形结构中单向流动，不能反向流动：

这种单向数据流结构的MVI有什么优缺点呢？

优点

UI的所有变化来自State，所以只需聚焦State，架构更简单、易于调试

数据单向流动，很容易对状态变化进行跟踪和回溯

state实例都是不可变的，确保线程安全

UI只是反应State的变化，没有额外逻辑，可以被轻松替换或复用

缺点

所有的操作最终都会转换成State，所以当复杂页面的State容易膨胀

state是不变的，每当state需要更新时都要创建新对象替代老对象，这会带来一定内存开销

有些事件类的UI变化不适合用state描述，例如弹出一个toast或者snackbar

代码示例

// Added Dependencies
implementation "androidx.recyclerview:recyclerview:1.1.0"
implementation 'android.arch.lifecycle:extensions:1.1.1'
implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0'
implementation 'androidx.lifecycle:lifecycle-runtime-ktx:2.2.0'
implementation 'com.github.bumptech.glide:glide:4.11.0'

//retrofit
implementation 'com.squareup.retrofit2:retrofit:2.8.1'
implementation "com.squareup.retrofit2:converter-moshi:2.6.2"

//Coroutine
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.3.6"
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.6"

1. 数据层

定义User的data class

data class User(
    @Json(name = "id")
    val id: Int = 0,
    @Json(name = "first_name")
    val name: String = "",
    @Json(name = "email")
    val email: String = "",
    @Json(name = "avator")
    val avator: String = ""
)

ApiService

interface ApiService {

   @GET("users")
   suspend fun getUsers(): List<User>
}

Retrofit

object RetrofitBuilder {

    private const val BASE_URL = "https://reqres.in/api/user/1"

    private fun getRetrofit() = Retrofit.Builder()
        .baseUrl(BASE_URL)
        .addConverterFactory(MoshiConverterFactory.create())
        .build()


    val apiService: ApiService = getRetrofit().create(ApiService::class.java)

}

Repository

class MainRepository(private val apiService: ApiService) {

    suspend fun getUsers() = apiService.getUsers()

}

2. UI层

2.1 RecyclerView.Adapter

class MainAdapter(
    private val users: ArrayList<User>
) : RecyclerView.Adapter<MainAdapter.DataViewHolder>() {

    class DataViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        fun bind(user: User) {
            itemView.textViewUserName.text = user.name
            itemView.textViewUserEmail.text = user.email
            Glide.with(itemView.imageViewAvatar.context)
                .load(user.avatar)
                .into(itemView.imageViewAvatar)
        }
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) =
        DataViewHolder(
            LayoutInflater.from(parent.context).inflate(
                R.layout.item_layout, parent,
                false
            )
        )

    override fun getItemCount(): Int = users.size

    override fun onBindViewHolder(holder: DataViewHolder, position: Int) =
        holder.bind(users[position])

    fun addData(list: List<User>) {
        users.addAll(list)
    }

}

item_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="60dp">

    <ImageView
        android:id="@+id/imageViewAvatar"
        android:layout_width="60dp"
        android:layout_height="0dp"
        android:padding="4dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <androidx.appcompat.widget.AppCompatTextView
        android:id="@+id/textViewUserName"
        style="@style/TextAppearance.AppCompat.Large"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="4dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/imageViewAvatar"
        app:layout_constraintTop_toTopOf="parent"/>

    <androidx.appcompat.widget.AppCompatTextView
        android:id="@+id/textViewUserEmail"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="@+id/textViewUserName"
        app:layout_constraintTop_toBottomOf="@+id/textViewUserName" />

</androidx.constraintlayout.widget.ConstraintLayout>

2.2 Intent

定义Intent用来包装用户Action

sealed class MainIntent {

    object FetchUser : MainIntent()

}

2.3 State

定义UI层的State结构体

sealed class MainState {

    object Idle : MainState()
    object Loading : MainState()
    data class Users(val user: List<User>) : MainState()
    data class Error(val error: String?) : MainState()

}

2.4 ViewModel

ViewModel是MVI的核心，存放和管理State，同时接受Intent并进行数据请求

class MainViewModel(
    private val repository: MainRepository
) : ViewModel() {

    val userIntent = Channel<MainIntent>(Channel.UNLIMITED)
    private val _state = MutableStateFlow<MainState>(MainState.Idle)
    val state: StateFlow<MainState>
        get() = _state

    init {
        handleIntent()
    }

    private fun handleIntent() {
        viewModelScope.launch {
            userIntent.consumeAsFlow().collect {
                when (it) {
                    is MainIntent.FetchUser -> fetchUser()
                }
            }
        }
    }

    private fun fetchUser() {
        viewModelScope.launch {
            _state.value = MainState.Loading
            _state.value = try {
                MainState.Users(repository.getUsers())
            } catch (e: Exception) {
                MainState.Error(e.localizedMessage)
            }
        }
    }
}

我们在handleIntent中订阅userIntent并根据Action类型执行相应操作。本case中当出现FetchUser的Action时，调用fetchUser方法请求用户数据。用户数据返回后，会更新State，MainActivity订阅此State并刷新界面。

2.5 ViewModelFactory

构造ViewModel需要Repository，所以通过ViewModelFactory注入必要的依赖

class ViewModelFactory(private val apiService: ApiService) : ViewModelProvider.Factory {

    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(MainViewModel::class.java)) {
            return MainViewModel(MainRepository(apiService)) as T
        }
        throw IllegalArgumentException("Unknown class name")
    }

}


class MainActivity : AppCompatActivity() {

    private lateinit var mainViewModel: MainViewModel
    private var adapter = MainAdapter(arrayListOf())

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setupUI()
        setupViewModel()
        observeViewModel()
        setupClicks()
    }

    private fun setupClicks() {
        buttonFetchUser.setOnClickListener {
            lifecycleScope.launch {
                mainViewModel.userIntent.send(MainIntent.FetchUser)
            }
        }
    }


    private fun setupUI() {
        recyclerView.layoutManager = LinearLayoutManager(this)
        recyclerView.run {
            addItemDecoration(
                DividerItemDecoration(
                    recyclerView.context,
                    (recyclerView.layoutManager as LinearLayoutManager).orientation
                )
            )
        }
        recyclerView.adapter = adapter
    }


    private fun setupViewModel() {
        mainViewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(
                    RetrofitBuilder.apiService
                )
            )
        ).get(MainViewModel::class.java)
    }

    private fun observeViewModel() {
        lifecycleScope.launch {
            mainViewModel.state.collect {
                when (it) {
                    is MainState.Idle -> {

                    }
                    is MainState.Loading -> {
                        buttonFetchUser.visibility = View.GONE
                        progressBar.visibility = View.VISIBLE
                    }

                    is MainState.Users -> {
                        progressBar.visibility = View.GONE
                        buttonFetchUser.visibility = View.GONE
                        renderList(it.user)
                    }
                    is MainState.Error -> {
                        progressBar.visibility = View.GONE
                        buttonFetchUser.visibility = View.VISIBLE
                        Toast.makeText(this@MainActivity, it.error, Toast.LENGTH_LONG).show()
                    }
                }
            }
        }
    }

    private fun renderList(users: List<User>) {
        recyclerView.visibility = View.VISIBLE
        users.let { listOfUsers -> listOfUsers.let { adapter.addData(it) } }
        adapter.notifyDataSetChanged()
    }
}

MainActivity中订阅mainViewModel.state，根据State处理各种UI显示和刷新。

activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".ui.main.view.MainActivity">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:visibility="gone" />

    <ProgressBar
        android:id="@+id/progressBar"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:visibility="gone"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/buttonFetchUser"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/fetch_user"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

MVI在MVVM的基础上，规定了数据的单向流动和状态的不可变性，这类似于前端的Redux思想，非常适合UI展示类的场景。MVVM也好，MVI也好都不是架构的最终形态，世界上没有完美的架构，要根据项目情况选择适合的架构进行开发。