[Android Kotlin-Flow]Kotlin-Flow如何在Android项目中使用

前言：

Flow是kotlin提供的一个重要的异步工具，类似于RxJava的响应式编程框架，使用协程封装成生产者-消费者模式，上流来负责生产，下流来接收消耗。

关于这个项目：

如果您想开始使用 Kotlin Flow，那么这个项目就是为您准备的。
本项目已实现 Android 中 Kotlin Flow 的常见用例。
这是您学习 Kotlin Flow for Android 开发的一站式解决方案。
通过该项目中的示例学习 Kotlin Flow 的步骤
- 首先，要学习 Kotlin Flow 的概念
- 了解 Kotlin Flow 中的终端运算符。
- 在 Kotlin 中使用 Flow Builder 创建流。
- Kotlin Flow 中的异常处理。
- Kotlin 中的 StateFlow API。
- 使用 Kotlin 流运算符实现即时搜索。
- 具有指数退避延迟的 Kotlin 流重试运算符。
- 用于并行多个网络调用的 Zip 运算符。
- 然后，只需克隆、构建、运行项目并开始通过示例学习 Kotlin Flow。

这个 Kotlin Flow 示例项目将帮助您学习以下 Android 应用程序开发：

什么是 Kotlin 流？
如何在 Android 中使用 Kotlin Flow？
关于如何在 Android 中实现 Kotlin 流程的分步指南？
在 Kotlin Flow 中完成简单的任务
使用 filter、map、reduce、flatMapConcat、zip 等运算符。
Kotlin Flow 中的异常
如何在 Flow 中使用 onCompletion？
在 Flow 中使用重试运算符重试任务
在流程中使用指数退避重试任务
将 Kotlin Flow 与 Retrofit 结合使用。
将 Kotlin Flow 与房间数据库结合使用。
使用 Kotlin Flow 并行进行两个网络调用。
使用 Kotlin Flow 连续执行任务。
Flow 中的单元测试。

Android 开发的 Kotlin 流程示例：

单一网络调用： 了解如何使用 Kotlin Flow 进行网络调用。这是 Android 应用程序开发中一个非常简单的用例。

活动代码

包 com.mindorks.kotlinFlow.learn.retrofit.single 

导入 android.os.Bundle
导入 android.view.View
导入 android.widget.Toast
导入 androidx.appcompat.app.AppCompatActivity
导入 androidx.lifecycle.Observer
导入 androidx.lifecycle.ViewModelProviders
导入 androidx.recyclerview.widget.DividerItemDecoration
导入 androidx.recyclerview.widget.LinearLayoutManager
导入 com.mindorks.kotlinFlow.R
导入 com.mindorks.kotlinFlow.data.api.ApiHelperImpl
导入 com.mindorks.kotlinFlow.data.api.RetrofitBuilder
导入 com.mindorks.kotlinFlow.data.local.DatabaseBuilder
导入 com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
导入 com.mindorks.kotlinFlow.data.model.ApiUser
导入 com.mindorks.kotlinFlow.learn.base.ApiUserAdapter
导入 com.mindorks.kotlinFlow.utils.Status
导入 com.mindorks.kotlinFlow.utils.ViewModelFactory
导入 kotlinx.android.synthetic.main.activity_recycler_view.*

类 SingleNetworkCallActivity : AppCompatActivity () {

    私有 lateinit  var viewModel ： SingleNetworkCallViewModel
    私有 lateinit  var适配器： ApiUserAdapter
    
    覆盖 fun  onCreate ( savedInstanceState :  Bundle ? ) {
        超级.onCreate(savedInstanceState)
        设置内容视图（R.layout.activity_recycler_view）
        设置界面（）
        设置视图模型（）
        设置观察者（）
    }
    
    私人 乐趣 setupUI () {
        recyclerView.layoutManager =  LinearLayoutManager (这个)
        适配器=
            ApiUserAdapter (
                数组列表()
            )
        recyclerView.addItemDecoration(
            分隔项装饰(
                recyclerView.context,
                (recyclerView.layoutManager作为 LinearLayoutManager ).orientation
            )
        )
        recyclerView.adapter =适配器
    }
    
    私人 乐趣 setupObserver () {
        viewModel.getUsers().observe(这个，观察者{
            当（it.status）{
                状态。成功 -> {
                    progressBar.visibility = 查看。消失了
                    it.data?. 让{ 用户->渲染列表（用户） }
                    recyclerView.visibility = 查看。可见的
                }
                状态。加载 -> {
                    progressBar.visibility = 查看。可见的
                    recyclerView.visibility = 查看。消失了
                }
                状态。错误 -> {
                    //处理错误
                    progressBar.visibility = 查看。消失了
                    Toast .makeText( this , it.message, Toast . LENGTH_SHORT ).show()
                }
            }
        })
    }
    
    私人 乐趣 渲染列表（用户：列表< ApiUser >）{ 
        适配器.addData（用户）
        适配器.notifyDataSetChanged()
    }
    
    私人 乐趣 setupViewModel () {
        viewModel =  ViewModelProviders .of (
            这个，
            视图模型工厂(
                ApiHelperImpl ( RetrofitBuilder .apiService),
                DatabaseHelperImpl ( DatabaseBuilder .getInstance(applicationContext))
            )
        ).get( SingleNetworkCallViewModel ::类.java)
    }

}

视图模型代码

package com.mindorks.kotlinFlow.learn.retrofit.single

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.data.model.ApiUser
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.InternalCoroutinesApi
import kotlinx.coroutines.flow.catch
import kotlinx.coroutines.flow.collect
import kotlinx.coroutines.launch

class SingleNetworkCallViewModel(
    private val apiHelper: ApiHelper,
    private val dbHelper: DatabaseHelper
) : ViewModel() {

    private val users = MutableLiveData>>()

    init {
        fetchUsers()
    }

    private fun fetchUsers() {
        viewModelScope.launch {
            users.postValue(Resource.loading(null))
            apiHelper.getUsers()
                .catch { e ->
                    users.postValue(Resource.error(e.toString(), null))
                }
                .collect {
                    users.postValue(Resource.success(it))
                }
        }
    }

    fun getUsers(): LiveData>> {
        return users
    }

}

系列网络调用： 了解如何使用 Kotlin Flow 进行系列网络调用。当您要进行依赖于另一个网络调用的网络调用时，这很有用。
活动代码

package com.mindorks.kotlinFlow.learn.retrofit.series

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import androidx.recyclerview.widget.DividerItemDecoration
import androidx.recyclerview.widget.LinearLayoutManager
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.data.model.ApiUser
import com.mindorks.kotlinFlow.learn.base.ApiUserAdapter
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_recycler_view.*

class SeriesNetworkCallsActivity : AppCompatActivity() {

    private lateinit var viewModel: SeriesNetworkCallsViewModel
    private lateinit var adapter: ApiUserAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_recycler_view)
        setupUI()
        setupViewModel()
        setupObserver()
    }

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

    private fun setupObserver() {
        viewModel.getUsers().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    it.data?.let { users -> renderList(users) }
                    recyclerView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    recyclerView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
    }

    private fun renderList(users: List) {
        adapter.addData(users)
        adapter.notifyDataSetChanged()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(SeriesNetworkCallsViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.retrofit.series

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.data.model.ApiUser
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.flow.catch
import kotlinx.coroutines.flow.collect
import kotlinx.coroutines.flow.flatMapConcat
import kotlinx.coroutines.flow.flowOn
import kotlinx.coroutines.launch

class SeriesNetworkCallsViewModel(
    private val apiHelper: ApiHelper,
    private val dbHelper: DatabaseHelper
) : ViewModel() {

    private val users = MutableLiveData>>()

    init {
        fetchUsers()
    }

    private fun fetchUsers() {
        viewModelScope.launch {
            users.postValue(Resource.loading(null))
            val allUsersFromApi = mutableListOf()
            apiHelper.getUsers()
                .flatMapConcat { usersFromApi ->
                    allUsersFromApi.addAll(usersFromApi)
                    apiHelper.getMoreUsers()
                }
                .flowOn(Dispatchers.Default)
                .catch { e ->
                    users.postValue(Resource.error(e.toString(), null))
                }
                .collect { moreUsersFromApi ->
                    allUsersFromApi.addAll(moreUsersFromApi)
                    users.postValue(Resource.success(allUsersFromApi))
                }
        }
    }

    fun getUsers(): LiveData>> {
        return users
    }

}

并行网络调用： 了解如何使用 Kotlin Flow 并行进行网络调用。当您要进行依赖于另一个网络调用的网络调用时，这很有用。
活动代码

package com.mindorks.kotlinFlow.learn.retrofit.parallel

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import androidx.recyclerview.widget.DividerItemDecoration
import androidx.recyclerview.widget.LinearLayoutManager
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.data.model.ApiUser
import com.mindorks.kotlinFlow.learn.base.ApiUserAdapter
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_recycler_view.*

class ParallelNetworkCallsActivity : AppCompatActivity() {

    private lateinit var viewModel: ParallelNetworkCallsViewModel
    private lateinit var adapter: ApiUserAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_recycler_view)
        setupUI()
        setupViewModel()
        setupObserver()
    }

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

    private fun setupObserver() {
        viewModel.getUsers().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    it.data?.let { users -> renderList(users) }
                    recyclerView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    recyclerView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
    }

    private fun renderList(users: List) {
        adapter.addData(users)
        adapter.notifyDataSetChanged()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(ParallelNetworkCallsViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.retrofit.parallel

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.data.model.ApiUser
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.flow.catch
import kotlinx.coroutines.flow.collect
import kotlinx.coroutines.flow.flowOn
import kotlinx.coroutines.flow.zip
import kotlinx.coroutines.launch

class ParallelNetworkCallsViewModel(
    private val apiHelper: ApiHelper,
    private val dbHelper: DatabaseHelper
) : ViewModel() {

    private val users = MutableLiveData>>()

    init {
        fetchUsers()
    }

    private fun fetchUsers() {
        viewModelScope.launch {
            users.postValue(Resource.loading(null))
            apiHelper.getUsers()
                .zip(apiHelper.getMoreUsers()) { usersFromApi, moreUsersFromApi ->
                    val allUsersFromApi = mutableListOf()
                    allUsersFromApi.addAll(usersFromApi)
                    allUsersFromApi.addAll(moreUsersFromApi)
                    return@zip allUsersFromApi
                }
                .flowOn(Dispatchers.Default)
                .catch { e ->
                    users.postValue(Resource.error(e.toString(), null))
                }
                .collect {
                    users.postValue(Resource.success(it))
                }
        }
    }

    fun getUsers(): LiveData>> {
        return users
    }

}

房间数据库操作：了解如何使用 Kotlin Flow 在数据库中获取或插入实体。当您在 Android 应用程序中使用房间数据库时，这很有用。
活动代码

package com.mindorks.kotlinFlow.learn.room

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import androidx.recyclerview.widget.DividerItemDecoration
import androidx.recyclerview.widget.LinearLayoutManager
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.data.local.entity.User
import com.mindorks.kotlinFlow.learn.base.UserAdapter
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_recycler_view.*

class RoomDBActivity : AppCompatActivity() {

    private lateinit var viewModel: RoomDBViewModel
    private lateinit var adapter: UserAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_recycler_view)
        setupUI()
        setupViewModel()
        setupObserver()
    }

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

    private fun setupObserver() {
        viewModel.getUsers().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    it.data?.let { users -> renderList(users) }
                    recyclerView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    recyclerView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
    }

    private fun renderList(users: List) {
        adapter.addData(users)
        adapter.notifyDataSetChanged()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(RoomDBViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.room

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.data.local.entity.User
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.launch

class RoomDBViewModel(private val apiHelper: ApiHelper, private val dbHelper: DatabaseHelper) :
    ViewModel() {

    private val users = MutableLiveData>>()

    init {
        fetchUsers()
    }

    private fun fetchUsers() {
        viewModelScope.launch {
            users.postValue(Resource.loading(null))
            dbHelper.getUsers()
                .flatMapConcat { usersFromDb ->
                    if (usersFromDb.isEmpty()) {
                        return@flatMapConcat apiHelper.getUsers()
                            .map { apiUserList ->
                                val userList = mutableListOf()
                                for (apiUser in apiUserList) {
                                    val user = User(
                                        apiUser.id,
                                        apiUser.name,
                                        apiUser.email,
                                        apiUser.avatar
                                    )
                                    userList.add(user)
                                }
                                userList
                            }
                            .flatMapConcat { usersToInsertInDB ->
                                dbHelper.insertAll(usersToInsertInDB)
                                    .flatMapConcat {
                                        flow {
                                            emit(usersToInsertInDB)
                                        }
                                    }
                            }
                    } else {
                        return@flatMapConcat flow {
                            emit(usersFromDb)
                        }
                    }
                }
                .flowOn(Dispatchers.Default)
                .catch { e ->
                    users.postValue(Resource.error(e.toString(), null))
                }
                .collect {
                    users.postValue(Resource.success(it))
                }
        }
    }

    fun getUsers(): LiveData>> {
        return users
    }

}

长时间运行的任务：了解如何使用 Kotlin Flow 执行长时间运行的任务。如果您想使用 Kotlin Flow 在后台线程中执行任何任务，那么这很有用。
活动代码

package com.mindorks.kotlinFlow.learn.task.onetask

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_long_running_task.*
import kotlinx.android.synthetic.main.activity_recycler_view.progressBar

class LongRunningTaskActivity : AppCompatActivity() {

    private lateinit var viewModel: LongRunningTaskViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_long_running_task)
        setupViewModel()
        setupLongRunningTask()
    }

    private fun setupLongRunningTask() {
        viewModel.getStatus().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    textView.text = it.data
                    textView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    textView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
        viewModel.startLongRunningTask()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(LongRunningTaskViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.task.onetask

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.launch

class LongRunningTaskViewModel(
    private val apiHelper: ApiHelper,
    private val dbHelper: DatabaseHelper
) : ViewModel() {

    private val status = MutableLiveData>()

    fun startLongRunningTask() {
        viewModelScope.launch {
            status.postValue(Resource.loading(null))
            // do a long running task
            doLongRunningTask()
                .flowOn(Dispatchers.Default)
                .catch {
                    status.postValue(Resource.error("Something Went Wrong", null))
                }
                .collect {
                    status.postValue(Resource.success("Task Completed"))
                }
        }
    }

    fun getStatus(): LiveData> {
        return status
    }

    private fun doLongRunningTask(): Flow {
        return flow {
            // your code for doing a long running task
            // Added delay to simulate
            delay(5000)
            emit(0)
        }
    }
}

两个长时间运行的任务：了解如何使用 Kotlin Flow 并行运行两个长时间运行的任务。
活动代码

package com.mindorks.kotlinFlow.learn.task.twotasks

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_long_running_task.*
import kotlinx.android.synthetic.main.activity_recycler_view.progressBar

class TwoLongRunningTasksActivity : AppCompatActivity() {

    private lateinit var viewModel: TwoLongRunningTasksViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_long_running_task)
        setupViewModel()
        setupLongRunningTask()
    }

    private fun setupLongRunningTask() {
        viewModel.getStatus().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    textView.text = it.data
                    textView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    textView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
        viewModel.startLongRunningTask()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(TwoLongRunningTasksViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.task.twotasks

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.launch

class TwoLongRunningTasksViewModel(
    private val apiHelper: ApiHelper,
    private val dbHelper: DatabaseHelper
) : ViewModel() {

    private val status = MutableLiveData>()

    fun startLongRunningTask() {
        viewModelScope.launch {
            status.postValue(Resource.loading(null))
            doLongRunningTaskOne()
                .zip(doLongRunningTaskTwo()) { resultOne, resultTwo ->
                    return@zip resultOne + resultTwo
                }
                .flowOn(Dispatchers.Default)
                .catch { e ->
                    status.postValue(Resource.error(e.toString(), null))
                }
                .collect {
                    status.postValue(Resource.success(it))
                }
        }
    }

    fun getStatus(): LiveData> {
        return status
    }

    private fun doLongRunningTaskTwo(): Flow {
        return flow {
            // your code for doing a long running task
            // Added delay to simulate
            delay(5000)
            emit("Two")
        }
    }

    private fun doLongRunningTaskOne(): Flow {
        return flow {
            // your code for doing a long running task
            // Added delay to simulate
            delay(5000)
            emit("One")
        }
    }

}

Catch 错误处理：了解如何使用 Catch 处理 Kotlin Flow 中的错误。
活动代码

package com.mindorks.kotlinFlow.learn.errorhandling.catch

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import androidx.recyclerview.widget.DividerItemDecoration
import androidx.recyclerview.widget.LinearLayoutManager
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.data.model.ApiUser
import com.mindorks.kotlinFlow.learn.base.ApiUserAdapter
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_recycler_view.*

class CatchActivity : AppCompatActivity() {

    private lateinit var viewModel: CatchViewModel
    private lateinit var adapter: ApiUserAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_recycler_view)
        setupUI()
        setupViewModel()
        setupObserver()
    }

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

    private fun setupObserver() {
        viewModel.getUsers().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    it.data?.let { users -> renderList(users) }
                    recyclerView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    recyclerView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
    }

    private fun renderList(users: List) {
        adapter.addData(users)
        adapter.notifyDataSetChanged()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(CatchViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.errorhandling.catch

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.data.model.ApiUser
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.flow.catch
import kotlinx.coroutines.flow.collect
import kotlinx.coroutines.launch

class CatchViewModel(
    private val apiHelper: ApiHelper,
    private val dbHelper: DatabaseHelper
) : ViewModel() {

    private val users = MutableLiveData>>()

    init {
        fetchUsers()
    }

    private fun fetchUsers() {
        viewModelScope.launch {
            users.postValue(Resource.loading(null))
            apiHelper.getUsersWithError()
                .catch { e ->
                    users.postValue(Resource.error(e.toString(), null))
                }
                .collect {
                    users.postValue(Resource.success(it))
                }
        }
    }

    fun getUsers(): LiveData>> {
        return users
    }

}

EmitAll 错误处理：了解如何使用 emitAll 处理 Kotlin Flow 中的错误。
活动代码

package com.mindorks.kotlinFlow.learn.errorhandling.emitall

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import androidx.recyclerview.widget.DividerItemDecoration
import androidx.recyclerview.widget.LinearLayoutManager
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.data.model.ApiUser
import com.mindorks.kotlinFlow.learn.base.ApiUserAdapter
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_recycler_view.*

class EmitAllActivity : AppCompatActivity() {

    private lateinit var viewModel: EmitAllViewModel
    private lateinit var adapter: ApiUserAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_recycler_view)
        setupUI()
        setupViewModel()
        setupObserver()
    }

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

    private fun setupObserver() {
        viewModel.getUsers().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    it.data?.let { users -> renderList(users) }
                    recyclerView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    recyclerView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
    }

    private fun renderList(users: List) {
        adapter.addData(users)
        adapter.notifyDataSetChanged()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(EmitAllViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.errorhandling.emitall

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.data.model.ApiUser
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.launch

class EmitAllViewModel(
    private val apiHelper: ApiHelper,
    private val dbHelper: DatabaseHelper
) : ViewModel() {

    private val users = MutableLiveData>>()

    init {
        fetchUsers()
    }

    private fun fetchUsers() {
        viewModelScope.launch {
            users.postValue(Resource.loading(null))
            apiHelper.getUsers()
                .zip(
                    apiHelper.getUsersWithError()
                        .catch { emitAll(flowOf(emptyList())) }) { usersFromApi, moreUsersFromApi ->
                    val allUsersFromApi = mutableListOf()
                    allUsersFromApi.addAll(usersFromApi)
                    allUsersFromApi.addAll(moreUsersFromApi)
                    return@zip allUsersFromApi
                }
                .flowOn(Dispatchers.Default)
                .catch { e ->
                    users.postValue(Resource.error(e.toString(), null))
                }
                .collect {
                    users.postValue(Resource.success(it))
                }
        }
    }

    fun getUsers(): LiveData>> {
        return users
    }

}

完成：
活动代码

package com.mindorks.kotlinFlow.learn.completion

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.learn.base.ApiUserAdapter
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_long_running_task.*
import kotlinx.android.synthetic.main.activity_recycler_view.progressBar

class CompletionActivity : AppCompatActivity() {

    private lateinit var viewModel: CompletionViewModel
    private lateinit var adapter: ApiUserAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_completion)
        setupViewModel()
        setupObserver()
    }

    private fun setupObserver() {
        viewModel.getStatus().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    textView.text = it.data
                    textView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    textView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(CompletionViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.completion

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.flow.catch
import kotlinx.coroutines.flow.collect
import kotlinx.coroutines.flow.onCompletion
import kotlinx.coroutines.launch

class CompletionViewModel(
    private val apiHelper: ApiHelper,
    private val dbHelper: DatabaseHelper
) : ViewModel() {

    private val status = MutableLiveData>()

    init{
        fetchUsers()
    }
    private fun fetchUsers() {
        viewModelScope.launch {
            status.postValue(Resource.loading(null))
            apiHelper.getUsers()
                .catch { e ->
                    status.postValue(Resource.error(e.toString(), null))
                }
                .onCompletion {
                    status.postValue(Resource.success("Task Completed"))
                }
                .collect {
                }
        }
    }

    fun getStatus(): LiveData> {
        return status
    }
    
}

减少：
活动代码

package com.mindorks.kotlinFlow.learn.reduce

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_long_running_task.*

class ReduceActivity : AppCompatActivity() {

    private lateinit var viewModel: ReduceViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_long_running_task)
        setupViewModel()
        setupLongRunningTask()
    }

    private fun setupLongRunningTask() {
        viewModel.getStatus().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    textView.text = it.data
                    textView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    textView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
        viewModel.startReduceTask()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(ReduceViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.reduce

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.flow.asFlow
import kotlinx.coroutines.flow.reduce
import kotlinx.coroutines.launch

class ReduceViewModel(
    val apiHelper: ApiHelper,
    dbHelper: DatabaseHelper
) : ViewModel() {

    private val status = MutableLiveData>()

    fun startReduceTask() {
        viewModelScope.launch {
            status.postValue(Resource.loading(null))
            val result = (1..5).asFlow()
                .reduce { a, b -> a + b }

            status.postValue(Resource.success(result.toString()))
        }
    }

    fun getStatus(): LiveData> {
        return status
    }

}

地图：
活动代码

package com.mindorks.kotlinFlow.learn.map


import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import androidx.recyclerview.widget.DividerItemDecoration
import androidx.recyclerview.widget.LinearLayoutManager
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.data.local.entity.User
import com.mindorks.kotlinFlow.learn.base.UserAdapter
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_recycler_view.*

class MapActivity : AppCompatActivity() {

    private lateinit var viewModel: MapViewModel
    private lateinit var adapter: UserAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_recycler_view)
        setupUI()
        setupViewModel()
        setupObserver()
    }

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

    private fun setupObserver() {
        viewModel.getUsers().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    it.data?.let { users -> renderList(users) }
                    recyclerView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    recyclerView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
    }

    private fun renderList(users: List) {
        adapter.addData(users)
        adapter.notifyDataSetChanged()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(MapViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.map

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.data.local.entity.User
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.flow.catch
import kotlinx.coroutines.flow.collect
import kotlinx.coroutines.flow.map
import kotlinx.coroutines.launch


class MapViewModel(
    val apiHelper: ApiHelper,
    dbHelper: DatabaseHelper
) : ViewModel() {

    private val users = MutableLiveData>>()

    init {
        fetchUsers()
    }

    private fun fetchUsers() {
        viewModelScope.launch {
            users.postValue(Resource.loading(null))
            apiHelper.getUsers()
                .map { apiUserList ->
                    val userList = mutableListOf()
                    for (apiUser in apiUserList) {
                        val user = User(
                            apiUser.id,
                            apiUser.name,
                            apiUser.email,
                            apiUser.avatar
                        )
                        userList.add(user)
                    }
                    userList
                }
                .catch { e ->
                    users.postValue(Resource.error(e.toString(), null))
                }
                .collect {
                    users.postValue(Resource.success(it))
                }
        }
    }

    fun getUsers(): LiveData>> {
        return users
    }
}

筛选：
活动代码

package com.mindorks.kotlinFlow.learn.filter

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_long_running_task.*
import kotlinx.android.synthetic.main.activity_recycler_view.progressBar

class FilterActivity : AppCompatActivity() {

    private lateinit var viewModel: FilterViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_long_running_task)
        setupViewModel()
        setupLongRunningTask()
    }

    private fun setupLongRunningTask() {
        viewModel.getStatus().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    textView.text = it.data
                    textView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    textView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
        viewModel.startFilterTask()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(FilterViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.filter

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.flow.asFlow
import kotlinx.coroutines.flow.filter
import kotlinx.coroutines.flow.toList
import kotlinx.coroutines.launch

class FilterViewModel(
    apiHelper: ApiHelper,
    dbHelper: DatabaseHelper
) : ViewModel() {
    private val status = MutableLiveData>()

    fun startFilterTask() {
        viewModelScope.launch {
            status.postValue(Resource.loading(null))
            val result = mutableListOf()
            (1..5).asFlow()
                .filter {
                    it % 2 == 0
                }
                .toList(result)

            status.postValue(Resource.success(result.toString()))
        }
    }

    fun getStatus(): LiveData> {
        return status
    }

}

搜索功能：使用 Kotlin Flow Operators 实现搜索 - Debounce、Filter、DistinctUntilChanged、FlatMapLatest。
- 活动代码

package com.mindorks.kotlinFlow.learn.search

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.utils.getQueryTextChangeStateFlow
import kotlinx.android.synthetic.main.activity_search.*
import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
import kotlin.coroutines.CoroutineContext

class SearchActivity : AppCompatActivity(), CoroutineScope {

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

    private lateinit var job: Job

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_search)
        job = Job()
        setUpSearchStateFlow()
    }

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

    private fun setUpSearchStateFlow() {
        launch {
            searchView.getQueryTextChangeStateFlow()
                .debounce(300)
                .filter { query ->
                    if (query.isEmpty()) {
                        textViewResult.text = ""
                        return@filter false
                    } else {
                        return@filter true
                    }
                }
                .distinctUntilChanged()
                .flatMapLatest { query ->
                    dataFromNetwork(query)
                        .catch {
                            emitAll(flowOf(""))
                        }
                }
                .flowOn(Dispatchers.Default)
                .collect { result ->
                    textViewResult.text = result
                }
        }
    }

    /**
     * Simulation of network data
     */
    private fun dataFromNetwork(query: String): Flow {
        return flow {
            delay(2000)
            emit(query)
        }
    }

}

重试：
活动代码

package com.mindorks.kotlinFlow.learn.retry

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_retry.*

class RetryActivity : AppCompatActivity() {

    private lateinit var viewModel: RetryViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_retry)
        setupViewModel()
        setupLongRunningTask()
    }

    private fun setupLongRunningTask() {
        viewModel.getStatus().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    textView.text = it.data
                    textView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    textView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
        viewModel.startTask()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(RetryViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.retry

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.launch
import java.io.IOException

class RetryViewModel(
    val apiHelper: ApiHelper,
    dbHelper: DatabaseHelper
) : ViewModel() {

    private val status = MutableLiveData>()

    fun startTask() {
        viewModelScope.launch {
            status.postValue(Resource.loading(null))
            // do a long running task
            doLongRunningTask()
                .flowOn(Dispatchers.Default)
                .retry(retries = 3) { cause ->
                    if (cause is IOException) {
                        delay(2000)
                        return@retry true
                    } else {
                        return@retry false
                    }
                }
                .catch {
                    status.postValue(Resource.error("Something Went Wrong", null))
                }
                .collect {
                    status.postValue(Resource.success("Task Completed"))
                }
        }
    }

    fun getStatus(): LiveData> {
        return status
    }

    private fun doLongRunningTask(): Flow {
        return flow {
            // your code for doing a long running task
            // Added delay, random number, and exception to simulate

            delay(2000)

            val randomNumber = (0..2).random()

            if (randomNumber == 0) {
                throw IOException()
            } else if (randomNumber == 1) {
                throw IndexOutOfBoundsException()
            }

            delay(2000)
            emit(0)
        }
    }

}

重试时间：
活动代码

package com.mindorks.kotlinFlow.learn.retrywhen

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_retry.*

class RetryWhenActivity : AppCompatActivity() {

    private lateinit var viewModel: RetryWhenViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_retry)
        setupViewModel()
        setupLongRunningTask()
    }

    private fun setupLongRunningTask() {
        viewModel.getStatus().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    textView.text = it.data
                    textView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    textView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
        viewModel.startTask()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(RetryWhenViewModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.retrywhen

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.launch
import java.io.IOException

class RetryWhenViewModel(
    val apiHelper: ApiHelper,
    dbHelper: DatabaseHelper
) : ViewModel() {

    private val status = MutableLiveData>()

    fun startTask() {
        viewModelScope.launch {
            status.postValue(Resource.loading(null))
            // do a long running task
            doLongRunningTask()
                .flowOn(Dispatchers.Default)
                .retryWhen { cause, attempt ->
                    if (cause is IOException && attempt < 3) {
                        delay(2000)
                        return@retryWhen true
                    } else {
                        return@retryWhen false
                    }
                }
                .catch {
                    status.postValue(Resource.error("Something Went Wrong", null))
                }
                .collect {
                    status.postValue(Resource.success("Task Completed"))
                }
        }
    }

    fun getStatus(): LiveData> {
        return status
    }

    private fun doLongRunningTask(): Flow {
        return flow {
            // your code for doing a long running task
            // Added delay, random number, and exception to simulate

            delay(2000)

            val randomNumber = (0..2).random()

            if (randomNumber == 0) {
                throw IOException()
            } else if (randomNumber == 1) {
                throw IndexOutOfBoundsException()
            }

            delay(2000)
            emit(0)
        }
    }

}

使用指数退避重试：
活动代码

package com.mindorks.kotlinFlow.learn.retryexponentialbackoff

import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import com.mindorks.kotlinFlow.R
import com.mindorks.kotlinFlow.data.api.ApiHelperImpl
import com.mindorks.kotlinFlow.data.api.RetrofitBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseBuilder
import com.mindorks.kotlinFlow.data.local.DatabaseHelperImpl
import com.mindorks.kotlinFlow.utils.Status
import com.mindorks.kotlinFlow.utils.ViewModelFactory
import kotlinx.android.synthetic.main.activity_retry.*

class RetryExponentialBackoffActivity : AppCompatActivity() {

    private lateinit var viewModel: RetryExponentialBackoffModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_retry)
        setupViewModel()
        setupLongRunningTask()
    }

    private fun setupLongRunningTask() {
        viewModel.getStatus().observe(this, Observer {
            when (it.status) {
                Status.SUCCESS -> {
                    progressBar.visibility = View.GONE
                    textView.text = it.data
                    textView.visibility = View.VISIBLE
                }
                Status.LOADING -> {
                    progressBar.visibility = View.VISIBLE
                    textView.visibility = View.GONE
                }
                Status.ERROR -> {
                    //Handle Error
                    progressBar.visibility = View.GONE
                    Toast.makeText(this, it.message, Toast.LENGTH_SHORT).show()
                }
            }
        })
        viewModel.startTask()
    }

    private fun setupViewModel() {
        viewModel = ViewModelProviders.of(
            this,
            ViewModelFactory(
                ApiHelperImpl(RetrofitBuilder.apiService),
                DatabaseHelperImpl(DatabaseBuilder.getInstance(applicationContext))
            )
        ).get(RetryExponentialBackoffModel::class.java)
    }
}

视图模型代码

package com.mindorks.kotlinFlow.learn.retryexponentialbackoff

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.mindorks.kotlinFlow.data.api.ApiHelper
import com.mindorks.kotlinFlow.data.local.DatabaseHelper
import com.mindorks.kotlinFlow.utils.Resource
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.launch
import java.io.IOException

class RetryExponentialBackoffModel(
    val apiHelper: ApiHelper,
    dbHelper: DatabaseHelper
) : ViewModel() {

    private val status = MutableLiveData>()

    fun startTask() {
        viewModelScope.launch {
            status.postValue(Resource.loading(null))
            // do a long running task
            var currentDelay = 1000L
            val delayFactor = 2
            doLongRunningTask()
                .flowOn(Dispatchers.Default)
                .retry(retries = 3) { cause ->
                    if (cause is IOException) {
                        delay(currentDelay)
                        currentDelay = (currentDelay * delayFactor)
                        return@retry true
                    } else {
                        return@retry false
                    }
                }
                .catch {
                    status.postValue(Resource.error("Something Went Wrong", null))
                }
                .collect {
                    status.postValue(Resource.success("Task Completed"))
                }
        }
    }

    fun getStatus(): LiveData> {
        return status
    }

    private fun doLongRunningTask(): Flow {
        return flow {
            // your code for doing a long running task
            // Added delay, random number, and exception to simulate

            delay(2000)

            val randomNumber = (0..2).random()

            if (randomNumber == 0) {
                throw IOException()
            } else if (randomNumber == 1) {
                throw IndexOutOfBoundsException()
            }

            delay(2000)
            emit(0)
        }
    }

}

作者：amitshekhariitbhu
链接：https://github.com/MindorksOpenSource/Kotlin-Flow-Android-Examples

[Android Kotlin-Flow]Kotlin-Flow如何在Android项目中使用

前言：

关于这个项目：

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