kotlin学习-Coroutines(协程)

协程（又名纤程），轻量级线程（建立在线程基础上，属于用户态调用），非阻塞式编程（像同步编写一样），在用户态内进行任务调度，避免与内核态过多交互问题，提高程序快速响应。协程使用挂起当前上下文替代阻塞，被挂起后的协程可以去运行其它active task,即协程可以被复用，相比于线程，减少了线程资源的大量浪费。

备注

挂起：保存当前运行状态，释放资源，此时协程可去做其它工作，可充分利用资源
阻塞：占用资源未释放，等待状态

基本使用：

fun runAsync()= runBlocking {
    val time = measureTimeMillis {//系统函数统计时间
        val one = async { doSomethingUsefulOne() }//异步调用，返回结果
        val two = async { doSomethingUsefulTwo() }
        println("The answer is ${one.await() + two.await()}")//等待异步执行完成（await调用会挂起当前线程,等待执行结果完成后，通过调用resume恢复挂起前状态）
    }
    println("Completed in $time ms")
}

//协程coroutines 调用的方法需要用suspend修饰，告诉编译器此函数可以被挂起
suspend fun doSomethingUsefulOne(): Int {
    delay(1000L) // pretend we are doing something useful here
    return 13
}

suspend fun doSomethingUsefulTwo(): Int {
    delay(1000L) // pretend we are doing something useful here, too
    return 29
}

这里面没有使用异步+回调，直接像写同步代码一样，简洁

launch 异步执行没有返回结果，产生Job对象用于cancel，join处理

fun cancelCoroutine() = runBlocking {
    val startTime = System.currentTimeMillis()
    val job = launch(Dispatchers.Default) {
        var nextPrintTime = startTime
        var i = 0
        while (isActive) { // cancellable computation loop
            // print a message twice a second
            if (System.currentTimeMillis() >= nextPrintTime) {
                println("job: I'm sleeping ${i++} ...")
                nextPrintTime += 500L
            }
        }
    }
    delay(1300L) // delay a bit
    println("main: I'm tired of waiting!")
    job.cancelAndJoin() // cancels the job and waits for its completion
    println("main: Now I can quit.")
}

线程之间切换，使用withContext

fun log(msg: String) = println("[${Thread.currentThread().name}] $msg")
fun jumpCor(){//创建单线程coroutines
    newSingleThreadContext("Ctx1").use { ctx1 ->
        newSingleThreadContext("Ctx2").use { ctx2 ->
            runBlocking(ctx1) {
                log("Started in ctx1")
                withContext(ctx2) {
                    log("Working in ctx2")
                }
                log("Back to ctx1")
            }
        }
    }
}

协程必须关联CoroutineScope以便于管理追踪，方法内创建Scope

suspend fun showSomeData() = coroutineScope {//此处coroutineScope属于out scope的child scop
      val data = async(Dispatchers.IO) { // IO task  io线程调用操作
//          ... load some UI data for the Main thread ...
       }

    withContext(Dispatchers.Main){//UI task  UI更新
        val result = data.await()
//        display(result)
    }
}

协程上下文环境，CoroutineScope，CoroutineContext

每个协程运行需要在指定Scope内才能使用协程相关方法delay,asyc,launch，创建CoroutineScope ，runBlocking函数内部会创建CoroutineScope，系统提供GlobalScope,MainScope等辅助类创建Scope

也可以通过CoroutineContext和Job创建自己的CoroutineScope

fun sampleCreateCorountine(){
    //create corountine scope
    //自定义CoroutineScope
    val coroutineContext = Dispatchers.Default
    val job = Job()
    val coroutineScope = CoroutineScope(coroutineContext + job)
    //创建child scope
    coroutineScope.launch {

    }
    //创建全局Scope
    GlobalScope.launch (Dispatchers.Default+CoroutineName("global background thread")){

    }
    //创建主线程分发处理Scope
    MainScope().launch {

    }

}

类内部定义协程

1，直接继承CoroutineScope

class SomethingWithLifecycle : CoroutineScope {
    // 使用job来管理你的SomethingWithLifecycle的所有子协程
    private val job = Job()
    override val coroutineContext: CoroutineContext
        get() = Dispatchers.Main + job

    fun destory(){//退出取消
        job.cancel()
    }
}

2，直接使用已定义Scope

class CorMyActivity : AppCompatActivity(), CoroutineScope by MainScope() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        showSomeData()
    }

    /**
     * Note how coroutine builders are scoped: if activity is destroyed or any of the launched coroutines
    in this method throws an exception, then all nested coroutines are cancelled.
     */
    fun showSomeData() = launch {
        // <- extension on current activity, launched in the main thread
        // ... here we can use suspending functions or coroutine builders with other dispatchers
//        draw(data) // draw in the main thread
    }

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

}

Dispatchers，协程分发器：

fun dispatchTask()= runBlocking<Unit> {
    // it inherits the context (and thus dispatcher) from the CoroutineScope that it is being launched from.
        launch { // context of the parent, main runBlocking coroutine
            println("main runBlocking      : I'm working in thread ${Thread.currentThread().name}")
        }
    //执行coroutine是在调用者的线程，但是当在coroutine中第一个挂起之后，后面所在的线程将完全取决于
    // 调用挂起方法的线程(如delay一般是由kotlinx.coroutines.DefaultExecutor中的线程调用)
    //Unconfined在挂起后在delay的调用线程DefaultExecutor执行
        launch(context = Dispatchers.Unconfined) { // not confined -- will work with main thread
            println("Unconfined            : I'm working in thread ${Thread.currentThread().name}")
        }
    // coroutines are launched in GlobalScope,uses shared background pool of threads
    //uses the same dispatcher as GlobalScope.launch
　　//Dispatchers.Default 处理cup密集型任务，线程数为cpu内核数，最少为2，Dispatchers.IO 处理阻塞性IO，socket密集度任务，数量随任务多少变化，默认最大数量64
        launch(context = Dispatchers.Default) { // will get dispatched to DefaultDispatcher
            println("Default               : I'm working in thread ${Thread.currentThread().name}")
        }
    //creates a thread for the coroutine to run
        launch(newSingleThreadContext("MyOwnThread")) { // will get its own new thread
            println("newSingleThreadContext: I'm working in thread ${Thread.currentThread().name}")
        }

}

suspend 是如何工作的? 
Kotlin 使用堆栈帧来管理要运行哪个函数以及所有局部变量。暂停协程时，
会复制并保存当前的堆栈帧以供稍后使用。恢复协程时，调度器会将堆栈帧从其保存位置复制回来，然后函数再次开始运行

协程间通信之channel

协程之间通过channel进行数据传递，生产者->消费者模式

例：

fun channelTest()= runBlocking {
    val channel = Channel<Int>()
    launch {//生产数据
        for (x in 1..5) channel.send(x * x)
        channel.close() //关闭停止
    }
    // 循环接收直到channnel close
    for (y in channel) println(y)
    println("Done!")
}

生产者每生产一个数据就发送到channel里，消费者等待接收数据，

channel分类：

SendChannel：创建的producers类型属于sendChannel实例

ReceiveChannel：创建的consumers类型属于receiveChannel实例

Channel：继承SendChannel和ReceiveChannel即可send，又可以receive数据

channel类型：

Unlimited channel：容量无限制，producer不断生产数据，可能会产生OutOfMemoryException，consumer接收数据时，如果channel内数据为空则会挂起
Buffered channel：指定 channel size，当生产者的数据达到buffer size大小则send会挂起，直到channel内数据量小于size才能继续生产数据
Rendezvous：是bufferred channel size=0，当producer生成数据send时如果没有consumer接受，则producer会挂起直到consumer取走数据，才继续send下一个数据，即实现同步传递数据功能
Conflated channel：producer不停地send数据，后面的数据会覆盖前面已经存在的数据，consumer始终取到最新的数据

    val rendezvousChannel = Channel<String>()//同步传递
    val bufferedChannel = Channel<String>(10)//指定size pool
    val conflatedChannel = Channel<String>(Channel.CONFLATED)//channel内数据实时更新
    val unlimitedChannel = Channel<String>(Channel.UNLIMITED)//无容量限制

协程结合Architecture ViewModel使用

class NewsViewModel: ViewModel() {

    private val mApi:WebServer
    init {
        mApi = WebServer()
    }

    val dataNews: MutableLiveData<DataResource<NewsDataRsp>> by lazy {
//        MutableLiveData<DataResource<NewsDataRsp>>().also {
//            loadNewsData(minId=null)
//        }
        MutableLiveData<DataResource<NewsDataRsp>>()
    }

     fun loadNewsData(pageIndex:Int =1,countItem:Int = 20,minId:String?=null){
        runCoroutine(dataNews){
            val mp = mutableMapOf("encode" to "ywjh","source" to "app","sys" to "android","banner" to "banner",
                    "limit" to countItem.toString(),"version" to "7002000")
            if(pageIndex>1 && false==minId.isNullOrEmpty()){
                mp.put("min_id",minId)
            }
            val response = mApi.commonDataSourceApi.getNewsData(mp).execute()
            return@runCoroutine response.body()!!
        }
    }

     fun fetchNews(pageIndex:Int =1,countItem:Int = 20,minId:String){
         val mp = mutableMapOf("encode" to "ywjh","source" to "app","sys" to "android","banner" to "banner",
                 "limit" to countItem.toString(),"version" to "7002000")
         if(pageIndex>1 && false==minId.isNullOrEmpty()){
             mp.put("min_id",minId)
         }

         val cor = CoroutineScope(Dispatchers.IO)
         cor.launch {
             try {
                 val response = mApi.commonDataSourceApi.getNewsData(mp).execute()
                 dataNews.postValue(DataResource(DataResource.Status.COMPLETED, response.body(), null))
             } catch (exception: Exception) {
                 dataNews.postValue(DataResource(DataResource.Status.COMPLETED, null, exception))
             }
         }
    }

    suspend fun simpleGetData(pageIndex:Int =1,countItem:Int = 20,minId:String) = withContext(Dispatchers.IO) {
        val mp = mutableMapOf("encode" to "ywjh","source" to "app","sys" to "android","banner" to "banner",
                "limit" to countItem.toString(),"version" to "7002000")
        if(pageIndex>1 && false==minId.isNullOrEmpty()){
            mp.put("min_id",minId)
        }

        try {
            val response = mApi.commonDataSourceApi.getNewsData(mp).execute()
            dataNews.postValue(DataResource(DataResource.Status.COMPLETED, response.body(), null))
        } catch (exception: Exception) {
            dataNews.postValue(DataResource(DataResource.Status.COMPLETED, null, exception))
        }
    }

    private fun <T> runCoroutine(correspondenceLiveData: MutableLiveData<DataResource<T>>, block: suspend () -> T) {
        correspondenceLiveData.value = DataResource(DataResource.Status.LOADING, null, null)

        GlobalScope.launch(Dispatchers.IO) {
            try {
                val result = block()
                correspondenceLiveData.postValue(DataResource(DataResource.Status.COMPLETED, result, null))
            } catch (exception: Exception) {
//                val error = ErrorConverter.convertError(exception)
                correspondenceLiveData.postValue(DataResource(DataResource.Status.COMPLETED, null, exception))
            }
        }
    }

}