本篇文章舉例協程的各種方法的使用,并簡單闡述各個方法的一些注意事項。
協程作用域的創建
1.通過工廠函數創建自定義上下文的作用域
// 舉個例子:
// 協程異常處理器(也是一個上下文元素)
private val exceptionHandler =
CoroutineExceptionHandler { _, throwable ->
onError(throwable)
}
val myScope = CoroutineScope(exceptionHandler + SupervisorJob() + Dispatchers.IO)
// 源碼
public fun CoroutineScope(
context: CoroutineContext
): CoroutineScope = ContextScope(
if (context[Job] != null) context
else context + Job()
)
internal class ContextScope(
context: CoroutineContext
) : CoroutineScope {
override val coroutineContext: CoroutineContext = context
override fun toString(): String =
"CoroutineScope(coroutineContext=$coroutineContext)"
}
2.通過工廠函數MainScope()創建主線程調度器的作用域
// 舉個例子:
val mainScope = MainScope()
// 源碼
public fun MainScope(): CoroutineScope = ContextScope(SupervisorJob() + Dispatchers.Main)
3.官方架構提供的一些作用域viewModelScope和lifecycleScope
// Android JetPack ViewModel里的viewModelScope,ViewModeld的擴展變量
// 來自androidx.lifecycle:lifecycle-viewmodel-ktx:2.3.1
public val ViewModel.viewModelScope: CoroutineScope
get() {
val scope: CoroutineScope? = this.getTag(JOB_KEY)
if (scope != null) {
return scope
}
return setTagIfAbsent(
JOB_KEY,
CloseableCoroutineScope(
SupervisorJob() +
Dispatchers.Main.immediate
)
)
}
internal class CloseableCoroutineScope(
context: CoroutineContext
) : Closeable, CoroutineScope {
override val coroutineContext: CoroutineContext = context
override fun close() {
coroutineContext.cancel()
}
}
// Android JetPack lifecycle里的lifecycleScope,LifecycleOwner(Activity實現了此接口,可以直接用)的擴展變量
// 來自androidx.lifecycle:lifecycle-runtime-ktx:2.2.0
val LifecycleOwner.lifecycleScope: LifecycleCoroutineScope
get() = lifecycle.coroutineScope
val Lifecycle.coroutineScope: LifecycleCoroutineScope
get() {
while (true) {
val existing = mInternalScopeRef.get() as LifecycleCoroutineScopeImpl?
if (existing != null) {
return existing
}
val newScope = LifecycleCoroutineScopeImpl(
this,
SupervisorJob() + Dispatchers.Main.immediate
)
if (mInternalScopeRef.compareAndSet(null, newScope)) {
newScope.register()
return newScope
}
}
}
協程的創建
方式一:launch
創建一個協程,并返回協程的引用job,最常用的方式
val job = MainScope().launch (Dispatchers.IO){
delay(1000)
println("run in coroutine")
}
job.cancel() // 取消協程
job.join() // 等待協程執行完畢
job.cancelAndJoin() // 取消并等待協程執行完畢(取消操作并不等于會立馬響應,這里其實是分別執行了cancel和join)
job.cancelChildren() // 取消子協程
方式二:async
創建一個協程,并返回協程的引用Deferred(也是個job),通過Deferred的await拿到返回結果,此時協程會掛起
MainScope().launch (Dispatchers.Main){
// 啟動協程
val deferred = async {
"async result"
}
// 掛起等待協程結果
val result = deferred.await()
// 恢復掛起
println("result = $result")
}
作用域構建器
runBlocking
runBlocking {} 會在開啟一個新的協程,并且阻塞主線程,直到操作完成。這個函數不應該在協程里面使用,它是用來橋接需要阻塞的掛起函數,主要用于 main function 和 junit 測試。注意他只是一個普通函數。
coroutineScope
runBlocking {
val result = coroutineScope {
println("run thread = ${Thread.currentThread().name}")
launch {
delay(1000)
println("run launch success1")
}
launch {
delay(2000)
println("run launch success2")
}
"coroutineScope result"
}
println("result = $result")
}
// 運行結果(看得出來使用的是父協程的線程調度器)
run thread = main
run launch success1
run launch success2
result = coroutineScope result
// 注意這是一個掛起函數,它會掛起執行一直到里面的子協程執行完畢、代碼執行完畢然后返回結果,這是他與runBlocking不同的地方,runBlocking只是個普通函數,里面的協程體會阻塞線程。
public suspend fun <R> coroutineScope(block: suspend CoroutineScope.() -> R): R {
contract {
callsInPlace(block, InvocationKind.EXACTLY_ONCE)
}
return suspendCoroutineUninterceptedOrReturn { uCont ->
val coroutine = ScopeCoroutine(uCont.context, uCont)
coroutine.startUndispatchedOrReturn(coroutine, block)
}
}
coroutineScope經常用來把一個長耗時的任務拆分成多個子任務,使這些子任務并行執行,這就是所謂的結構化并發:
suspend fun showSomeData() = coroutineScope {
val data1 = async { //子任務1
delay(2000)
100
}
val data2 = async { //子任務2
delay(3000)
20
}
withContext(Dispatchers.Default) { //合并結果并返回
delay(3000)
val random = Random(10)
data1.await() + data2.await() + random.nextInt(100)
}
}
supervisorScope
supervisorScope功能與coroutinScope類似,不同之處在于,它可以讓里面的子協程發生異常而崩潰的時候不影響自身和其他的子協程的執行。
runBlocking {
val result = supervisorScope {
println("run thread = ${Thread.currentThread().name}")
launch {
delay(1000)
// 主動拋異常
throw RuntimeException()
println("run launch success1")
}
launch {
delay(2000)
println("run launch success2")
}
"coroutineScope result"
}
println("result = $result")
}
// 運行結果
run thread = main
run launch success2
result = coroutineScope result
說到supervisorScope順便提一嘴SupervisorJob,創建作用域或者啟動協程的時候如果上下文帶上SupervisorJob()也可以達到自身協程處理異常而不影響父協程和兄弟協程的運行。
常用函數
withTimeout
suspend fun timeOut(){
try {
withTimeout(1000){
println("在此做一些耗時操作,超過設定的時間就拋異常")
delay(2500)
}
}catch (e:TimeoutCancellationException){
println(e.message)
// 處理超時異常
}
}
withContext
// 掛起函數,適合一些耗時異步操作
suspend fun timeOut(){
val result = withContext(Dispatchers.IO){
delay(2500)
"result"
}
println(result)
}
flow
private fun flowEmit() = flow<Int> {
// 這段代碼塊實際運行在 flowEmit().collect所處的協程代碼塊里
println("...$coroutineContext")
for (k in 1 .. 3){
delay(1000)
emit(k)
}
}
@Test
fun flowTest() = runBlocking {
println("...${this.coroutineContext}")
// flowEmit()只是創建了個對象
// 調用collect的時候才開始執行上面定義的代碼塊
flowEmit().collect(object : FlowCollector<Int> {
// 這個代碼其實也是運行在runBlocking協程域里
override suspend fun emit(value: Int) {
println("...$value")
}
})
//
println("...end")
}
輸出
...[CoroutineId(1), "coroutine#1":BlockingCoroutine{Active}@2bdd8394, BlockingEventLoop@5f9edf14]
...[CoroutineId(1), "coroutine#1":BlockingCoroutine{Active}@2bdd8394, BlockingEventLoop@5f9edf14]
...1
...2
...3
...end