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Diagram Coroutines: suspend

1. Preface

Coroutines is the most amazing feature in Kotlin.

In this post, I'll briefly introduce Kotlin coroutines, and then explain Coroutines in the form of Diagram + Animation. After reading this post, you may find out that Coroutines is not that difficult.

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// Have fun!
@Nullable
public static final Object testCoroutine(@NotNull Continuation $completion) {
    Object $continuation;
    label37: {
        if ($completion instanceof <TestSuspendKt$testCoroutine$1>) {
            $continuation = (<TestSuspendKt$testCoroutine$1>)$completion;
            if ((((<TestSuspendKt$testCoroutine$1>)$continuation).label & Integer.MIN_VALUE) != 0) {
                ((<TestSuspendKt$testCoroutine$1>)$continuation).label -= Integer.MIN_VALUE;
                break label37;

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// pseudocode
// marked by label
// ↓      ↓
label: whenStart
when (continuation.label) {
    0 -> {
        ...
    }

    1 -> {

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if (suspendReturn == sFlag) {
} else {
    // What is the specific code?
    //       ↓
    //go to next state
}

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when (continuation.label) {
    0 -> {
        ...
    }

    1 -> {
        ...
        //            function name changed
        //                      ↓
        suspendReturn = noSuspendFriendList(user, continuation)

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// "fake" suspending function
// Although it has suspend modifier, 
// it will not really suspend when executed,
// because there is no other suspending in its body
//  ↓
suspend fun noSuspendFriendList(user: String): String{
    return "Tom, Jack"
}

suspend fun testNoSuspend() {

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when (continuation.label) {
    0 -> {
        // check exceptions
        throwOnFailure(result)

        log("start")
        // Assign 1 to label, ready for the next state
        continuation.label = 1

        // Call getUserInfo()

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// Three variables, corresponding to the three variables of the original function
lateinit var user: String
lateinit var friendList: String
lateinit var feedList: String

// The running result of the coroutine
var result = continuation.result

// The return of the suspending function
var suspendReturn: Any? = null

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//                    ↓
fun testCoroutine(completion: Continuation<Any?>): Any? {
    ...
    val continuation = if (completion is TestContinuation) {
        completion
    } else {
        //               As parameter
        //                   ↓
        TestContinuation(completion)
    }

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fun testCoroutine(completion: Continuation<Any?>): Any? {

    class TestContinuation(completion: Continuation<Any?>?) : ContinuationImpl(completion) {
        // Represents the current state of the coroutines state machine
        var label: Int = 0
        // Coroutines results
        var result: Any? = null

        // Used to save the calculation results of the previous coroutine
        var mUser: Any? = null