chaxiu

suspend fun testCoroutine() {
    log("start")
    val user = getUserInfo()
    log(user)
    val friendList = getFriendList(user)
    log(friendList)
    val feedList = getFeedList(friendList)
    log(feedList)
}

chaxiu

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

chaxiu

fun getUserInfo(completion: Continuation<Any?>): Any?{}
fun getFriendList(user: String, completion: Continuation<Any?>): Any?{}
fun getFeedList(friendList: String, completion: Continuation<Any?>): Any?{}

chaxiu

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

chaxiu

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

chaxiu

// 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

chaxiu

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

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

        // Call getUserInfo()

chaxiu

// "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() {

chaxiu

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

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

chaxiu

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