如何从协程 scope 返回值

Question

Is there a way to return value from a coroutine scope? for example like this:

suspend fun signUpUser(signUpData : SignUpUserRequest) : User {

CoroutineScope(Dispatchers.IO).launch() {

        val response = retrofitInstance.socialSignUP(signUpData)
        if (response.success) {
            obj = response.data
        } else {
                obj = response.message
        }
    }.invokeOnCompletion {
        obj = response.message
    }

    return obj
}

the problem is as soon as signupUser function called, return obj suspend statement run right away, not with response value.. its not running sequentially, what I expect is first response come and then function return obj but its not happening. why?

I tried run blocking, it served the purpose but is there better approach to do same task without run blocking? or is it alright?

Thanks in advance!

Answer 1

You can use withContext function to run a code in background thread and return some result:

suspend fun signUpUser(signUpData : SignUpUserRequest): User = withContext(Dispatchers.IO) {    
    val response = retrofitInstance.socialSignUP(signUpData)
    if (response.success) response.data else response.message
}

Answer 2

If you need to run works in parallel then you can use coroutineScope or async coroutine builder functions. However, in your case you don't have more than a request and so it could be sequantial . As mentioned in previous answer you can use withContext . Here is the small explaination from docs :

Calls the specified suspending block with a given coroutine context, suspends until it completes, and returns the result.

suspend fun getUser(signUpData: SignUpUserRequest): User = withContext(Dispatchers.IO) {
    // do your network request logic here and return the result
}

Please note that, when you are working with expression body with functions, always try to annotate return type explicitly.

EDIT

Coroutines use the regular Kotlin syntax for handling exceptions: try/catch or built-in helper functions like runCatching (which uses try/catch internally). Beware of that uncaught exceptions will always be thrown . However, different coroutines builders treat exceptions in different ways. It's always good to read official documentation or other sources (like this ).

Answer 3

According to Android's tutorial: Kotlin coroutines on Android

Assume that the code snippet of the HTTP request as below

1. Remember to add the 'suspend' for the function
2. Add @withContext to return the Result object

// Function that makes the network request, blocking the current thread
suspend fun makeLoginRequest(
    jsonBody: String
): Result<LoginResponse> {

    // Move the execution of the coroutine to the I/O dispatcher
    return withContext(Dispatchers.IO) {
        // Blocking network request code
        val url = URL(loginUrl)
        (url.openConnection() as? HttpURLConnection)?.run {
            requestMethod = "POST"
            setRequestProperty("Content-Type", "application/json; utf-8")
            setRequestProperty("Accept", "application/json")
            doOutput = true
            outputStream.write(jsonBody.toByteArray())
            return@withContext Result.Success(responseParser.parse(inputStream))
        }
        return@withContext Result.Error(Exception("Cannot open HttpURLConnection"))
    }

}

Answer 4

In the Google codelab "Load and display images from the Internet" , there's a nice and very elegant example with MutableLiveData.

Outline:

• You make a ViewModel with LiveData (internal) and MutableLiveData (external)
• Now you can use the data directly in views, fragments or activities

The benefit being that this data is lifecycle-aware, and you can use the returned values with observers.

class OverviewViewModel : ViewModel() {

    enum class MarsApiStatus { LOADING, ERROR, DONE }

    private val _status = MutableLiveData<MarsApiStatus>()
    val status: LiveData<MarsApiStatus> = _status

    private val _photos = MutableLiveData<List<MarsPhoto>>()
    val photos: LiveData<List<MarsPhoto>> = _photos

    /**
     * Call getMarsPhotos() on init so we can display status immediately.
     */
    init {
        getMarsPhotos()
    }

    /**
     * Gets Mars photos information from the Mars API Retrofit service and updates the
     * [MarsPhoto] [List] [LiveData].
     */
    private fun getMarsPhotos() {
        viewModelScope.launch {
            _status.value = MarsApiStatus.LOADING
            Log.d(TAG, "loading")
            try {
                _photos.value = MarsApi.retrofitService.getPhotos()
                _status.value = MarsApiStatus.DONE
                Log.d(TAG, "done")
            } catch (e: Exception) {
                _status.value = MarsApiStatus.ERROR
                _photos.value = listOf()
                Log.d(TAG, "error")
            }
        }
    }
}

You can use the status or photos values by observing them or directly using them in a view like this:

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/photos_grid"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layoutManager="androidx.recyclerview.widget.GridLayoutManager"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:listData="@{viewModel.photos}"
        app:spanCount="2"
        tools:itemCount="16"
        tools:listitem="@layout/grid_view_item" />

...after you've "registered" and wired everything in the f.ex. regarding fragment, like this:

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {

    try {
        val binding = FragmentOverviewBinding.inflate(inflater)
        //val binding = GridViewItemBinding.inflate(inflater)

        // Allows Data Binding to Observe LiveData with the lifecycle of this Fragment
        binding.lifecycleOwner = this

        // Giving the binding access to the OverviewViewModel
        binding.viewModel = viewModel

        binding.photosGrid.adapter = PhotoGridAdapter()

        return binding.root
    } catch (e: Exception) {
        Log.e("OverviewFragment", "${e.localizedMessage}")
    }
    return null
}

Answer 5

Return value using return@coroutineScope . It will work.

suspend fun signUpUser(signUpData : SignUpUserRequest) : User = coroutineScope {

launch() {

    val response = retrofitInstance.socialSignUP(signUpData)
    if (response.success) {
        obj = response.data
    } else {
            obj = response.message
    }
}.invokeOnCompletion {
    obj = response.message
}

return@coroutineScope obj
}