Suspend Coroutines's execution to wait for callback

Question

Recently, I applied Coroutines into my project everything seems to be fine but today I meet a problem Upload file/image into server using Coroutine + Retrofit.

It seems that there is no solution for upload file using coroutine + Retrofit, so we must use callback for retrofit.

//Api interface
interface UploadFileApiKotlin {
    @Multipart
    @POST("/uploadFileApi")
    fun uploadFiles(
        @Part listUri: List<MultipartBody.Part>
    ): Call<Response<List<FileResponse>?>>
}
//ViewModel class
serviceScope.launch {
                //Insert into db
                repository.insertNewExtAct()
                //Send data into server.
                val call = RequestHelper.getUpLoadFilesKotlinRequest().uploadFiles(partList)
                call.enqueue(object : Callback<Response<List<FileResponse>?>> {
                    override fun onResponse(
                        call: Call<Response<List<FileResponse>?>>,
                        response: Response<Response<List<FileResponse>?>>
                    ) {
                        TODO("Not yet implemented")
                    }

                    override fun onFailure(
                        call: Call<Response<List<FileResponse>?>>,
                        t: Throwable
                    ) {
                        TODO("Not yet implemented")
                    }
                })
                //Wait for response from server and do logic
            }

My question is: How can I suspend coroutines's execution to wait for retrofit's callback ?

Answer 1

You can use suspendCoroutine or suspendCancellableCoroutine to work with callbacks (to convert a callback to a suspend function):

serviceScope.launch {
    //Insert into db
    repository.insertNewExtAct()
    //Send data into server.
    uploadFile()
    //Wait for response from server and do logic
}

suspend fun uploadFile() = suspendCoroutine<Response<List<FileResponse>?>> { continuation ->
    val call = RequestHelper.getUpLoadFilesKotlinRequest().uploadFiles(partList)
    call.enqueue(object : Callback<Response<List<FileResponse>?>> {
        override fun onResponse(
            call: Call<Response<List<FileResponse>?>>,
            response: Response<Response<List<FileResponse>?>>
        ) {
            // Resume coroutine with a value provided by the callback
            continuation.resumeWith(response.data)
        }

        override fun onFailure(
            call: Call<Response<List<FileResponse>?>>,
            t: Throwable
        ) {
            continuation.resumeWithException(t)
        }
    })
}

suspendCoroutine suspends coroutine in which it executed until we decide to continue by calling appropriate methods - Continuation.resume.... suspendCoroutine mainly used when we have some legacy code with callbacks.

There is also suspendCancellableCoroutine builder function, it behaves similar to suspendCoroutine with additional feature - provides an implementation of CancellableContinuation to the block.