1.0.0 (Mar 1, 2017)
Aug 19, 2016
Feb 16, 2018 (Retired)
MetaLab (metalabdesign)
Oleksii Masnyi (pilgr)
Louis CAD (LouisCAD)
Tsuharesu (tsuharesu)
Ryan Lee (ryanleecode)
Source code


A Kotlin library for Android to write asynchronous code in a simpler and more reliable way using async/await approach, like:

async {
   progressBar.visibility = View.VISIBLE
   // Release main thread and wait until text is loaded in background thread
   val loadedText = await { loadFromServer() }
   // Loaded successfully, come back in UI thread and show the result
   txtResult.text = loadedText
   progressBar.visibility = View.INVISIBLE

As you see in the example above, you can write asynchronous code in a imperative style, step by step. Calling await to run code in background doesn't lock the UI thread. And execution continues in UI thread after background work is finished. There is no magic, see how it works.


compile 'co.metalab.asyncawait:asyncawait:1.0.0'



Coroutine code has to be passed as a lambda in async function

async {
   // Coroutine body


Long running code has to be passed as a lambda in await function

async {
   val result = await {
      //Long running code
   // Use result

You may have many await calls inside async block, or have await in a loop

async {
   val repos = await { github.getRepos() }
   repos.forEach { repo ->
      val stats = await { github.getStats( }
      showStats(repo, stats)


Use it to show loading progress, its second parameter is a progress handler.

val loadedText = awaitWithProgress(::loadTextWithProgress) {
         // Called in UI thread
         progressBar.progress = it
         progressBar.max = 100

A data loading function (like the loadTextWithProgress above) should have a functional parameter of type (P) -> Unit which can be called in order to push progress value. For example, it could be like:

private fun loadTextWithProgress(handleProgress: (Int) -> Unit): String {
   for (i in 1..10) {
      handleProgress(i * 100 / 10) // in %
   return "Loaded Text"

Handle exceptions using try/catch

async {
   try {
      val loadedText = await {
         // throw exception in background thread
      // Process loaded text
   } catch (e: Exception) {
      // Handle exception in UI thread

Handle exceptions in onError block

Could be more convenient, as resulting code has fewer indents. onError called only if exception hasn't been handled in try/catch.

async {
   val loadedText = await {
      // throw exception in background thread
   // Process loaded text
}.onError {
   // Handle exception in UI thread

Unhandled exceptions and exception delivered in onError wrapped by AsyncException with convenient stack trace to the place where await been called originally in UI thread

finally execution

finally always executed after calling onError or when the coroutine finished successfully.

async {
   // Show progress
   await { }
}.onError {
   // Handle exception
}.finally {
   // Hide progress

Safe execution

The library has Activity.async and Fragment.async extension functions to produce more safe code. So when using async inside Activity/Fragment, coroutine won't be resumed if Activity is in finishing state or Fragment is detached.

Avoid memory leaks

Long running background code referencing any view/context may produce memory leaks. To avoid such memory leaks, call async.cancelAll() when all running coroutines referencing current object should be interrupted, like

override fun onDestroy() {

The async is an extension property for Any type. So calling [this.]async.cancelAll intrerrupts only coroutines started by [this.]async {} function.

Common extensions

The library has a convenient API to work with Retrofit and rxJava.


  • awaitSuccessful(retrofit2.Call)

Returns Response<V>.body() if successful, or throws RetrofitHttpError with error response otherwise.

async {
   reposList = awaitSuccessful(github.listRepos(userName))


  • await(Observable)

Waits until observable emits first value.

async {
   val observable = Observable.just("O")
   result = await(observable)

How to create custom extensions

You can create your own await implementations. Here is example of rxJava extension to give you idea. Just return the result of calling AsyncController.await with your own lambda implementation. The code inside await block will be run on a background thread.

suspend fun <V> AsyncController.await(observable: Observable<V>): V = this.await {

How it works

The library is built upon coroutines introduced in Kotlin 1.1.

The Kotlin compiler responsibility is to convert coroutine (everything inside async block) into a state machine, where every await call is a non-blocking suspension point. The library is responsible for thread handling, error handling and managing state machine. When background computation is done the library delivers result back into UI thread and resumes coroutine execution.