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Kotlin Coroutines Tutorial for Android: Getting Began

Replace word: Luka Kordić up to date this tutorial for Android Studio Flamingo, Kotlin 1.8 and Android 13. Amanjeet Singh wrote the unique.

Asynchronous programming is essential for contemporary apps. Utilizing it will increase the quantity of labor your app can carry out in parallel. This, in flip, lets you run heavy-duty duties within the background, away from the UI thread. By doing so, you keep away from UI freezes and supply a fluid expertise in your customers.

The Android ecosystem has a number of mechanisms builders can select from in relation to asynchronous programming: IntentService, Handlers, Executors, RxJava and ListenableFutures, to call just a few. But it surely’s troublesome to choose essentially the most acceptable one to make use of. Some mechanisms have a steep studying curve. Others require a ton of boilerplate code to implement and aren’t that concise or intuitive to make use of. Asynchronous programming is complicated sufficient by itself, so builders naturally search for the only answer to assist scale back the complexity. Meet Kotlin Coroutines!

Why Use Kotlin Coroutines?

Kotlin Coroutines are nice as a result of they’re simple to start out with. Their greatest benefit over the opposite options is that they let you write your asynchronous code sequentially. This makes the code a lot simpler to know. Whereas simple to start out with, they’re fairly highly effective and provide the instruments to deal with virtually each downside associated to concurrency or threading that you just may encounter whereas constructing trendy Android apps.

All through this tutorial, you’ll develop a photograph modifying app, Snowy. It lets you obtain a picture after which apply a snow filter to it. To obtain the photographs and course of them, you’ll must carry out asynchronous duties.

Alongside the way in which, you’ll study:

  • About completely different components of the Coroutine API.
  • The right way to create your individual Kotlin Coroutines.
  • The right way to execute a number of duties in parallel.
  • Exception dealing with with Kotlin Coroutines.
Notice: This tutorial assumes you’re already acquainted with the fundamentals of Android improvement. In case you’re fully new to creating apps on Android, learn our Newbie Android Sequence. You’ll additionally want some fundamental information of asynchronous programming and the way threads work.

Getting Began

To start out, obtain the supplies for this tutorial by clicking Obtain supplies on the high or backside of the tutorial. Then, open the starter undertaking in Android Studio Electrical Eel or later, and look by way of its content material.

You’ll see:

  • mannequin package deal with Tutorial mannequin, which has 4 properties: the tutorial’s title, the description and two URLs for photographs.
  • utils package deal with SnowFilter, which has a perform referred to as applySnowEffect. applySnowEffect takes a Bitmap as an argument and returns a processed Bitmap with a snow filter.
  • MainActivity, which hosts 4 tabs: Kotlin, Android, RxKotlin and Kitura.
  • TutorialFragment, which exhibits particulars of various tutorials.
  • TutorialPagerAdapter: A FragmentStateAdapter to arrange the tabs and ViewPager.

Construct and run the starter undertaking.

Starter app initial state

You’ll see 4 tabs with their names. Every tab comprises a title, description and a placeholder picture. You’ll substitute the placeholder with a picture downloaded from the web quickly. Earlier than doing that, you want to add Kotlin Coroutines to the undertaking and study some fundamental ideas. You’ll try this within the subsequent part.

Including Kotlin Coroutines Help

Earlier than you possibly can create Kotlin Coroutines, it’s a must to add the dependencies to your Android undertaking. Navigate to the app module’s construct.gradle file, and add the next two strains contained in the dependencies block:

implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:1.6.4'
implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-android:1.6.4'

Despite the fact that Kotlin has native assist for coroutines, you want to add these two dependencies when engaged on Android. It’s because the built-in language assist offers solely low-level primitives. The library comprises all of the higher-level APIs that you just’ll work with.

Introduction to Kotlin Coroutines

A coroutine is a mechanism just like a thread. Despite the fact that they’re just like threads, coroutines are less expensive to create. That’s why they’re also known as “light-weight threads”, and that’s why you possibly can simply create many coroutines with none reminiscence overhead. You may as well consider a coroutine as a computation or a bit of labor that may be paused — suspended at a sure level after which resumed at a later time limit.

You’re most likely conscious {that a} piece of code may be blocking or non-blocking. Kotlin Coroutines convey a brand new idea of suspension into the combination. Within the subsequent part, you’ll see how suspending habits differs from blocking habits.

Suspending vs. Blocking

Suspension and blocking could sound comparable, however they’re truly fairly completely different ideas. It’s all the time simpler to clarify issues utilizing photographs, so take a look at the one beneath:

Blocking function

A blocking name to a perform signifies that the thread the perform is operating in received’t have the ability to do anything till the perform completes. Following up, because of this in the event you make a blocking perform name on the primary thread, you successfully freeze the UI. Till that blocking name finishes, the person will see a static display and received’t have the ability to work together with the app. If left on this state for 5 seconds or extra, the app will crash with the ANR (Utility Not Responding) error.

However, suspending capabilities have a particular means to pause their execution and resume it at a later time. To make a perform suspendable, you should add the droop modifier earlier than the perform.

droop enjoyable myFirstSuspendingFunction() {...}

One vital factor to know is that suspending capabilities can solely be referred to as from inside a coroutine or from different suspending capabilities. That’s as a result of they droop the coroutine — not a thread — they’re operating in. This means of suspending leaves the present thread free to do different work till the droop perform returns a end result. See the picture beneath to get a greater understanding of the idea.

Suspending function

Within the instance above, Operate B does some work and suspends on the primary thread. Whereas it’s suspended, the primary thread isn’t blocked and is free to execute usually. Operate B merely resumes its execution as soon as it’s prepared to take action. Within the subsequent part, you’ll see tips on how to use the ability of droop capabilities and coroutines to obtain a picture.

Creating Your First Coroutine

Open TutorialFragment.kt, and navigate to downloadSingleImage. Exchange // TODO: Not applied with the next code:

lifecycleScope.launch {
 val originalBitmap = getOriginalBitmap(tutorial)
 val snowFilterBitmap = loadSnowFilter(originalBitmap)

On this methodology, you mix two ideas from the Kotlin Coroutines API to launch a brand new coroutine and execute some code inside it. lifecycleScope is a coroutine scope and launch is a coroutine builder. You merely name launch on an occasion of CoroutineScope and cross a block of code that you just need to execute. That is all it takes to create a brand new coroutine and execute some code in it. Easy, proper? :]

An excellent factor when utilizing Kotlin Coroutines is that the code you write is sequential and appears just about like common blocking code. When you name getOriginalBitmap, the coroutine will droop till the bitmap is prepared. In the meantime, the thread this code runs in is free to do different work. When the bitmap turns into obtainable, the coroutine will resume and can execute loadSnowFilter and loadImage after that.

Earlier than you proceed to the following part, the place you’ll study extra about coroutine builders, study getOriginalBitmap:

non-public droop enjoyable getOriginalBitmap(tutorial: Tutorial): Bitmap =
withContext(Dispatchers.IO) {
  URL(tutorial.imageUrl).openStream().use {
    return@withContext BitmapFactory.decodeStream(it)

This methodology is invoked from contained in the coroutine you’ve simply created. Right here’s a breakdown of what it does:

  1. Discover that the strategy is marked with the droop modifier. Because of this the strategy has the flexibility to droop the execution of a coroutine it’s at the moment operating in. That is actually vital on this case as a result of the strategy is doing a heavy operation that might doubtlessly block the primary thread.
  2. withContext(Dispatchers.IO) makes positive that you just swap the heavy work to a employee thread. You’ll find out about Dispatchers and withContext within the following sections. For now, simply do not forget that that is used to dump the work to a different thread.
  3. You open a connection to the desired URL, which returns an occasion of InputStream for studying from that connection. This piece of code downloads the picture.

Construct and run the app to see what you’ve finished to this point:

Kotlin tab screenshot

You’ll see a Kotlin picture with the snow filter utilized within the first tab. In case you attempt to navigate to different tabs, you’ll solely see a placeholder picture. That’s OK for now — you’ll repair it later. Proper now, it’s time to study a bit extra about coroutine builders.

Coroutine Builders

You may create new coroutines in a few other ways. The API has a number of constructs at your disposal, every supposed for a distinct function. Since that is an introduction to Kotlin Coroutines, you’ll study solely concerning the important ones:

  • launch: Essentially the most usually used coroutine builder. It creates a brand new coroutine and returns a deal with to the newly created coroutine as a Job object. You’ll learn to use jobs for canceling a coroutine in a later part. You utilize this builder whenever you don’t must return a worth from the coroutine.
  • async: Used whenever you need to return a worth from a coroutine in a postponed means. It launches a brand new coroutine and returns a Deferred object, which comprises the operation’s end result. To get a worth from the Deferred object, you want to name await on it. You may as well use this builder to do issues in parallel. You’ll see this later within the tutorial whenever you obtain two photographs.

Discover that you just invoke the launch builder on lifecycleScope. Each async and launch are outlined as extension capabilities on CoroutineScope. Test the next part for extra particulars concerning the coroutine scope.

Coroutine Scope

A coroutine scope determines how lengthy a coroutine lives. It does that by offering a mother or father context to coroutines created inside it. That’s why each coroutine builder is outlined as an extension perform on the scope.

In Android apps, you will have two predefined scopes prepared for use: lifecycleScope and viewModelScope. In downloadSingleImage, you used lifecycleScope, which is tied to the lifecycle of the present lifecycle proprietor. Because of this all coroutines created in lifecycleScope will likely be canceled as soon as this fragment is destroyed. It is a nice idea since you don’t must maintain observe of the coroutines manually. All the things’s finished routinely for you. As a basic rule, you must use the predefined scopes to create your coroutines.


Coroutines API additionally comprises GlobalScope. This scope stays lively so long as an app is alive. It’s thought-about a fragile API as a result of it may be simply misused and trigger reminiscence leaks. You may examine the official docs for extra details about it. You received’t use it on this tutorial.

Downloading Photographs in Parallel With async

For the Kotlin tutorial, you solely downloaded one picture. Different tutorials within the app have two picture URLs. On this part, you’ll use the async coroutine builder to obtain each photographs in parallel.

Open TutorialFragment.kt, and navigate to downloadTwoImages. Exchange // TODO: Not applied with this code:

// 1
lifecycleScope.launch {
  // 2
  val deferredOne = lifecycleScope.async {
  // 3
  val deferredTwo = lifecycleScope.async {
    val originalBitmap = getOriginalBitmap(tutorial)
  // 4
  loadTwoImages(deferredOne.await(), deferredTwo.await())

Right here’s what the code does:

  1. Launches a brand new coroutine in lifecyleScope. This is identical as within the earlier instance.
  2. Creates a brand new coroutine in lifecycleScope, returns an implementation of Deferred and shops it in deferredOne‘s worth. This coroutine will obtain and present the unique picture.
  3. Creates a brand new coroutine in lifecycleScope, returns an implementation of Deferred and shops it in deferredTwo‘s worth. This coroutine will obtain and present the unique picture with the snow filter utilized.
  4. Calls await on each deferredOne and deferredTwo. This suspends the coroutine till each of the values are totally computed.

While you create a brand new coroutine through the use of async, the system begins its execution instantly, but it surely additionally returns a future worth wrapped in a Deferred object.

To get the worth, you want to name await on the deferred occasion. If the worth isn’t prepared but, the coroutine will droop. If it’s prepared, you’ll get it again instantly.

It is a very highly effective idea, and it could possibly considerably velocity up your code when you want to carry out a number of long-running operations. However what you probably have just one piece of labor and you want to return its end result? You’ll discover that reply within the subsequent part.

Construct and run the app and navigate to the Android tab to see the 2 photographs:

two images in snowy app

Returning a Single Worth From a Coroutine

After all, you should use the async builder to get a single worth, however that’s not its supposed function. As an alternative, you must use withContext. It’s a suspending perform that takes in a CoroutineContext and a block of code to execute as its parameters. An instance utilization can seem like this:

droop enjoyable getTestValue(): String = withContext(Dispatchers.Fundamental) { 
  "Take a look at" 

As a result of withContext is a suspending perform, you want to mark getTestValue with droop as properly. The primary parameter to withContext is Dispatchers.Fundamental, which implies this code will likely be executed on the primary thread. The second parameter is a lambda perform that merely returns the "Take a look at" string.

withContext isn’t used solely to return a worth. You may as well use it to modify the execution context of a coroutine. That’s why it accepts CoroutineContext as a parameter.

Coroutine Context

CoroutineContext is a set of many parts, however you received’t undergo all of them. You’ll concentrate on just some on this tutorial. One vital aspect you’ve already used is CoroutineDispatcher.

Coroutine Dispatchers

The title “dispatchers” hints at their function. They’re liable for dispatching work to 1 a thread pool. You’ll use three dispatchers most frequently:

  • Default: Makes use of a predefined pool of background threads. Use this for computationally costly coroutines that use CPU assets.
  • IO: Use this for offloading blocking IO operations to a pool of threads optimized for this sort of work.
  • Fundamental: This dispatcher is confined to Android’s essential thread. Use it when you want to work together with the UI from inside a coroutine.

Enhancing Snowy’s Efficiency

You’ll use your information about dispatchers to enhance the efficiency of your code by shifting applySnowEffect‘s execution to a different thread.

Exchange the prevailing implementation of loadSnowFilter with the next:

non-public droop enjoyable loadSnowFilter(originalBitmap: Bitmap): Bitmap =
 withContext(Dispatchers.Default) {

applySnowEffect is a CPU-heavy operation as a result of it goes by way of each pixel of a picture and does sure operations on it. To maneuver the heavy work from the primary thread, you wrap the decision with withContext(Dispatchers.Default). You’re utilizing the Default dispatcher as a result of it’s optimized for duties which can be intensive on the CPU.

Construct and run the undertaking now.

two images in snowy app

You received’t see any distinction on the display, however you possibly can connect a debugger and put a breakpoint on applySnowEffect. When the execution stops, you’ll see one thing like this:

worker thread example with debugger

You may see within the marked space that the strategy is executing in a employee thread. Because of this the primary thread is free to do different work.

Nice progress to this point! Now, it’s time to learn to cancel a operating coroutine.

Canceling a Coroutine

Cancellation performs a giant function within the Coroutines API. You all the time need to create coroutines in a means that lets you cancel them when their work is not wanted. This implies you’ll principally create coroutines in ViewModel courses or within the view layer. Each of them have well-defined lifecycles. That offers you the flexibility to cancel any work that’s not wanted when these courses are destroyed. You may cancel a number of coroutines operating in a scope by canceling all the scope. You do that by calling scope.cancel(). Within the subsequent part, you’ll learn to cancel a single coroutine.

Coroutine Job

A Job is without doubt one of the CoroutineContext parts that acts like a deal with for a coroutine. Each coroutine you launch returns a type of a Job. launch builder returns Job, whereas async builder returns Deferred. Deferred is only a Job with a end result. Thus, you possibly can name cancel on it. You’ll use jobs to cancel the execution of a single coroutine. To run the next instance, open it within the Kotlin Playground. It ought to seem like this:

import kotlinx.coroutines.*

enjoyable essential() = runBlocking {
 val printingJob = launch {
  repeat(10) { quantity ->
 println("I canceled the printing job!")

This instance does the next:

  1. Creates a brand new coroutine and shops its job to the printingJob worth.
  2. Repeats the desired block of code 10 occasions.
  3. Delays the execution of the mother or father coroutine by one second.
  4. Cancels printingJob after one second.

While you run the instance, you’ll see output like beneath:

I canceled the printing job!

Jobs aren’t used only for cancellation. They can be used to kind parent-child relationships. Have a look at the next instance within the Kotlin Playground:

import kotlinx.coroutines.*

enjoyable essential() = runBlocking {
 val parentJob = launch {
  repeat(10) { quantity -> 
   println("Dad or mum coroutine $quantity")
  launch {
   repeat(10) { quantity -> 
    println("Youngster coroutine $quantity")    

This instance does the next:

  1. Creates a mother or father coroutine and shops its job in parentJob.
  2. Creates a baby coroutine.
  3. Delays the execution of the basis coroutine by one second.
  4. Cancels the mother or father coroutine.

The output ought to seem like this:

Dad or mum coroutine 0
Dad or mum coroutine 1
Dad or mum coroutine 2
Dad or mum coroutine 3

You may see that the kid coroutine by no means obtained to execute its work. That’s as a result of whenever you cancel a mother or father coroutine, it cancels all of its youngsters as properly.

Now that you understand how to cancel coroutines, there’s yet one more vital matter to cowl — error dealing with.

Error Dealing with in Coroutines

The strategy to exception dealing with in coroutines is barely completely different relying on the coroutine builder you employ. The exception could get propagated routinely, or it might get deferred till the patron consumes the end result.

Have a look at how exceptions behave for the builders you utilized in your code and tips on how to deal with them:

  • launch: Exceptions are thrown as quickly as they occur and are propagated as much as the mother or father. Exceptions are handled as uncaught exceptions.
  • async: When async is used as a root coroutine builder, exceptions are solely thrown whenever you name await.

Coroutine Exception Handler

CoroutineExceptionHandler is one other CoroutineContext aspect that’s used to deal with uncaught exceptions. Because of this solely exceptions that weren’t beforehand dealt with will find yourself within the handler. Typically, uncaught exceptions may end up solely from root coroutines created utilizing launch builder.

Open TutorialFragment.kt, and substitute // TODO: Insert coroutineExceptionHandler with the code beneath:

non-public val coroutineExceptionHandler: CoroutineExceptionHandler =
 CoroutineExceptionHandler { _, throwable ->
  showError("CoroutineExceptionHandler: ${throwable.message}")
  println("Caught $throwable")

This code creates an occasion of CoroutineExceptionHandler and handles the incoming exception. To put in the handler, add this code instantly beneath it:

non-public val tutorialLifecycleScope = lifecycleScope + coroutineExceptionHandler

This piece of code creates a brand new coroutine scope referred to as tutorialLifecycleScope. It combines the predefined lifecycleScope with the newly created coroutineExceptionHandler.

Exchange lifecycleScope with tutorialLifecycleScope in downloadSingleImage.

non-public enjoyable downloadSingleImage(tutorial: Tutorial) {
  tutorialLifecycleScope.launch {
    val originalBitmap = getOriginalBitmap(tutorial)
    val snowFilterBitmap = loadSnowFilter(originalBitmap)

Earlier than you do this, be sure you activate airplane mode in your cellphone. That’s the best strategy to set off an exception within the code. Construct and run the app. You’ll see a display with an error message and a reload button, like beneath:

screenshot with an error

CoroutineExceptionHandler ought to solely be used as a worldwide catch-all mechanism as a result of you possibly can’t get well from an exception in it. The coroutine that threw an exception has already completed at that time.


On the subject of dealing with exceptions for a selected coroutine, you should use a strive/catch block to catch exceptions and deal with them as you’d do in regular synchronous programming with Kotlin. To see this in motion, navigate to downloadTwoImages and wrap the loadTwoImages invocation with the strive/catch block.

strive {
  loadTwoImages(deferredOne.await(), deferredTwo.await())
} catch (e: Exception) {
  showError("Strive/catch: ${e.message}")

Discover that you just didn’t wrap the async builder itself with the strive/catch block as a result of the exception is simply thrown whenever you name await().

Construct and run the app once more, and navigate to the Android tab. You’ll see this display:

screenshow with an error

The place to Go From Right here?

Good job ending the tutorial! In the long run, you realized that Kotlin Coroutines aren’t simply one other software in a dusty shed referred to as asynchronous programming. The API is far more than that. It’s a brand new means to consider async programming total, which is humorous as a result of the idea dates again to the ’50s and ’60s. You noticed how simple it was to modify between threads and return values asynchronously, and also you additionally noticed how dealing with exceptions may be simple and the way cleansing up assets takes one perform name.

You may obtain the ultimate undertaking by clicking Obtain supplies on the high or backside of this tutorial.

If you wish to study extra about coroutines, take a look at our Kotlin Coroutines by Tutorials e-book. It brings an much more in-depth have a look at Kotlin coroutines and affords extra tutorials. You may as well take a look at Kotlin Coroutines Tutorial for Android: Superior. Lastly, the official coroutines information is a good studying useful resource as properly.

I hope you loved this tutorial. Be a part of us within the boards to share your experiences with Kotlin Coroutines!

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