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๐ŸŽ“ Learning Kotlin Coroutines for Android by example. ๐Ÿš€ Sample implementations for real-world Android use cases. ๐Ÿ›  Unit tests included!

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Kotlin Coroutines - Use Cases on Android

๐ŸŽ“ Learning Kotlin Coroutines for Android by example.

๐Ÿš€ Sample implementations for real-world Android use cases.

๐Ÿ›  Unit tests included!

This repository is intended to be a "Playground Project". You can quickly look up and play around with the different Coroutine Android implementations. In the

package you can play around with Coroutines examples that run directly on the JVM.

๐Ÿ”ง Project Setup

Every use case is using its own

JetPack ViewModel
. The
s contain all the interesting Coroutine related code.
listen to
events of the
and render received

This project is using retrofit/okhttp together with a

. This lets you define how the API should behave. Everything can be configured: http status codes, response data and delays. Every use case defines a certain behaviour of the Mock API. The API has 2 endpoints. One returns the names of the most recent Android versions and the other one returns the features of a certain Android version.

Unit Tests exist for most use cases.

๐Ÿฟ๏ธ Related Videos

  • Kotlin Coroutines Fundamentals Playlist [link]
  • Kotlin Coroutines Exception Handling explained [link]

โœ๏ธ Related blog posts

  • 7 Common Mistakes you might be making when using Kotlin Coroutines [link]
  • Why exception handling with Kotlin Coroutines is so hard and how to successfully master it! [link]
  • Kotlin Coroutines exception handling cheat sheet [link]
  • Understanding Kotlin Coroutines with this mental model [link]
  • Do I need to call suspend functions of Retrofit and Room on a background thread? [link]
  • Comparing Kotlin Coroutines with Callbacks and RxJava [link]
  • How to run an expensive calculation with Kotlin Coroutines on the Android Main Thread without freezing the UI [link]

Sign up to my newsletter to never miss a new blog post. I will publish new blog posts about Coroutines on a regular basis.

๐ŸŽ“ Online Course

This project is the foundation of a comprehensive Online Course about Mastering Kotlin Coroutines for Android Development


๐Ÿ“ข Sharing is Caring

If you like this project, please tell other developers about it! โค๏ธ

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โญ๏ธ Use Cases

  1. Perform single network request
  2. Perform two sequential network requests
  3. Perform several network requests concurrently
  4. Perform variable amount of network requests
  5. Perform a network request with timeout
  6. Retrying network requests
  7. Network requests with timeout and retry
  8. Room and Coroutines
  9. Debugging Coroutines
  10. Offload expensive calculation to background thread
  11. Cooperative Cancellation
  12. Offload expensive calculation to several Coroutines
  13. Exception Handling
  14. Continue Coroutine execution even when the user leaves the screen
  15. Using WorkManager with Coroutines
  16. Performance analysis of dispatchers, number of coroutines and yielding
  17. Perform expensive calculation on Main Thread without freezing the UI

๐Ÿ“„ Description

1. Perform single network request

This use case performs a single network request to get the latest Android Versions and displays them on the screen.


2. Perform two sequential network requests

This use case performs two network requests sequentially. First it retrieves recent Android Versions and then it requests the features of the latest version.

There are also 2 alternative implementations included. One is using old-school callbacks. The other one uses RxJava. You can compare each implementation. If you compare all three implementations, it is really interesting to see, in my opinion, how simple the Coroutine-based version actually is.


3. Perform several network requests concurrently

Performs three network requests concurrently. It loads the feature information of the 3 most recent Android Versions. Additionally, an implementation that performs the requests sequentially is included. The UI shows how much time each implementation takes to load the data so you can see that the network requests in the concurrent version are actually performed in parallel. The included unit test is also interesting, as it shows how you can use virtual time to verify that the concurrent version really gets performed in parallel.


4. Perform variable amount of network requests

Demonstrates the simple usage of

to perform a dynamic amount of network requests. At first, this use case performs a network request to load all Android versions. Then it performs a network request for each Android version to load its features. It contains an implementation that performs the network requests sequentially and another one that performs them concurrently.


5. Perform network request with timeout

This use case uses the suspending function

from the coroutines-core library. It throws a
if the timeout was exceeded. You can set the duration of the request in the UI and check the behaviour when the response time is bigger than the timeout.

General networking timeouts can also be configured in the okhttp client.


6. Retrying network requests

Demonstrates the usage of higher order functions together with coroutines. The higher order function

retries a certain suspending operation for a given amount of times. It uses an exponential backoff for retries, which means that the delay between retries increases exponentially. The behavior of the Mock API is defined in a way that it responses with 2 unsuccessful responses followed by a successful response.


Unit tests verify the amount of request that are performed in different scenarios. Furthermore they check if the exponential backoff is working properly by asserting the amount of elapsed virtual time.

7. Network requests with timeout and retry

Composes higher level functions

. Demonstrates how simple and readable code written with Coroutines can be. The mock API first responds after the timeout and then returns an unsuccessful response. The third attempt is then successful.

Take a look at the included callback-based implementation to see how tricky this use case is to implement without Coroutines.

I also implemented the use case with RxJava.


8. Room and Coroutines

This example stores the response data of each network request in a Room database. This is essential for any "offline-first" app. If the

requests data, the
first checks if there is data available in the database. If so, this data is returned before performing a network request to get fresh data.


9. Debugging Coroutines

This is not really a use case, but I wanted to show how you can add additional debug information about the Coroutine that is currently running to your logs. It will add the Coroutine name next to the thread name when calling
This is done by enabling Coroutine Debug mode by setting the property


10. Offload expensive calculation to background thread

This use case calculates the factorial of a number. The calculation is performed on a background thread using the default Dispatcher.

Attention: This use case does not support cancellation! UseCase#11 fixes this!


In the respective unit test, we have to pass the testDispatcher to the ViewModel, so that the calculation is not performed on a background thread but on the main thread.

11. Cooperative cancellation

UseCase#10 has a problem. It is not able to prematurely cancel the calculation because it is not cooperative regarding cancellation. This leads to wasted device resources and memory leaks, as the calculation is not stopped and ViewModel is retained longer than necessary. This use case now fixes this issue. The UI now also has a "Cancel Calculation" Button. Note: Only the calculation can be cancelled prematurely but not the


There are several ways to make your coroutines cooperative regarding cancellation: You can use either use

. More information about cancellation can be found here


12. Offload expensive calculation to several Coroutines

The factorial calculation here is not performed by a single coroutine, but by an amount of coroutines that can be defined in the UI. Each coroutine calculates the factorial of a sub-range.

[code viewmodel] [code factorial calculator]

13. Exception Handling

This use case demonstrates different ways of handling exceptions using

. It also demonstrates when you should to use
: In situations when you don't want a failing coroutine to cancel its sibling coroutines. In one implementation of this use case, the results of the successful responses are shown even tough one response wasn't successful.


14. Continue Coroutine execution when the user leaves the screen

Usually, when the user leaves the screen, the

gets cleared and all the coroutines launched in
get cancelled. Sometimes, however, we want a certain coroutine operation to be continued when the user leaves the screen. In this use case, the network request keeps running and the results still get inserted into the database cache when the user leaves the screen. This makes sense in real world application as we don't want to cancel an already started background "cache sync".

You can test this behavior in the UI by clearing the database, then loading the Android version and instantly close the screen. You will see in LogCat that the response still gets executed and the result still gets stored. The respective unit test

also verifies this behavior. Check out this blogpost for details of the implementation.

[code viewmodel] [code repository]

15. Using WorkManager with Coroutines

Demonstrates how you can use WorkManager together with Coroutines. When creating a subclass of

instead of
, the
function is now a
suspend function
which means that we can now call other suspend functions. In this example, we are sending an analytics request when the user enters the screen, which is a nice use case for using WorkManager.

[code viewmodel] [code worker]

16. Performance analysis of dispatchers, number of coroutines and yielding

This is an extension of use case #12 (Offload expensive calculation to several coroutines). Here it is possible to additionally define the dispatcher type you want the calculation to be performed on. Additionally, you can enable or disable the call to

during the calculation. A list of calculations is displayed on the bottom in order to be able to compare them in a convenient way.

17. Perform expensive calculation on Main Thread without freezing the UI

This example shows how you can perform an expensive calculation on the main thread in a non-blocking fashion. It uses

for every step in the calculation so that other work, like drawing the UI, can be performed on the main thread. It is more a "showcase" rather than a use case for a real application, because of performance reasons you should always perform expensive calculations on a background thread (See UseCase#10). See [this blog post] for more information!

You can play around and check the performance of different configurations!

๐Ÿ‘ทโ€โ™€๏ธ Contributing ๐Ÿ‘ทโ€โ™‚๏ธ

I am currently learning Coroutines myself. So if you have any ideas for or improvements or other use cases, feel free to create a pull request or an issue.



Lukas Lechner



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