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https://github.com/algoflows/mastering-kotlin-coroutines

Masting Kotlin coroutines use cases
https://github.com/algoflows/mastering-kotlin-coroutines

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Masting Kotlin coroutines use cases

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README 

          
![CoroutineUsecasesOnAndroid](documentation/images/Logo-new.png)



# Kotlin Coroutines - Use Cases on Android



🎓 Learning Kotlin Coroutines for Android by example. 



🚀 Sample implementations for real-world Android use cases. 



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



## 🔧 Project Setup



Every use case is using its own `Activity` and `JetPack ViewModel`. The `ViewModel`s contain all the interesting Coroutine related code.

`Activities` listen to `LiveData` events of the `ViewModel` and render received `UiState`s.



This project is using retrofit/okhttp together with a `MockNetworkInterceptor`. 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.



## ✍️ Related blog posts

* Why exception handling with Kotlin Coroutines is so hard and how to successfully master it! [[link](https://www.lukaslechner.com/why-exception-handling-with-kotlin-coroutines-is-so-hard-and-how-to-successfully-master-it/)]

* Understanding Kotlin Coroutines with this mental model [[link](https://www.lukaslechner.com/understanding-kotlin-coroutines-with-this-mental-model/)]

* Do I need to call suspend functions of Retrofit and Room on a background thread? [[link](https://www.lukaslechner.com/do-i-need-to-call-suspend-functions-of-retrofit-and-room-on-a-background-thread/)]

* Comparing Kotlin Coroutines with Callbacks and RxJava [[link](https://www.lukaslechner.com/comparing-kotlin-coroutines-with-callbacks-and-rxjava/)]

* How to run an expensive calculation with Kotlin Coroutines on the Android Main Thread without freezing the UI [[link](https://www.lukaslechner.com/how-to-run-an-expensive-calculation-with-kotlin-coroutines-on-the-android-main-thread-without-freezing-the-ui/)]



Sign up to my [newsletter](https://www.lukaslechner.com/newsletter/) to never miss a new blog post. I will publish new blog posts about Coroutines on a regular basis.



## 🎓 Online Course



This project is the foundation of a comprehensive Online Course about "Mastering Kotlin Coroutines for Android Development",on which I am currently working on.



Sign up to my [newsletter](https://www.lukaslechner.com/newsletter/) to get more information once it is released!



## ⭐️ Use Cases

1. [Perform single network request](#1-perform-single-network-request)

2. [Perform two sequential network requests](#2-perform-two-sequential-network-requests)

3. [Perform several network requests concurrently](#3-perform-several-network-requests-concurrently)

4. [Perform variable amount of network requests](#4-perform-variable-amount-of-network-requests)

5. [Perform a network request with timeout](#5-perform-network-request-with-timeout)

6. [Retrying network requests](#6-retrying-network-requests)

7. [Network requests with timeout and retry](#7-network-requests-with-timeout-and-retry)

8. [Room and Coroutines](#8-room-and-coroutines)

9. [Debugging Coroutines](#9-debugging-coroutines)

10. [Offload expensive calculation to background thread](#10-offload-expensive-calculation-to-background-thread)

11. [Cooperative Cancellation](#11-cooperative-cancellation)

12. [Offload expensive calculation to several Coroutines](#12-offload-expensive-calculation-to-several-coroutines)

13. [Exception Handling](#13-exception-handling)

14. [Continue Coroutine execution even when the user leaves the screen](#14-continue-coroutine-execution-when-the-user-leaves-the-screen)

15. [Using WorkManager with Coroutines](#15-using-workmanager-with-coroutines)

16. [Performance analysis of dispatchers, number of coroutines and yielding](#16-performance-analysis-of-dispatchers-number-of-coroutines-and-yielding)

17. [Perform expensive calculation on Main Thread without freezing the UI](#17-perform-expensive-calculation-on-main-thread-without-freezing-the-ui)



## 📄 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.



[[code](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase1/PerformSingleNetworkRequestViewModel.kt)]



### 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](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase2/callbacks/SequentialNetworkRequestsCallbacksViewModel.kt).

The other one uses [RxJava](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase2/rx/SequentialNetworkRequestsRxViewModel.kt). 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.



[[code](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase2/Perform2SequentialNetworkRequestsViewModel.kt)]



### 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.



[[code](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase3/PerformNetworkRequestsConcurrentlyViewModel.kt)]



### 4. Perform variable amount of network requests



Demonstrates the simple usage of `map()` 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.



[[code](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase4/VariableAmountOfNetworkRequestsViewModel.kt)]



### 5. Perform network request with timeout



This use case uses the suspending function `withTimeout()` from the coroutines-core library. It throws a `TimeoutCancellationException` 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](https://square.github.io/okhttp/recipes/#timeouts-kt-java).



[[code](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase5/NetworkRequestWithTimeoutViewModel.kt)]



### 6. Retrying network requests



Demonstrates the usage of higher order functions together with coroutines. The higher order function `retry()` 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.



[[code](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase6/RetryNetworkRequestViewModel.kt)]



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 `retry()` and `withTimeout()`. 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](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase7/callbacks/TimeoutAndRetryCallbackViewModel.kt) to see how tricky this use case is to implement without Coroutines.



I also implemented the use case with [RxJava](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase7/rx/TimeoutAndRetryRxViewModel.kt).



[[code](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase7/TimeoutAndRetryViewModel.kt)]



### 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 `View` requests data, the `ViewModel` 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.



[[code](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase8/RoomAndCoroutinesViewModel.kt)]



### 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 `Thread.currentThread.name()`

This is done by enabling Coroutine Debug mode by setting the property `kotlinx.coroutines.debug` to `true`.



[[code](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase9/DebuggingCoroutinesViewModel.kt)]



### 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!**



[[code](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase10/CalculationInBackgroundViewModel.kt)]



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 `toString()` conversion.



There are several ways to make your coroutines cooperative regarding cancellation: You can use either use `isActive()`, `ensureActive()` or `yield()`.

More information about cancellation can be found [here](https://medium.com/androiddevelopers/exceptions-in-coroutines-ce8da1ec060c)



[[code](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase11/CooperativeCancellationViewModel.kt)]



### 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](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase12/CalculationInSeveralCoroutinesViewModel.kt)]

[[code factorial calculator](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase12/FactorialCalculator.kt)]



### 13. Exception Handling



This use case demonstrates different ways of handling exceptions using `try/catch` and `CoroutineExceptionHandler`. It also demonstrates when you should to use `supervisorScope{}`: 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.



[[code](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase13/ExceptionHandlingViewModel.kt)]



### 14. Continue Coroutine execution when the user leaves the screen



Usually, when the user leaves the screen, the `ViewModel` gets cleared and all the coroutines launched in `viewModelScope` 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 `AndroidVersionRepositoryTest` also verifies this behavior. Check out this [blogpost](https://medium.com/androiddevelopers/coroutines-patterns-for-work-that-shouldnt-be-cancelled-e26c40f142ad) for details of the implementation.



[[code viewmodel](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase14/ContinueCoroutineWhenUserLeavesScreenViewModel.kt)]

[[code repository](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase14/AndroidVersionRepository.kt)]



### 15. Using WorkManager with Coroutines



Demonstrates how you can use WorkManager together with Coroutines. When creating a subclass of `CoroutineWorker` instead of `Worker`,

the `doWork()` 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](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase15/WorkManagerViewModel.kt)]

[[code worker](app/src/main/java/com/lukaslechner/coroutineusecasesonandroid/usecases/coroutines/usecase15/AnalyticsWorker.kt)]



### 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 `yield()` 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 `yield()` 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](https://www.lukaslechner.com/how-to-run-an-expensive-calculation-with-kotlin-coroutines-on-the-android-main-thread-without-freezing-the-ui/)] for more information!



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



## 👷‍♀️ Contributing 👷‍♂️



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.



## Author



![LukasLechner](documentation/images/lukle.png)



[Lukas Lechner](https://www.lukaslechner.com)



[Twitter](https://twitter.com/LukasLechnerDev) 



## License



Licensed under the Apache License, Version 2.0 (the "License").

You may obtain a copy of the License at



   http://www.apache.org/licenses/LICENSE-2.0



Unless required by applicable law or agreed to in writing, software

distributed under the License is distributed on an "AS IS" BASIS,

WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

See the License for the specific language governing permissions and

limitations under the License.



You agree that all contributions to this repository, in the form of fixes, pull-requests, new examples etc. follow the above-mentioned license.