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Kotlin Coroutines: An Intro Practical Usage on Android Nick Capurso / @nickcapurso

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whoami Nick Capurso @nickcapurso

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whoareyou This talk is intended for those who: ● Are Android developers* with Kotlin experience ● Have familiarity with asynchronous programming, especially for making API calls. ● Have some experience at least one of: ○ AsyncTask (Android) ○ Executor ○ Future ○ RxJava

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Outline ● Framing example ● Issues with current async solutions ● Intro to Kotlin Coroutines ○ What is a “coroutine”? ○ Main usage & concepts ○ Thread pools ○ Cancellation ○ Error handling ○ Testing ● Tips for usage in an existing app

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Take an example... Simple Email App To retrieve user’s emails: ● Retrieve an OAuth token. ● Make the Emails API call.

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Take an example... Simple Email App What makes this an interesting example?

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Take an example... Simple Email App What makes this an interesting example? ● An OAuth token can be retrieved asynchronously from the network…

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Take an example... Simple Email App What makes this an interesting example? ● An OAuth token can be retrieved asynchronously from the network… ● … or synchronously if stored in a cache.

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Take an example... Simple Email App What makes this an interesting example? ● An OAuth token can be retrieved asynchronously from the network… ● … or synchronously if stored in a cache. ● The “retrieve emails” call can potentially kick off two API calls (OAuth + Emails)...

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Take an example... Simple Email App What makes this an interesting example? ● An OAuth token can be retrieved asynchronously from the network… ● … or synchronously if stored in a cache. ● The “retrieve emails” call can potentially kick off two API calls (OAuth + Emails)... ● … and either one could fail.

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Take an example... Simple Email App What makes this an interesting example? ● An OAuth token can be retrieved asynchronously from the network… ● … or synchronously if stored in a cache. ● The “retrieve emails” call can potentially kick off two API calls (OAuth + Emails)... ● … and either one could fail. ● Finally, you may want to cancel either call.

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Take an example... UI Layer Repository Cache Network

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Take an example… UI Layer Repository Cache Network Retrieve Emails

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Take an example... UI Layer Repository Cache Network Do OAuth Retrieve Emails

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Take an example… UI Layer Repository Cache Network Render result on UI

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Take an example... UI Layer Repository Cache Network

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Take an example... class Repository(private val cache: Cache, private val network: Network) { fun retrieveOAuthToken() {} fun retrieveEmails() {} }

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Take an example... fun retrieveEmails() { // Get an OAuth token (could be cached else involves a network call) // Use token to call the Emails API }

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Take an example... How might we implement this? 1. Need to notify caller of an asynchronous result. 2. Need an abstraction for writing asynchronous code. 3. Other needs: a. Support making one call dependent on another (OAuth -> Emails). b. Support cancellation.

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Take an example... How might we implement this? 1. Need to notify caller of an asynchronous result. 2. Need an abstraction for writing asynchronous code. 3. Other needs: a. Support making one call dependent on another (OAuth -> Emails). b. Support cancellation.

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Take an example... How might we implement this? 1. Need to notify caller of an asynchronous result. 2. Need an abstraction for writing asynchronous code. 3. Other needs: a. Support making one call dependent on another (OAuth -> Emails). b. Support cancellation.

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Take an example… with callbacks fun retrieveOAuthToken(callback: (OAuthToken) -> Unit): Unit fun retrieveEmails(callback: (List) -> Unit): Unit

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Take an example… with callbacks fun retrieveEmails(callback: (List) -> Unit): Unit { // Get an OAuth Token retrieveOAuthToken { token -> network.callEmailsApi(token) { emails -> // Maybe do some processing on the list ... // ... callback.invoke(emails) } } }

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Take an example… with callbacks fun retrieveEmails(callback: (List) -> Unit): Unit { // Get an OAuth Token retrieveOAuthToken { token -> network.callEmailsApi(token) { emails -> // Maybe do some processing on the list ... // ... callback.invoke(emails) } } }

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Take an example… with callbacks fun retrieveEmails(callback: (List) -> Unit): Unit { // Get an OAuth Token retrieveOAuthToken { token -> network.callEmailsApi(token) { emails -> // Maybe do some processing on the list ... // ... callback.invoke(emails) } } }

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Take an example… with callbacks fun retrieveEmails(callback: (List) -> Unit): Unit { // Get an OAuth Token retrieveOAuthToken { token -> network.callEmailsApi(token) { emails -> // Maybe do some processing on the list ... // ... callback.invoke(emails) } } }

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Take an example… with callbacks fun retrieveEmails(callback: (List) -> Unit): Unit { // Get an OAuth Token retrieveOAuthToken { token -> network.callEmailsApi(token) { emails -> // Maybe do some processing on the list ... // ... callback.invoke(emails) } } } How do we “cancel”?

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Take an example... fun retrieveEmails() { // Get an OAuth token (could be cached else involves a network call) // Use token to call the Emails API }

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Take an example… with Future fun retrieveOAuthToken(): Future fun retrieveEmails(): Future>

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Take an example… with Future fun retrieveOAuthToken(): Future fun retrieveEmails(): Future> ● Support out-of-the box for Android is lacking (Java 7 Futures): ○ No way to (easily) compose multiple operations. ● Supports cancelation and waiting with a timeout.

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Take an example… with RxJava fun retrieveOAuthToken(): Single fun retrieveEmails(): Single>

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Take an example… with RxJava fun retrieveOAuthToken(): Single fun retrieveEmails(): Single> ● Powerful, easy to compose multiple operations, supports cancelation, thread pools, error handling, etc. ● High learning curve. ● See the extra links at the end of the talk -- RxJava vs. Coroutines

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Coroutines

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What is a coroutine? “Coroutines are computer-program components that … [allows] multiple entry points for suspending and resuming execution at certain locations.” - Wikipedia

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What is a coroutine? A function that can stop executing (“suspend”) and continue where it left off* later. *implies some kind of state is kept

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What is a coroutine? A function that can stop executing (“suspend”) and continue where it left off* later. *implies some kind of state is kept

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What is a coroutine? A function that can stop executing (“suspend”) and continue where it left off* later. *implies some kind of state is kept

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What is a coroutine? A function that is “blocking” stops the thread from performing other work. Diagrams: https://medium.com/@elye.project/understanding-suspend-function-of-coroutines-de26b070c5ed

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What is a coroutine? A function that is “suspending” is not blocking. Other work can be done on the same thread. Diagrams: https://medium.com/@elye.project/understanding-suspend-function-of-coroutines-de26b070c5ed

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What is a coroutine? A function that is “suspending” is not blocking. Other work can be done on the same thread. Diagrams: https://medium.com/@elye.project/understanding-suspend-function-of-coroutines-de26b070c5ed

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What is a coroutine? Because coroutines can suspend and resume execution:

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What is a coroutine? Because coroutines can suspend and resume execution: ● A single thread can potentially do more work.

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What is a coroutine? Because coroutines can suspend and resume execution: ● A single thread can potentially do more work. ● You can write asynchronous code that looks sequential.

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What is a coroutine? Because coroutines can suspend and resume execution: ● A single thread can potentially do more work. ● You can write asynchronous code that looks sequential. ○ Writable ○ Maintainable ○ Testable

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails }

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11 T

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11 suspend at line 3

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11 suspend at line 3

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What is a coroutine? While the function is suspended waiting for the response, we can actually run something else on the same thread. suspend at line 3

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What is a coroutine? fun retrieveProfilePicture(): Bitmap { // ... } While the function is suspended waiting for the response, we can actually run something else on the same thread. suspend at line 3

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11 resume at line 3

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11 resume at line 3

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11 suspend at line 6

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11 suspend at line 6

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11 resume at line 6

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11

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What is a coroutine? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11

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What are Kotlin Coroutines? fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11

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What are Kotlin Coroutines? suspend fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } 1 2 3 4 5 6 7 8 9 10 11

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What are Kotlin Coroutines? compile 'org.jetbrains.kotlinx:kotlinx-coroutines-core:0.23.4' // More on this later... kotlin { experimental { coroutines 'enable' } } Added to your project via Gradle:

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What are Kotlin Coroutines? suspend fun retrieveEmails(): List Enables the use of the keyword “suspend” to mark functions that may suspend (e.x. to do an asynchronous action)

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What are Kotlin Coroutines? suspend fun retrieveEmails(): List Suspend functions can also directly return data even though the data may be retrieved asynchronously.

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What are Kotlin Coroutines? suspend fun retrieveEmails(): List A combination of generated code + the coroutines dependencies is used to efficiently manage threads and suspend / resume coroutines.

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What are Kotlin Coroutines? suspend fun retrieveOAuthToken(): OAuthToken suspend fun retrieveEmails(): List class Network { suspend fun callEmailsApi(token: OAuthToken): List The catch: suspend functions can only be called from other suspend functions.

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What are Kotlin Coroutines? suspend fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails }

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What are Kotlin Coroutines? suspend fun retrieveEmails(search: String): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing … val filtered = emails.filter { it.title.contains(search) } return filtered }

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Coroutine Builders non-suspend -> suspend Problem: you can’t call suspend functions from non-suspend functions. fun onClick() { val emails = repository.retrieveEmails() // ... }

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Coroutine Builders non-suspend -> suspend In other words, only coroutines can call suspend functions. How do you start a coroutine in the first place? fun onClick() { val emails = repository.retrieveEmails() // ... }

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Coroutine Builders non-suspend -> suspend Use “coroutine builders” to start coroutines.

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Coroutine Builders non-suspend -> suspend Use “coroutine builders” to start coroutines. All coroutine builders takes in a lambda and runs that lambda as a coroutine.

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Coroutine Builders non-suspend -> suspend Use “coroutine builders” to start coroutines. All coroutine builders takes in a lambda and runs that lambda as a coroutine -- i.e. enables you to call suspend functions.

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Coroutine Builders non-suspend -> suspend Use “coroutine builders” to start coroutines and allow you to call suspend functions: ● launch ● async ● runBlocking

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Coroutine Builders - launch non-suspend -> suspend launch - returns immediately and executes the lambda on a separate thread* (as a coroutine) fun onClick() { // Want to call retrieveEmails(), but need to be in a suspend function / // coroutine first }

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Coroutine Builders - launch non-suspend -> suspend launch - returns immediately and executes the lambda on a separate thread* (as a coroutine) fun onClick() { // Want to call retrieveEmails(), but need to be in a suspend function / // coroutine first launch { // I'm now within a coroutine, so I can call suspend functions val emails = repository.retrieveEmails() // Do something with the emails... } }

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Coroutine Builders - launch non-suspend -> suspend fun onClick() { // ... launch { // This code executes as a coroutine val emails = repository.retrieveEmails() // Process emails / display on UI } // ... } 1 2 3 4 5 6 7 8 9 10 11 12

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Coroutine Builders - launch non-suspend -> suspend fun onClick() { // ... launch { // This code executes as a coroutine val emails = repository.retrieveEmails() // Process emails / display on UI } // ... } 1 2 3 4 5 6 7 8 9 10 11 12

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Coroutine Builders - launch non-suspend -> suspend fun onClick() { // ... launch { // This code executes as a coroutine val emails = repository.retrieveEmails() // Process emails / display on UI } // ... } 1 2 3 4 5 6 7 8 9 10 11 12

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Coroutine Builders - launch non-suspend -> suspend fun onClick() { // ... launch { // This code executes as a coroutine val emails = repository.retrieveEmails() // Process emails / display on UI } // ... } 1 2 3 4 5 6 7 8 9 10 11 12

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Coroutine Builders - launch non-suspend -> suspend fun onClick() { // ... launch { // This code executes as a coroutine val emails = repository.retrieveEmails() // Process emails / display on UI } // ... } 1 2 3 4 5 6 7 8 9 10 11 12 launch

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Coroutine Builders - launch non-suspend -> suspend fun onClick() { // ... launch { // This code executes as a coroutine val emails = repository.retrieveEmails() // Process emails / display on UI } // ... } 1 2 3 4 5 6 7 8 9 10 11 12 launch

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Coroutine Builders - launch non-suspend -> suspend fun onClick() { // ... launch { // This code executes as a coroutine val emails = repository.retrieveEmails() // Process emails / display on UI } // ... } 1 2 3 4 5 6 7 8 9 10 11 12 launch suspend at line 6

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Coroutine Builders - launch non-suspend -> suspend fun onClick() { // ... launch { // This code executes as a coroutine val emails = repository.retrieveEmails() // Process emails / display on UI } // ... } 1 2 3 4 5 6 7 8 9 10 11 12 launch suspend at line 6

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Coroutine Builders - launch non-suspend -> suspend fun onClick() { // ... launch { // This code executes as a coroutine val emails = repository.retrieveEmails() // Process emails / display on UI } // ... } 1 2 3 4 5 6 7 8 9 10 11 12 launch resume at line 6

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Coroutine Builders - launch non-suspend -> suspend fun onClick() { // ... launch { // This code executes as a coroutine val emails = repository.retrieveEmails() // Process emails / display on UI } // ... } 1 2 3 4 5 6 7 8 9 10 11 12 launch

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Coroutine Builders - async non-suspend -> suspend async - returns a Deferred, which acts just like a Future. fun foo() { async { repository.retrieveEmails() } }

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Coroutine Builders - async non-suspend -> suspend async - returns a Deferred, which acts just like a Future. fun foo() { val pendingEmails: Deferred> = async { repository.retrieveEmails() } }

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Coroutine Builders - async non-suspend -> suspend async - returns a Deferred, which acts just like a Future. Call await() to get the data. val emails = pendingEmails.await() // Do something with emails ...

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Coroutine Builders - async non-suspend -> suspend async - returns a Deferred, which acts just like a Future. Call await() to get the data. Though, await() is a suspend function. launch { val emails = pendingEmails.await() // Do something with emails ... }

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Coroutine Builders - async non-suspend -> suspend Async is useful for running suspend functions in parallel. val pendingEmails = async { repository.retrieveEmails() } val pendingPicture = async { repository.retrieveProfilePicture() } val emails = pendingEmails.await() val picture = pendingPicture.await() // ...

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Coroutine Builders - async non-suspend -> suspend Async is useful for running suspend functions in parallel. val pendingEmails = async { repository.retrieveEmails() } val pendingPicture = async { repository.retrieveProfilePicture() } val emails = pendingEmails.await() val picture = pendingPicture.await() // ...

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Coroutine Builders - async non-suspend -> suspend Async is useful for running suspend functions in parallel. val pendingEmails = async { repository.retrieveEmails() } val pendingPicture = async { repository.retrieveProfilePicture() } val emails = pendingEmails.await() val picture = pendingPicture.await() // ...

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Coroutine Builders - async non-suspend -> suspend Async is useful for running suspend functions in parallel. launch { val pendingEmails = async { repository.retrieveEmails() } val pendingPicture = async { repository.retrieveProfilePicture() } val emails = pendingEmails.await() val picture = pendingPicture.await() // ... }

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Coroutine Builders - async non-suspend -> suspend … whereas this would just execute both calls sequentially launch { val emails = repository.retrieveEmails() val picture = repository.retrieveProfilePicture() // ... }

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Coroutine Builders - runBlocking non-suspend -> suspend Blocks the current thread until the coroutine completes. runBlocking { // Maybe I'm expecting these to be cached, so no suspending will be done... val emails = repository.retrieveEmails() // Do something with emails ... }

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Coroutine Builders - runBlocking non-suspend -> suspend Blocks the current thread until the coroutine completes. runBlocking { // Maybe I'm expecting these to be cached, so no suspending will be done... val emails = repository.retrieveEmails() // Do something with emails ... } // Code down here doesn’t run until runBlocking completes

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What are Kotlin Coroutines? suspending function calls non-suspend function Start coroutine: launch / async / runBlocking

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Next ● Using thread pools ● Cancellation ● Error Handling ● Testing coroutines

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Using Thread Pools You can specify a thread pool (“Coroutine Dispatcher”) to use when using a coroutine builder. The default is the “CommonPool” builtin (of size numProcessors - 1).

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Using Thread Pools You can specify a thread pool (“Coroutine Dispatcher”) to use when using a coroutine builder. The default is the “CommonPool” builtin (of size numProcessors - 1). launch(CommonPool) { // Code in here runs on the CommonPool }

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Using Thread Pools You can specify a thread pool (“Coroutine Dispatcher”) to use when using a coroutine builder. The default is the “CommonPool” builtin (of size numProcessors - 1). launch(CommonPool) { // Code in here runs on the CommonPool }

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Using Thread Pools You can specify a thread pool (“Coroutine Dispatcher”) to use when using a coroutine builder. The default is the “CommonPool” builtin (of size numProcessors - 1). On Android -- careful with single & dual-core devices + CommonPool! launch(CommonPool) { // Code in here runs on the CommonPool }

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Using Thread Pools You can specify a thread pool (“Coroutine Dispatcher”) to use when using a coroutine builder. The default is the “CommonPool” builtin (of size numProcessors - 1). On Android -- careful with single & dual-core devices + CommonPool! ● Solution - use your own thread pool of an appropriate size. launch(CommonPool) { // Code in here runs on the CommonPool }

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Using Thread Pools If you already have a thread pool, you can convert it to a Coroutine Dispatcher with a built-in extension function. val executor = AsyncTask.THREAD_POOL_EXECUTOR launch(executor.asCoroutineDispatcher()) { }

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Using Thread Pools You can also create your own dispatcher of a fixed pool size. val fixedPool = newFixedThreadPoolContext(3, "fixed pool") launch(fixedPool) { }

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A note about threading... Recall: using coroutines means functions are suspended (not blocked) and the same thread can be reused for something else.

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A note about threading... Assume we have a single thread pool: val singlePool = newFixedThreadPoolContext(1, "single thread pool")

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A note about threading... launch(singlePool) { // Blocking sleep Thread.sleep(5000) Log.d("Test", "1") } launch(singlePool) { Log.d("Test", "2") }

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A note about threading... launch(singlePool) { // Blocking sleep Thread.sleep(5000) Log.d("Test", "1") } launch(singlePool) { Log.d("Test", "2") }

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A note about threading... launch(singlePool) { // Blocking sleep Thread.sleep(5000) Log.d("Test", "1") } launch(singlePool) { Log.d("Test", "2") } // Output: [5 second delay] 1 2

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A note about threading... launch(singlePool) { // Part of coroutines -- suspend-version of sleep delay(5000) Log.d("Test", "1") } launch(singlePool) { Log.d("Test", "2") }

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A note about threading... launch(singlePool) { // Part of coroutines -- suspend-version of sleep delay(5000) Log.d("Test", "1") } launch(singlePool) { Log.d("Test", "2") }

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A note about threading... launch(singlePool) { // Part of coroutines -- suspend-version of sleep delay(5000) Log.d("Test", "1") } launch(singlePool) { Log.d("Test", "2") } // Output: 2 [5 second delay] 1

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Using Thread Pools Finally, there is “Unconfined” which basically gives you synchronous behavior*. // Code up here runs on the same thread ... launch(Unconfined) { // ... as code in here }

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Using Thread Pools Finally, there is “Unconfined” which basically gives you synchronous behavior*. *Note: Unconfined does not switch back to the original thread if the thread ever changes throughout execution of your coroutine.

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Using Thread Pools The kotlinx-coroutines-android dependency adds the “UI” dispatcher launch(UI) { // Basically acts as runOnMainThread }

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Using Thread Pools The UI dispatcher is also useful for updating views after running suspend functions. launch(UI) { val emails = repository.retrieveEmails() emailsAdapter.addAll(emails) emailsAdapter.notifyDataSetChanged() }

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Using Thread Pools Though be careful... launch(UI) { val emails = repository.retrieveEmails() emailsAdapter.addAll(emails) emailsAdapter.notifyDataSetChanged() }

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Using Thread Pools suspend fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails } … this code is now going to run on the UI thread as well, risking a NetworkOnMainThreadException

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Switching Threads in a Coroutine launch { val emails = repository.retrieveEmails() withContext(UI) { emailsAdapter.addAll(emails) emailsAdapter.notifyDataSetChanged() } } If you do need to switch threads within a coroutine, use withContext()

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Cancelling Coroutines Coroutine builders also return an object to represent the work-in-progress and also give you access to a cancel() function. val job: Job = launch { ... } job.cancel() val deferred: Deferred = async { ... } deferred.cancel()

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Cancelling Coroutines You can also use these objects to check the status of the coroutine. // Can check status, if needed job.isActive job.isCompleted job.isCancelled // Or cancel the job job.cancel()

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Cancelling Coroutines What does “cancelling” do? Built-in suspend functions (await(), delay(), etc.) understand cancellation and will stop suspending & quit the coroutine on cancellation.

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Cancelling Coroutines val job = launch { repeat(5) { delay(2500) Log.d("Test", "Test") } } launch { delay(3000) job.cancel() } // Only prints "Test" once

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Cancelling Coroutines What does “cancelling” do? Otherwise, you can check the isActive flag and exit yourself.

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Cancelling Coroutines val job = launch { while (isActive) { sleep(2500) // Blocking Log.d("Test", "Test") } } launch { delay(3000) job.cancel() } // Actually prints "Test" twice before quitting

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Cancelling Coroutines “Advanced” cancelation ● Canceling nested jobs together ● Canceling multiple, separate jobs at the same time (i.e. RxJava disposables).

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Cancelling Coroutines You can also set up a “master” parent and have all your coroutines draw from it, for easy cancellation. // All coroutines will have this as its parent. val rootParent = Job() // ... launch(parent = rootParent) { … } launch(parent = rootParent) { … } // Cancels both of the above job rootParent.cancel()

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Error Handling Since you’re writing “sequentially executing” code now, you don’t have “onError” callbacks.

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Error Handling Since you’re writing “sequentially executing” code now, you don’t have “onError” callbacks. You might want to think about coming up with a data wrapper to represent “success” and “failure”.

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Error Handling Since you’re writing “sequentially executing” code now, you don’t have “onError” callbacks. You might want to think about coming up with a data wrapper to represent “success” and “failure”. sealed class Result { data class Success(val result: T) : Result() data class Error(val exception: Exception) : Result() }

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Error Handling suspend fun retrieveEmails(): Result> { … } launch { val emailResult = repository.retrieveEmails() when (emailResult) { is Result.Success -> { } is Result.Error -> { } } }

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Testing The added benefit of writing code that is sequential is that it’s easy to test. suspend fun retrieveEmails(): List { // Get an OAuth token val token = retrieveOAuthToken() // Use token to call the Emails API val emails = network.callEmailsApi(token) // Do some processing ... return emails }

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Testing The added benefit of writing code that is sequential is that it’s easy to test. @Test fun `Retrieve Emails`() { // Mock out OAuth too // ... val emailList = listOf(mock(), mock()) whenever(network.callEmailApi(any())).thenReturn(emailList) assertEquals(repository.retrieveEmails(), emailList) }

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Testing The added benefit of writing code that is sequential is that it’s easy to test. @Test fun `Retrieve Emails`() { // Mock out OAuth too // ... val emailList = listOf(mock(), mock()) whenever(network.callEmailApi(any())).thenReturn(emailList) assertEquals(repository.retrieveEmails(), emailList) }

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Testing Just one thing to take care of...

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Testing Use runBlocking when you test suspend functions. @Test fun `Retrieve Emails`() = runBlocking { // Mock out OAuth too // ... val emailList = listOf(mock(), mock()) whenever(network.callEmailApi(any())).thenReturn(emailList) assertEquals(repository.retrieveEmails(), emailList) }

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Testing Instead, I like to use this helper method, to ensure Unit is returned. fun testSuspending(test: suspend () -> Unit): Unit = runBlocking { test() }

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Testing Instead, I like to use this helper method, to ensure Unit is returned. fun testSuspending(test: suspend () -> Unit): Unit = runBlocking { test() }

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Testing Instead, I like to use this helper method, to ensure Unit is returned. @Test fun `Retrieve Emails`() = testSuspending { // Mock out OAuth too // ... val emailList = listOf(mock(), mock()) whenever(network.callEmailApi(any())).thenReturn(emailList) assertEquals(repository.retrieveEmails(), emailList) }

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Testing Keep your CoroutineDispatcher somewhere easily accessible, like a Dagger graph, in case you need to replace it during tests.

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Testing Keep your CoroutineDispatcher somewhere easily accessible, like a Dagger graph, in case you need to replace it during tests. You may need to replace it with “Unconfined” during unit tests to get synchronous / sequential execution of background coroutines.

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Testing Keep your CoroutineDispatcher somewhere easily accessible, like a Dagger graph, in case you need to replace it during tests. You may need to replace it with “Unconfined” during unit tests to get synchronous / sequential execution of background coroutines. For Android Espresso UI tests, swap in: AsyncTask.THREAD_POOL_EXECUTOR.asCoroutineDispatcher()

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Tips for starting in an existing app Replace usages of Thread { … }.start() ● Efficiency benefit by letting Kotlin handle when to explicitly start a new thread. Thread { // .. }.start() launch { // .. }

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Tips for starting in an existing app Same for ExecutorService usages. ● Convert to a CoroutineDispatcher with asCoroutineDispatcher() ● execute -> launch ● submit -> async executor.execute { // ... } val future = executor.submit { // ... } val dispatcher = executor.asCoroutineDispatcher() launch(dispatcher) { // ... } val deferred = async(dispatcher) { // ... }

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Tips for starting in an existing app Same for ExecutorService usages. ● Convert to a CoroutineDispatcher with asCoroutineDispatcher() ● execute -> launch ● submit -> async executor.execute { // ... } val future = executor.submit { // ... } val dispatcher = executor.asCoroutineDispatcher() launch(dispatcher) { // ... } val deferred = async(dispatcher) { // ... }

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Tips for starting in an existing app Same for ExecutorService usages. ● Convert to a CoroutineDispatcher with asCoroutineDispatcher() ● execute -> launch ● submit -> async executor.execute { // ... } val future = executor.submit { // ... } val dispatcher = executor.asCoroutineDispatcher() launch(dispatcher) { // ... } val deferred = async(dispatcher) { // ... }

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Tips for starting in an existing app Interop with existing libraries Retrofit (Android): ● Jake Wharton’s Deferred adapter: https://github.com/JakeWharton/retrofit2-kotlin-coroutines-adapter

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Tips for starting in an existing app Interop with existing libraries If you want to adapt an existing library to coroutines, use suspendCoroutine to make it compatible to use with suspend functions.

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Tips for starting in an existing app Interop with existing libraries If you want to adapt an existing library to coroutines, use suspendCoroutine to make it compatible to use with suspend functions. ● Allows you to suspend a coroutine “manually” handle when to resume execution. ● https://www.youtube.com/watch?v=YrrUCSi72E8

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A Note on “Experimental” Status compile 'org.jetbrains.kotlinx:kotlinx-coroutines-core:0.23.4' // Need to “enable” the experimental feature to avoid warnings kotlin { experimental { coroutines 'enable' } }

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A Note on “Experimental” Status “Kotlin coroutines can and should be used in production.” - Roman Elizarov Tech Lead on Kotlin Coroutines @ Jetbrains

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A Note on “Experimental” Status ● “Experimental” means they can release new stuff in minor updates. ● New updates will maintain backwards compatibility. ● “Unstable” or WIP features are clearly marked / annotated.

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A Note on “Experimental” Status Roman Elizarov on production usage: ● https://stackoverflow.com/questions/46240236/can-experimental-kotli n-coroutines-be-used-in-production ● https://www.slideshare.net/elizarov/introduction-to-kotlin-coroutines/ 33 ● https://github.com/Kotlin/kotlinx.coroutines/issues/110

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Kotlin Coroutines on Android: Things I Wish I Knew at the Beginning: https://medium.com/capital-one-developers/kotlin-coroutines-on-android-things-i-wish-i-knew-at-the-begi nning-c2f0b1f16cff Coroutines and RxJava — An Asynchronicity Comparison (Part 1): https://medium.com/capital-one-developers/coroutines-and-rxjava-an-asynchronicity-comparison-part-1-a synchronous-programming-e726a925342a If you want more...

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Talks by Roman Elizarov @ KotlinConf ● Introduction to Coroutines ● Deep Dive into Coroutines on JVM Coroutines Guide on GitHub: https://github.com/Kotlin/kotlinx.coroutines/blob/master/coroutines-guide.md Parent-child coroutines / canceling coroutines together: https://github.com/Kotlin/kotlinx.coroutines/blob/master/coroutines-guide.md#children-of-a-coroutine https://github.com/Kotlin/kotlinx.coroutines/blob/master/coroutines-guide.md#cancellation-via-explicit-jo b If you want more...

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Any Questions? @nickcapurso

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Any Questions? ● Write functions that suspend and resume execution -- perform asynchronous actions using code that looks sequential / synchronous. ● Asynchronous code is easier to write, test, and maintain. ● A single thread can perform more work in the same amount of time. ● Coroutines can work alongside existing asynchronous solutions and support can be written / provided for popular libraries. @nickcapurso