Kotlin Coroutines Cheat Sheet

Recently I was refreshing myself on kotlin coroutines, as I had worked with them before but never fulled grasped all the ins and outs of coroutines, and I decided to make some notes while I was reading through the docs (https://kotlinlang.org/docs/reference/coroutines/coroutines-guide.html).
I figured they would be helpful to other kotlin developers. Note that these are quick notes I jotted down and isn't a fully grammatical tutorial-esque article.

Summary

Coroutines are light-weight threads

CoroutineScope - Used to build coroutines, contains CoroutineContext
Scope Types:

• GlobalScope - Lifetime of the new coroutine is limited only by the lifetime of the whole application
• CoroutineScope - Is destroyed after all launched children are completed
• MainScope - Scope for UI applications and uses Dispatchers.Main

CoroutineContext - A collection of various elements. Contains Job and CoroutineDispatcher
CoroutineDispatcher - Determines the thread(s) to use
Dispatcher types:

• Unconfined - I’m working in thread main (appropriate for coroutines which neither consume CPU time nor update any shared data (like UI) confined to a specific thread)
• Default - I'm working in thread DefaultDispatcher-worker-1
• newSingleThreadContext - I’m working in thread MyOwnThread
• main runBlocking - I’m working in thread main
• newSingleThreadContext() - Creates new thread, can be used to switch between using withContext

Launch

• Launch is a coroutine builder that returns a Job
• Launch inherits the context from the scope it’s launched from
• Customizing Launch:
  • Can pass variables between threads using asContextElement
  • Can name Coroutines using CoroutineName

Async

• Async is like launch, but returns a Deferred (i.e. future)
• Async CoroutineStart.LAZY only starts when await is called

Cancellation

• Jobs can be cancelled or call join, which works like pthread_join
• Coroutine cancellation is cooperative, meaning if the coroutine isn’t checking if it’s cancelled then it won’t be cancelled
• SupervisorJob is a job where cancellation is only propagated downwards
• withTimeout creates a timeout for the coroutines (throws TimeoutCancellationException)
• When cancelled:
  • Job throws an exception which can be handled with finally or catch (CancellationException)
  • All child coroutines are cancelled as well

Suspend

• Suspend can be used inside coroutines, but can also use other suspending functions
• If a coroutine builder is inside suspend function a CoroutineScope needs to be added as well

Blocking

• runBlocking runs a new coroutine and blocks the current thread, interruptible until it’s completion
• Delay is non blocking

Flow

• Flow is an asynchronous stream of values
. Uses Emit and Collect to send and collect data
• Flow runs in the context that is provided the collector
• The code inside a flow isn’t run until the flow is collected
• Flows can be cancelled with withTimeoutOrNull
• Useful functions:
  • transform - customize data, such as emitting a header first
  • take - only take a certain amount of the flow
  • flowOn - change the context of preceding code
  • zip - combine multiple flows
  • catch - catch any exceptions of preceding code
  • onCompletion - perform any final tasks after the flow is done

Channel

• Similar to a blocking queue, a way to transfer values between coroutines
• Uses send and receive for normal values, produce and consume for streams
• Channels can be closed to indicate no more elements are coming

Actor

• Actor is an entity that can receive a class of messages using sealed classes. Meaning a consumer that takes in sealed classes

Let me know if I got anything wrong, or if I missed anything. Hope this helps!