Concurrency and coroutines
When working with mobile platforms, you may need to write multithreaded code that runs in parallel. For this, you can use the standard
kotlinx.coroutines library or its multithreaded version and alternative solutions.
Review the pros and cons of each solution and choose the one that works best for your situation.
Learn more about concurrency, the current approach, and future improvements.
Coroutines are light-weight threads that allow you to write asynchronous non-blocking code. Kotlin provides the
kotlinx.coroutines library with a number of high-level coroutine-enabled primitives.
The current version of
kotlinx.coroutines, which can be used for iOS, supports usage only in a single thread. You cannot send work to other threads by changing a dispatcher.
For Kotlin 1.4.30, the recommended coroutines version is
You can suspend execution and do work on other threads while using a different mechanism for scheduling and managing that work. However, this version of
kotlinx.coroutines cannot change threads on its own.
There is also another version of
kotlinx.coroutines that provides support for multiple threads.
Get acquainted with the main concepts for using coroutines:
- Asynchronous vs. parallel processing
- Dispatcher for changing threads
- Frozen captured data
- Frozen returned data
Asynchronous vs. parallel processing
Asynchronous and parallel processing are different.
Within a coroutine, the processing sequence may be suspended and resumed later. This allows for asynchronous, non-blocking code, without using callbacks or promises. That is asynchronous processing, but everything related to that coroutine can happen in a single thread.
The following code makes a network call using Ktor. In the main thread, the call is initiated and suspended, while another underlying process performs the actual networking. When completed, the code resumes in the main thread.
That is different from parallel code that needs to be run in another thread. Depending on your purpose and the libraries you use, you may never need to use multiple threads.
Dispatcher for changing threads
Coroutines are executed by a dispatcher that defines which thread the coroutine will be executed on. There are a number of ways in which you can specify the dispatcher, or change the one for the coroutine. For example:
withContext takes both a dispatcher as an argument and a code block that will be executed by the thread defined by the dispatcher. Learn more about coroutine context and dispatchers.
To perform work on a different thread, specify a different dispatcher and a code block to execute. In general, switching dispatchers and threads works similar to the JVM, but there are differences related to freezing captured and returned data.
Frozen captured data
To run code on a different thread, you pass a
functionBlock, which gets frozen and then runs in another thread.
You will call that function as follows:
As described in the concurrency overview, a state shared between threads in Kotlin/Native must be frozen. A function argument is a state itself, which will be frozen along with anything it captures.
Coroutine functions that cross threads use the same pattern. To allow function blocks to be executed on another thread, they are frozen.
In the following example, the data class instance
dc will be captured by the function block and will be frozen when crossing threads. The
println statement will print
When running parallel code, be careful with the captured state. Sometimes it's obvious when the state will be captured, but not always. For example:
The code inside
saveData runs on another thread. That will freeze
id, but because
id is a property of the parent class, it will also freeze the parent class.
Frozen returned data
Data returned from a different thread is also frozen. Even though it's recommended that you return immutable data, you can return a mutable state in a way that doesn't allow a returned value to be changed.
It may be a problem if a mutable state is isolated in a single thread and coroutine threading operations are used for communication. If you attempt to return data that retains a reference to the mutable state, it will also freeze the data by association.
Learn more about the thread-isolated state.
However, you can still use the multithreaded version of
kotlinx.coroutines in production, taking its specifics into account.
The current version for Kotlin 1.4.30 is
To use the multithreaded version, add a dependency for the
commonMain source set in
When using other libraries that also depend on
kotlinx.coroutines, such as Ktor, make sure to specify the multithreaded version of
kotlinx-coroutines. You can do this with
Because the main version of
kotlinx.coroutines is a single-threaded one, libraries will almost certainly rely on this version. If you see
InvalidMutabilityException related to a coroutine operation, it's very likely that you are using the wrong version.
Alternatives to kotlinx-coroutines
There are a few alternative ways to run parallel code.
CoroutinesWorker is a library published by AutoDesk that implements some features of coroutines across threads using the single-threaded version of
For simple suspend functions this is a pretty good option, but it does not support Flow and other structures.
Reaktive is an Rx-like library that implements Reactive extensions for Kotlin Multiplatform. It has some coroutine extensions but is primarily designed around RX and threads.
For simpler background tasks, you can create your own processor with wrappers around platform specifics. See a simple example.
In production, you can also rely on the platform to handle concurrency. This could be helpful if the shared Kotlin code will be used for business logic or data operations rather than architecture.
To share a state in iOS across threads, that state needs to be frozen. The concurrency libraries mentioned here will freeze your data automatically. You will rarely need to do so explicitly, if ever.
If you return data to the iOS platform that should be shared across threads, ensure that data is frozen before leaving the iOS boundary.
Kotlin has the concept of frozen only for Kotlin/Native platforms including iOS. To make
freeze available in common code, you can create expect and actual implementations for
freeze, or use
stately-common, which provides this functionality. In Kotlin/Native,
freeze will freeze your state, while on the JVM it'll do nothing.
stately-common, add a dependency for the
commonMain source set in
This material was prepared by Touchlab for publication by JetBrains.