Coroutines were added to Kotlin in version 1.1 and have been gaining popularity ever since. Numerous high-profile projects have switched to using Kotlin coroutines, including Google's Android Architecture Components, Spring Boot, and the ktor web framework.
Coroutines are a way of structuring concurrent code in a more natural way than threads. They allow you to write code that is more responsive, elastic, and maintainable. In this post, we'll take a look at how to use Kotlin coroutines to build concurrent applications.
A coroutine is a light-weight thread that is managed by the Kotlin runtime. Coroutines can be created with the launch or async functions from the kotlinx.coroutines package.
Coroutines are like regular threads, but they are much lighter-weight and consume less resources. They are also easier to work with, as we'll see later on.
Let's take a look at how to create a basic coroutine. We'll use the launch function to create a coroutine that runs in the background. The launch function takes a lambda as an argument, which contains the code that will be executed in the coroutine.
val job = launch {
// This code will be executed in the coroutine
}
When a coroutine is launched, it will run in the background and will not block the main thread. However, sometimes we'll want to wait for a coroutine to finish before continuing. This can be done with the join function.
job.join() // Wait for the coroutine to finish
A coroutine can be suspended with the suspend keyword. This keyword can only be used inside a coroutine. When a coroutine is suspended, it will not block the main thread.
suspend fun foo() {
// This code will be executed in the coroutine
}
A coroutine can be resumed with the resume function. This function can only be used inside a coroutine. When a coroutine is resumed, it will continue running from the point where it was last suspended.
foo.resume() // Resume the coroutine
A coroutine can be cancelled with the cancel function. This function will cause the coroutine to terminate. Any code that has not yet been executed in the coroutine will not be executed.
job.cancel() // Cancel the coroutine
A coroutine can be made to timeout with the withTimeout function. This function will cause the coroutine to terminate if it does not finish before the specified time limit.
withTimeout(1000L) {
// This code will be executed in the coroutine
}
Coroutines can be used with exception handling just like regular threads. The try/catch keywords can be used to catch exceptions that occur in a coroutine.
try {
// This code will be executed in the coroutine
} catch (e: Exception) {
// This code will be executed if an exception occurs
}
Data can be passed into a coroutine with the passData function. This function will cause the data to be available in the coroutine when it is resumed.
val data = passData("some data")
Data can be returned from a coroutine with the returnData function. This function will return the data from the coroutine to the caller.
val data = returnData()
A coroutine scope can be created with the coroutineScope function. This function will create a new coroutine scope. All coroutines created in the scope will be children of the scope. When the scope is cancelled, all of its children will be cancelled.
val scope = coroutineScope {
// This code will be executed in the scope
}
A child coroutine can be created with the launchChild function. This function will create a new coroutine that is a child of the current coroutine. When the parent coroutine is cancelled, the child coroutine will be cancelled.
val child = launchChild {
// This code will be executed in the child coroutine
}
The coroutine context can be accessed with the coroutineContext property. This property will return the current context of the coroutine. The context can be used to access information about the coroutine, such as its parent coroutine or its job.
val context = coroutineContext
The coroutine job can be accessed with the coroutineJob property. This property will return the current job of the coroutine. The job can be used to cancel the coroutine or to wait for it to finish.
val job = coroutineJob
The parent coroutine can be accessed with the coroutineParent property. This property will return the parent coroutine of the current coroutine. The parent coroutine can be used to cancel the current coroutine or to wait for it to finish.
val parent = coroutineParent
Dispatchers can be accessed with the coroutineDispatcher property. This property will return the dispatcher of the current coroutine. The dispatcher can be used to control how the coroutine runs.
val dispatcher = coroutineDispatcher
Different dispatchers can be used to control how a coroutine runs. The runBlocking function will run a coroutine in the blocking dispatcher. The withContext function will run a coroutine in the specified dispatcher.
// Run the coroutine in the blocking dispatcher
runBlocking {
// This code will be executed in the blocking dispatcher
}
// Run the coroutine in the Default dispatcher
withContext(Dispatchers.Default) {
// This code will be executed in the Default dispatcher
}
Dispatchers can be switched with the switchTo function. This function will cause the current coroutine to switch to the specified dispatcher.
// Switch to the Default dispatcher
switchTo(Dispatchers.Default)
// Switch to the blocking dispatcher
switchTo(Dispatchers.Main)
Thread-local data can be accessed with the threadLocalData property. This property will return the data for the current thread. The data can be used to store information about the thread, such as its name or its ID.
val data = threadLocalData
The coroutine name can be accessed with the coroutineName property. This property will return the name of the current coroutine. The name can be used for debugging or logging purposes.
val name = coroutineName
Coroutines can be named with the withName function. This function will cause the specified name to be associated with the current coroutine.
// Name the coroutine "foo"
withName("foo") {
// This code will be executed in the coroutine "foo"
}
The coroutine ID can be accessed with the coroutineId property. This property will return the ID of the current coroutine. The ID can be used for debugging or logging purposes.
val id = coroutineId
A coroutine can be terminated with the coroutineTerminate function. This function will cause the coroutine to terminate immediately. Any code that has not yet been executed in the coroutine will not be executed.
coroutineTerminate() // Terminate the coroutine