In this article, we'll introduce you to Kotlin Coroutines, explain how they work, and show you how to use them to write efficient and concise asynchronous code.
What are Coroutines?
Coroutines are a new way of writing asynchronous, non-blocking code. They are a lightweight alternative to threads, which can be used to perform long-running tasks without blocking the main thread or freezing the application's user interface. Coroutines are a type of cooperative multitasking, which means that they allow multiple tasks to be executed simultaneously, but the tasks themselves decide when to give up control and let other tasks run.
Coroutines are based on the concept of suspending functions. A suspending function is a function that can be paused and resumed later without blocking the thread on which it's running. When a suspending function is called, it can perform some work and then suspend its execution, allowing other code to run. When the work it was waiting for is complete, it can resume its execution from where it left off.
Coroutine Builders
To create a coroutine in Kotlin, you use a coroutine builder. A coroutine builder is a function that creates a new coroutine and returns a reference to it. There are several coroutine builders available in Kotlin, each with different features and use cases.
The most common coroutine builder is launch, which creates a new coroutine that runs on the same thread as the calling code. The launch function takes a suspending lambda as its argument, which contains the code to be executed in the coroutine. Here's an example:
GlobalScope.launch {
// Code to be executed in the coroutine
}
In this example, we're using the GlobalScope object to create a new coroutine. GlobalScope is a predefined object that can be used to launch coroutines that live as long as the application is running. When we call the launch function, it creates a new coroutine that runs the code inside the lambda.
Another coroutine builder is async, which creates a coroutine that returns a value when it completes. The async function also takes a suspending lambda as its argument, but it returns a Deferred object instead of a Job object. A Deferred object represents a value that may not be available yet, but will be in the future. Here's an example:
val result: Deferred<String> = GlobalScope.async {
// Code to be executed in the coroutine
"Hello, world!"
}
In this example, we're using async to create a new coroutine that returns the string "Hello, world!" when it completes. We're also storing the Deferred object returned by async in a variable named result.
Coroutine Scopes
Coroutines are tied to a particular scope, which defines their lifetime and their relationship with other coroutines. A scope is an object that manages the coroutines launched within it, and it can be used to cancel them all at once.
There are several types of coroutine scopes available in Kotlin. The most common ones are:
1. GlobalScope: This is a predefined scope that lives as long as the application is running. Coroutines launched in the GlobalScope are not tied to any particular activity or fragment and can be accessed from anywhere in the application.
2. MainScope: This is a predefined scope that lives as long as the main thread is running. Coroutines launched in the MainScope can update the user interface and interact with the user.
3. CoroutineScope: This is a custom scope that you can create to manage coroutines in a specific context. For example, you can create a CoroutineScope for a specific activity or fragment, so that any coroutines launched in that scope are automatically cancelled when the activity or fragment is destroyed.
Coroutine Context
A coroutine context is a set of parameters that define how a coroutine behaves. The context includes the coroutine's scope, the dispatcher it runs on, and other properties such as its name and exception handler.
The dispatcher is responsible for scheduling the coroutine's execution on a thread or thread pool. There are several dispatchers available in Kotlin, including:
1. Dispatchers.Main: This dispatcher runs coroutines on the main thread, which is the UI thread in Android applications.
2. Dispatchers.IO: This dispatcher runs coroutines on a shared pool of background threads, which are optimized for I/O operations such as network requests and disk I/O.
3. Dispatchers.Default: This dispatcher runs coroutines on a shared pool of background threads, which are optimized for CPU-bound operations such as complex calculations and sorting algorithms.
Here's an example of creating a CoroutineScope with a custom dispatcher:
val scope = CoroutineScope(Dispatchers.IO)
In this example, we're creating a CoroutineScope that uses the Dispatchers.IO dispatcher. Any coroutines launched within this scope will be scheduled to run on the I/O thread pool.
Cancellation and Exception Handling
One of the benefits of using coroutines is that they make it easy to cancel long-running operations. When a coroutine is cancelled, its execution is interrupted, and any resources it's using are released. This makes it easy to write code that gracefully handles cancellation and avoids memory leaks.
To cancel a coroutine, you can call its cancel method. Here's an example:
val job = GlobalScope.launch {
// Code to be executed in the coroutine
}
job.cancel()
In this example, we're creating a new coroutine using the launch function and storing a reference to the Job object returned by launch in a variable named job. We can then call the cancel method on the job to cancel the coroutine.
Coroutines also provide a way to handle exceptions that occur during their execution. By default, any unhandled exception in a coroutine will be propagated to its parent coroutine or the main thread, which can cause the application to crash. To handle exceptions gracefully, you can use a try-catch block inside the coroutine. Here's an example:
GlobalScope.launch {
try {
// Code that may throw an exception
} catch (e: Exception) {
// Handle the exception
}
}
In this example, we're using a try-catch block to catch any exceptions that occur during the execution of the coroutine.
Conclusion
Kotlin Coroutines offer a simpler and more efficient way of writing asynchronous code in Android applications. By using suspending functions, coroutine builders, coroutine scopes, coroutine contexts, and exception handling, you can create responsive and reliable applications without having to deal with the complexity of traditional asynchronous programming.
In this article, we've introduced you to the basics of Kotlin Coroutines, but there's still a lot more to learn. We encourage you to continue exploring coroutines and to try them out in your own applications. With coroutines, you can write code that is both more readable and more efficient, leading to better user experiences and happier users.