What is the Kotlin Flows?
Kotlin Flow is a library introduced in Kotlin 1.4 that provides a powerful and flexible way to build asynchronous and reactive applications. It is an improvement over the previous approaches such as RxJava and LiveData, and it is designed to be more performant, easier to use, and more flexible.
In this article, we will take a look at the basics of Kotlin Flow and how it can be used to build reactive and asynchronous applications.
What is Kotlin Flow?
Kotlin Flow is a cold, asynchronous, and backpressure-aware stream processing library. It is designed to be used with Kotlin’s coroutines, which are lightweight threads that can be suspended and resumed without blocking the thread they are running on.
A Flow is a sequence of values that are emitted sequentially and asynchronously, and it can be transformed, filtered, and combined using various operators. The flow of values is controlled by a flow collector, which can request a specific number of values at a time and cancel the flow if necessary. This allows the flow to adapt to the consumption rate of the collector and avoid overloading the system.
Creating a Flow
There are several ways to create a Flow in Kotlin. The most straightforward way is to use the flow builder function, which takes a suspending lambda as an argument and returns a Flow:
import kotlinx.coroutines.flow.*
fun createFlow(): Flow<Int> {
return flow {
for (i in 1..3) {
emit(i)
}
}
}In this example, the flow emits the values 1, 2, and 3. The emit function is used to send a value to the flow collector, and the flow builder function automatically suspends the coroutine whenever the collector is not ready to receive more values.
Collecting a Flow
To collect a flow, you can use the collect operator, which takes a lambda as an argument and passes a flow collector to it. The collector has several functions that can be used to control the flow, such as request, cancel, and isCancelled.
Here is an example of collecting a flow and printing its values:
fun main() {
val flow = createFlow()
flow.collect { value ->
println(value)
}
}This will print the values 1, 2, and 3.
Transforming a Flow
Kotlin Flow provides a variety of operators that can be used to transform a flow. For example, you can use the map operator to transform the values in a flow:
fun main() {
val flow = createFlow().map { it * 2 }
flow.collect { value ->
println(value)
}
}This will print the values 2, 4, and 6.
Advanced Flow Example
One of the main benefits of using Kotlin Flow is that it provides a lightweight and easy-to-use API for expressing async and reactive logic. It is built on top of the Kotlin coroutines library, which makes it very efficient and non-blocking.
Here is a simple example of how to use Kotlin Flow to perform a network request and process the response asynchronously:
fun fetchUserData(id: String): Flow<User> {
return flow {
val response = api.fetchUser(id)
if (response.isSuccessful) {
emit(response.body()!!)
} else {
throw Exception("Failed to fetch user data")
}
}
}In this example, the fetchUserData function returns a Flow of User objects. The flow builder creates a new flow that executes the code block inside it as a coroutine. In this case, the code makes a network request to the api.fetchUser function and emits the response body as a User object if the request was successful. If the request fails, it throws an exception.
To consume the data produced by this flow, you can use the collect operator:
fetchUserData(id).collect { user ->
// Process the user data here
}The collect operator will execute the code block passed to it for each value emitted by the flow. In this case, the user parameter will be a User object emitted by the flow.
Kotlin Flow also provides a number of operators for transforming and filtering the data produced by a flow. For example, you can use the map operator to transform the data emitted by a flow:
fetchUserData(id).map { user ->
user.email
}.collect { email ->
// Process the email here
}In this example, the map operator transforms each User object emitted by the flow into a string containing the user's email address. The resulting flow will emit a stream of email addresses, which can be collected and processed as before.
There are many other operators available in Kotlin Flow, such as filter, flatMap, and reduce, which can be used to manipulate the data produced by a flow in various ways.
Flow with runBlocking
Here is a simple example of how to create a Flow in Kotlin:
import kotlinx.coroutines.flow.*
fun getFlow(): Flow<Int> = flow {
for (i in 1..5) {
emit(i)
}
}The flow builder is a function that is used to define the flow. It takes a suspending lambda as an argument and can be used to emit values to the flow using the emit function. In this example, the flow emits the values from 1 to 5.
You can collect the values emitted by the flow using the collect function:
import kotlinx.coroutines.runBlocking
fun main() = runBlocking {
getFlow().collect {
println(it)
}
}This will print the values 1 to 5 on the console.
import kotlinx.coroutines.flow.*
fun main() = runBlocking {
getFlow()
.map { it * it }
.collect {
println(it)
}
}This will print the squares of the values from 1 to 5 on the console.
Flow also has support for error handling. You can use the catch operator to handle exceptions thrown by the flow:
import kotlinx.coroutines.flow.*
fun getErrorFlow(): Flow<Int> = flow {
for (i in 1..5) {
if (i == 3) throw Exception("error")
emit(i)
}
}
fun main() = runBlocking {
getErrorFlow()
.catch { e -> println(e) }
.collect {
println(it)
}
}This will print the values 1 and 2, and then it will print the exception message “error”.
Conclusion
In conclusion, Kotlin Flow is a powerful and easy-to-use library for reactive programming in Kotlin. It provides a lightweight and efficient API for expressing async and reactive logic, and offers a wide range of operators for manipulating and processing streams of data.
IBRAHIM CAN ERDOGAN
