Optimizing Asynchronous Programming with Flows in Kotlin: Introduction to Flow

1 – Introduction

As the complexity of applications grows, handling multiple asynchronous events in real time becomes essential. To this end, Kotlin offers a powerful tool called Flow, which simplifies the management of asynchronous data flows.

In this article, we will explore:

• What is a Flow and how does it differ from other approaches like suspend and async .
• Real use cases of Flow.
• Tools like Turbine to test data flows.

2 – What is a Flow?

Kotlin Flow is a tool for handling asynchronous data flows. It works like a pipeline that emits values over time, allowing you to handle data streams efficiently.

Main Features:

1. Asynchronous: Flow emits values in a non-blocking manner.
2. Cold Stream : A Flow only starts emitting values when there is a "sink" (i.e. when someone consumes the data).
3. Cancellable: If the collector is canceled, the flow is also automatically canceled.

3 – Creating a Flow

Simple Example of a Flow

import kotlinx.coroutines.* import kotlinx.coroutines.flow.* fun main() = runBlocking { val flow = flow { for (i in 1..3) { emit(i) // Emits a value delay(1000) // Simulates a delay between emissions } } flow.collect { value -> println("Received: $value") } } 

Console Output

Received: 1
Received: 2
Received: 3

Explanation:

• emit: Sends values to the stream.
• collect: Consumes the values emitted by the stream.

4 – Comparison: Flow vs. Other Approaches

5 – Transforming Data with Flow

The real power of Flow lies in its ability to transform the data output with operators.

Example: Using operators like map and filter

import kotlinx.coroutines.* import kotlinx.coroutines.flow.* fun main() = runBlocking { val flow = flow { for (i in 1..5) { emit(i) } } flow .filter { it % 2 == 0 } // Filter even values .map { it * 10 } // Multiplies the values by 10 .collect { value -> println("Transformed: $value") } } 

Console Output

Transformed: 20
Transformed: 40

6 – Testing Flows with Turbine

Turbine is a library that facilitates the validation of values emitted by a Flow.

Example with Turbine

import app.cash.turbine.test import kotlinx.coroutines.* import kotlinx.coroutines.flow.* import kotlinx.coroutines.test.* fun main() = runTest { // Creates a stream that emits two values with a delay between them val flow = flow { println("Emitting value 1") emit(1) // Emit the first value delay(500) // Wait 500ms println("Emitting value 2") emit(2) // Emit the second value } // Test the flow flow.test { val firstValue = awaitItem() // Wait for the first value println("Value received from stream: $ firstValue ") assert(firstValue == 1) // Validates the first value val secondValue = awaitItem() // Wait for the second value println("Value received from stream: $ secondValue ") assert(secondValue == 2) // Validates the second value awaitComplete() // Check if the stream has completed println("Flow completed successfully!") } } 

Expected departure at terminal

Emitting value 1
Value received from stream: 1
Emitting value 2
Value received from stream: 2
Flow completed successfully!

7 – Real Use Cases of Flow

7.1 – Real Time Update

In a chat application, you can use a Flow to stream new messages to the user interface as they arrive from the server.

import kotlinx.coroutines.* import kotlinx.coroutines.flow.* fun main() = runBlocking { val messages = flow { val newMessages = listOf("Hi", "How are you?", "See you soon!") for (message in newMessages) { emit(message) delay(1000) // Simulates interval between messages } } messages.collect { message -> println("New message: $message") } } 

7.2 – Batch Data Processing

Imagine a system that needs to process large batches of data at regular intervals. Flow makes this implementation easy.

import kotlinx.coroutines.* import kotlinx.coroutines.flow.* fun main() = runBlocking { val batchFlow = flow { val lots = listOf("Lot1", "Lot2", "Lot3") for (lot in lots) { issue(batch) delay(2000) // Simulates processing time } } batchFlow.collect { lot -> println("Processing: $batch") } } 

8 – Conclusion

Flow is a powerful tool for handling asynchronous data in Kotlin , allowing you to emit, transform, and consume values in real time. It is essential for building modern applications, especially in scenarios that require handling continuous streams of events.

Summary:

1. Flow is ideal for continuous, cancelable data streams.
2. Operators like map and filter make it easier to transform the outputted data.
3. Tools like Turbine simplify flow testing.

References:
Official Kotlin documentation on coroutines
Turbine Documentation