Kotlin Flow Essentials: 20 Must-Know Concepts — Part-1

Kotlin’s Flow API is a powerful tool for managing asynchronous data streams using Coroutines. Whether you’re new to reactive streams or looking to level up your expertise, here are 20 essential Flow concepts and operators every Kotlin developer should know.

In this first part, we cover 10 of these key concepts, while the remaining 10 will be discussed in Part 2.

Kotlin Flow

Flow Builders 🚀

• flow { … }: Creates a cold stream that only starts emitting when collected.
• flowOf(): Generates a flow from a fixed set of values.
• asFlow(): Converts collections or sequences into a Flow.

val simpleFlow = flow {
    emit(1) // ➡️ Emit value 1
    emit(2) // ➡️ Emit value 2
    emit(3) // ➡️ Emit value 3
}

Collecting Values 📥

collect { value -> … }: Terminal operator that triggers the flow and processes each emitted value.

simpleFlow.collect { value ->
    println("Received: $value") // 📬 Print each value
}

Transforming Emissions with map 🔄

Applies a transformation to each element in the stream.

simpleFlow
    .map { it * 2 } // ✖️ Multiply each value by 2
    .collect { println(it) } // 🖨️ Print the transformed values

Filtering Data with filter 🔍

Selects only the elements that satisfy the given predicate.

simpleFlow
    .filter { it % 2 == 1 } // 🍀 Only allow odd numbers
    .collect { println(it) }

Handling Side Effects with onEach 🎯

Executes an action for every emitted value without modifying the stream

simpleFlow
    .onEach { println("Emitting: $it") } // 🔊 Log each emission
    .collect()

Exception Handling with catch 🚨

Catches exceptions occurring upstream in the flow and allows you to handle them.

simpleFlow
    .catch { e ->
        println("Error: ${e.message}") // ⚠️ Log the error
        emit(-1) // 🚑 Emit a fallback value…