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๐ŸŒŠ Flow

๐Ÿ“– What is Flow?โ€‹

Flow is a data stream that asynchronously returns multiple values sequentially. You can think of it as a list version of suspend functions!

๐Ÿ’ก Basic Conceptsโ€‹

Single Value vs Multiple Valuesโ€‹

// suspend - single value
suspend fun fetchData(): String {
    delay(1000)
    return "data"
}

// Flow - multiple values
fun fetchDataStream(): Flow<String> = flow {
    emit("data1")
    delay(500)
    emit("data2")
    delay(500)
    emit("data3")
}

fun main() = runBlocking {
    // Single value
    println(fetchData())

    // Multiple values
    fetchDataStream().collect { value ->
        println(value)
    }
}

Creating Flowโ€‹

import kotlinx.coroutines.flow.*

fun main() = runBlocking {
    // 1. flow builder
    val flow1 = flow {
        emit(1)
        emit(2)
        emit(3)
    }

    // 2. flowOf
    val flow2 = flowOf(1, 2, 3)

    // 3. asFlow
    val flow3 = listOf(1, 2, 3).asFlow()

    flow1.collect { println(it) }
}

๐ŸŽฏ Practical Examplesโ€‹

Real-time Dataโ€‹

fun tickerFlow(): Flow<Int> = flow {
    var counter = 0
    while (counter < 5) {
        emit(counter++)
        delay(1000)
    }
}

fun main() = runBlocking {
    tickerFlow().collect { value ->
        println("$value seconds elapsed")
    }
}

Database Change Detectionโ€‹

data class User(val id: Int, val name: String)

fun observeUsers(): Flow<List<User>> = flow {
    repeat(3) { i ->
        delay(1000)
        val users = listOf(
            User(i * 2, "User${i * 2}"),
            User(i * 2 + 1, "User${i * 2 + 1}")
        )
        emit(users)
    }
}

fun main() = runBlocking {
    observeUsers().collect { users ->
        println("Updated users:")
        users.forEach { println("  - ${it.name}") }
    }
}

Sensor Dataโ€‹

fun sensorData(): Flow<Double> = flow {
    repeat(10) {
        val reading = (20..30).random() + Math.random()
        emit(reading)
        delay(500)
    }
}

fun main() = runBlocking {
    sensorData().collect { temperature ->
        println("Temperature: ${"%.1f".format(temperature)}ยฐC")
    }
}

๐Ÿ”ง Flow Operatorsโ€‹

map - Transformationโ€‹

fun main() = runBlocking {
    flowOf(1, 2, 3, 4, 5)
        .map { it * it }  // square
        .collect { println(it) }
    // 1, 4, 9, 16, 25
}

filter - Filteringโ€‹

fun main() = runBlocking {
    flowOf(1, 2, 3, 4, 5)
        .filter { it % 2 == 0 }  // even numbers only
        .collect { println(it) }
    // 2, 4
}

transform - Complex Transformationโ€‹

fun main() = runBlocking {
    flowOf(1, 2, 3)
        .transform { value ->
            emit("Transformed: $value")
            emit("Squared: ${value * value}")
        }
        .collect { println(it) }
}

take - Limiting Countโ€‹

fun main() = runBlocking {
    flowOf(1, 2, 3, 4, 5)
        .take(3)  // first 3 only
        .collect { println(it) }
    // 1, 2, 3
}

๐ŸŽจ Advanced Operatorsโ€‹

buffer - Bufferingโ€‹

fun main() = runBlocking {
    val time = measureTimeMillis {
        flow {
            repeat(3) {
                delay(100)
                emit(it)
            }
        }
            .buffer()  // improve performance with buffering
            .collect {
                delay(300)
                println(it)
            }
    }
    println("Time taken: ${time}ms")
}

conflate - Latest Value Onlyโ€‹

fun main() = runBlocking {
    flow {
        repeat(5) {
            delay(100)
            emit(it)
        }
    }
        .conflate()  // skip intermediate values if collector is slow
        .collect {
            delay(300)
            println("Collected: $it")
        }
}

zip - Combiningโ€‹

fun main() = runBlocking {
    val numbers = flowOf(1, 2, 3)
    val strings = flowOf("One", "Two", "Three")

    numbers.zip(strings) { num, str ->
        "$num - $str"
    }.collect { println(it) }
    // 1 - One
    // 2 - Two
    // 3 - Three
}

combine - Compositionโ€‹

fun main() = runBlocking {
    val flow1 = flow {
        emit("A")
        delay(500)
        emit("B")
    }

    val flow2 = flow {
        emit(1)
        delay(200)
        emit(2)
        delay(200)
        emit(3)
    }

    flow1.combine(flow2) { a, b ->
        "$a$b"
    }.collect { println(it) }
}

๐Ÿ”ฅ Practical Patternsโ€‹

Search Functionalityโ€‹

fun searchQuery(query: String): Flow<List<String>> = flow {
    delay(300)  // debounce
    val results = listOf(
        "${query}1",
        "${query}2",
        "${query}3"
    ).filter { it.contains(query) }
    emit(results)
}

fun main() = runBlocking {
    flowOf("kot", "kotlin", "kotli")
        .flatMapLatest { query ->
            searchQuery(query)
        }
        .collect { results ->
            println("Search results: $results")
        }
}

Paginationโ€‹

fun loadPage(page: Int): Flow<List<String>> = flow {
    delay(500)
    val items = List(10) { "Item${page * 10 + it}" }
    emit(items)
}

fun main() = runBlocking {
    flowOf(1, 2, 3)
        .flatMapConcat { page ->
            loadPage(page)
        }
        .collect { items ->
            println("Page loaded: $items")
        }
}

Error Handlingโ€‹

fun riskyFlow(): Flow<Int> = flow {
    emit(1)
    emit(2)
    throw Exception("Error occurred!")
    emit(3)
}

fun main() = runBlocking {
    riskyFlow()
        .catch { e ->
            println("Error caught: ${e.message}")
            emit(-1)  // default value
        }
        .collect { println("Value: $it") }
}

Retryโ€‹

var attemptCount = 0

fun unstableFlow(): Flow<String> = flow {
    attemptCount++
    if (attemptCount < 3) {
        throw Exception("Temporary error")
    }
    emit("Success!")
}

fun main() = runBlocking {
    unstableFlow()
        .retry(3) { cause ->
            println("Retrying... (${cause.message})")
            delay(1000)
            true
        }
        .collect { println(it) }
}

๐Ÿ› ๏ธ StateFlow and SharedFlowโ€‹

StateFlow - State Managementโ€‹

class Counter {
    private val _count = MutableStateFlow(0)
    val count: StateFlow<Int> = _count

    fun increment() {
        _count.value++
    }
}

fun main() = runBlocking {
    val counter = Counter()

    launch {
        counter.count.collect { value ->
            println("Count: $value")
        }
    }

    delay(100)
    counter.increment()
    delay(100)
    counter.increment()
    delay(100)
}

SharedFlow - Eventsโ€‹

class EventBus {
    private val _events = MutableSharedFlow<String>()
    val events: SharedFlow<String> = _events

    suspend fun emit(event: String) {
        _events.emit(event)
    }
}

fun main() = runBlocking {
    val bus = EventBus()

    // Subscriber 1
    launch {
        bus.events.collect { event ->
            println("Subscriber1: $event")
        }
    }

    // Subscriber 2
    launch {
        bus.events.collect { event ->
            println("Subscriber2: $event")
        }
    }

    delay(100)
    bus.emit("Event1")
    delay(100)
    bus.emit("Event2")
    delay(100)
}

๐Ÿค” Frequently Asked Questionsโ€‹

Q1. When does Flow execute?โ€‹

A: It executes when collect is called (Cold Stream)!

fun main() = runBlocking {
    val flow = flow {
        println("Flow started!")
        emit(1)
    }

    println("Flow created")
    delay(1000)

    println("collect called")
    flow.collect { println(it) }
}
// Flow created
// (wait 1 second)
// collect called
// Flow started!
// 1

Q2. What's the difference between List and Flow?โ€‹

A: List is immediate, Flow is progressive!

fun main() = runBlocking {
    // List - all data in memory
    val list = listOf(1, 2, 3)

    // Flow - generated as needed
    val flow = flow {
        repeat(3) {
            delay(1000)
            emit(it)
        }
    }
}

Q3. What happens if I collect Flow multiple times?โ€‹

A: It executes anew each time!

fun main() = runBlocking {
    val flow = flow {
        println("Executing!")
        emit(1)
    }

    flow.collect { println("First collection: $it") }
    flow.collect { println("Second collection: $it") }
}
// Executing!
// First collection: 1
// Executing!
// Second collection: 1

๐ŸŽฌ Conclusionโ€‹

Reactive programming with Flow!

Key Points:
โœ… Asynchronous data streams
โœ… Cold Stream (executes on collection)
โœ… Operators like map, filter
โœ… State management with StateFlow
โœ… Event handling with SharedFlow

Next Step: Learn about coroutine execution environments in Context and Dispatcher!