Combining flows (zip, combine) in Kotlin

Introduction

Combining flows lets you work with multiple streams of data together. It helps you get values from two flows at the same time and use them in one place.

When you want to pair items from two lists that come over time, like matching names and ages.

When you need to update UI based on two different data sources, like user input and network data.

When you want to react only when both flows have new values, like waiting for two sensors to send data.

When you want to merge two flows and create a new value from both, like combining temperature and humidity readings.

Syntax

Kotlin

val combinedFlow = flow1.zip(flow2) { value1, value2 ->
    // combine values here
}

val combinedFlow = flow1.combine(flow2) { value1, value2 ->
    // combine values here
}

zip pairs values from both flows one by one, stopping when one flow ends.

combine emits new values whenever either flow emits, using the latest from both.

Examples

This pairs 1 with A, 2 with B, and 3 with C, producing "1A", "2B", "3C".

Kotlin

val flow1 = flowOf(1, 2, 3)
val flow2 = flowOf("A", "B", "C")

val zipped = flow1.zip(flow2) { a, b -> "$a$b" }

This emits a new combined value whenever either flow emits, using the latest values from both.

Kotlin

val flow1 = flowOf(1, 2, 3)
val flow2 = flowOf("A", "B", "C")

val combined = flow1.combine(flow2) { a, b -> "$a$b" }

Sample Program

This program shows how zip pairs items one by one and combine merges latest values from both flows.

Kotlin

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*

fun main() = runBlocking {
    val numbers = flowOf(1, 2, 3)
    val letters = flowOf("A", "B", "C")

    println("Using zip:")
    numbers.zip(letters) { n, l -> "$n$l" }
        .collect { println(it) }

    println("Using combine:")
    numbers.combine(letters) { n, l -> "$n$l" }
        .collect { println(it) }
}

Important Notes

zip waits for both flows to emit before pairing values.

combine emits more often because it reacts to any new value from either flow.

Use zip when you want strict pairs, and combine when you want to react to any change.

Summary

zip pairs values one by one from two flows.

combine merges latest values from two flows whenever either emits.

Both help you work with multiple data streams together in Kotlin.

Practice

1. What does the zip function do when combining two Kotlin flows?

easy

A. Pairs values one by one from both flows, emitting pairs only when both have emitted.

B. Merges the latest values from both flows whenever either emits.

C. Combines flows by concatenating their emitted values into a single list.

D. Filters values from the first flow based on the second flow's emissions.

Solution

1. Step 1: Understand zip behavior

   The zip function pairs values from two flows one by one, waiting for both to emit before producing a pair.

2. Step 2: Compare with other options

   combine merges latest values on any emission, not zip. Other options describe different behaviors.

3. Final Answer:

   Pairs values one by one from both flows, emitting pairs only when both have emitted. -> Option A

4. Quick Check:

   zip pairs values one by one = A [OK]

Hint: Remember: zip waits for both flows to emit one value each [OK]

Common Mistakes:

• Confusing zip with combine behavior
• Thinking zip merges latest values on any emission
• Assuming zip concatenates values into a list

2. Which of the following is the correct syntax to combine two flows flowA and flowB using combine in Kotlin?

easy

A. flowA.combine(flowB) { a, b -> a + b }

B. flowA.zip(flowB) { a, b -> a + b }

C. combine(flowA flowB) { a, b -> a + b }

D. flowA.merge(flowB) { a, b -> a + b }

Solution

1. Step 1: Recall combine extension syntax

   combine is called as an extension on one flow with another flow as argument and a lambda combining values.

2. Step 2: Check options

   flowA.combine(flowB) { a, b -> a + b } matches the correct syntax. flowA.zip(flowB) { a, b -> a + b } uses zip, C uses top-level combine which is invalid, D uses non-existent merge with lambda.

3. Final Answer:

   flowA.combine(flowB) { a, b -> a + b } -> Option A

4. Quick Check:

   combine is an extension function = B [OK]

Hint: combine is called as flowA.combine(flowB) with lambda [OK]

Common Mistakes:

• Using zip instead of combine
• Trying to call combine as a top-level function
• Using merge with a lambda (not valid)

3. What will be the output of the following Kotlin code?

val flow1 = flowOf(1, 2, 3)
val flow2 = flowOf("A", "B", "C", "D")

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

medium

A. 1A
2B
3C
4D

B. 1A
2B
3C

C. 1A
2B
3C
D

D. A1
B2
C3
D4

Solution

1. Step 1: Understand zip behavior with unequal flow lengths

   zip pairs elements one by one until the shortest flow ends. Here, flow1 has 3 elements, flow2 has 4.

2. Step 2: Determine output pairs

   Pairs: (1, "A"), (2, "B"), (3, "C"). The extra "D" in flow2 is ignored.

3. Final Answer:

   1A 2B 3C -> Option B

4. Quick Check:

   zip stops at shortest flow length = C [OK]

Hint: zip stops when shortest flow ends, extra values ignored [OK]

Common Mistakes:

• Expecting zip to emit all values from longer flow
• Confusing zip with combine behavior
• Mixing order of concatenation in output

4. Identify the error in this Kotlin code using combine and fix it:

val flow1 = flowOf(1, 2)
val flow2 = flowOf("X", "Y")

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

medium

A. No error; code runs and prints combined values.

B. Lambda parameters should be reversed: { b, a -> ... }.

C. Cannot call combine on flowOf; use zip instead.

D. Missing import for combine; add import kotlinx.coroutines.flow.combine.

Solution

1. Step 1: Check for common mistakes with combine

   combine is an extension function requiring import from kotlinx.coroutines.flow.

2. Step 2: Identify missing import

   Without import kotlinx.coroutines.flow.combine, code won't compile. Other options are incorrect because combine works on flowOf and lambda parameter order is correct.

3. Final Answer:

   Missing import for combine; add import kotlinx.coroutines.flow.combine -> Option D

4. Quick Check:

   Missing import causes compile error = D [OK]

Hint: Always import combine from kotlinx.coroutines.flow [OK]

Common Mistakes:

• Forgetting to import combine function
• Thinking combine is not available on flowOf
• Swapping lambda parameters incorrectly

5. You want to combine two flows flowA and flowB so that every time either emits, you get the latest values from both. Which function should you use and why?

hard

A. Use merge because it merges emissions into a single flow without pairing.

B. Use zip because it pairs values one by one, ensuring synchronized pairs.

C. Use combine because it emits on any flow emission with latest values from both.

D. Use flatMapConcat to flatten emissions from both flows sequentially.

Solution

1. Step 1: Understand the requirement

   You want to emit combined latest values whenever either flow emits, not just pair one by one.

2. Step 2: Match function behavior

   combine merges latest values from both flows on any emission. zip waits for pairs, merge just merges emissions without combining, and flatMapConcat flattens sequences.

3. Final Answer:

   Use combine because it emits on any flow emission with latest values from both. -> Option C

4. Quick Check:

   combine merges latest values on any emission = A [OK]

Hint: combine emits latest from both on any emission, zip waits for pairs [OK]

Common Mistakes:

• Choosing zip instead of combine for latest values
• Confusing merge with combine
• Using flatMapConcat which flattens sequences, not combines