Flow basics in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Flow in Kotlin?

A Flow is a type that can emit multiple values sequentially, asynchronously. It is like a stream of data that you can collect over time.

beginner

How do you collect values from a Flow?

You use the collect terminal operator inside a coroutine to receive emitted values from the Flow.

intermediate

What is the difference between Flow and suspend functions?

suspend functions return a single value asynchronously, while Flow can emit multiple values over time.

beginner

What does the flow { } builder do?

It creates a Flow by defining a block where you can emit values using emit().

beginner

Why should you collect a Flow inside a coroutine?

Because Flow is asynchronous and suspending, collecting it requires a coroutine context to handle the asynchronous stream properly.

Which Kotlin type represents a stream of multiple asynchronous values?

A Flow

B suspend function

C List

D Deferred

Flow is designed to emit multiple values asynchronously over time.

How do you receive values emitted by a Flow?

A Using collect inside a coroutine

B Calling the Flow directly

C Using a for loop

D Using suspend functions only

You must call collect inside a coroutine to receive Flow emissions.

What keyword is used to create a Flow builder?

A emit

B async

C flow

D launch

The flow builder is created with the keyword flow { }.

Which operator is used inside a flow builder to send values?

A return

B send

C collect

D emit

emit() is used inside flow { } to emit values.

What is the main difference between a suspend function and a Flow?

A Suspend functions are faster than Flow

B Suspend functions return one value; Flow emits many

C Suspend functions are synchronous; Flow is asynchronous

D Flow cannot be collected

Suspend functions return a single asynchronous value, while Flow can emit multiple values over time.

Explain what a Flow is and how it differs from a suspend function in Kotlin.

Think about streams of data versus single results.

Describe how to create and collect values from a Flow in Kotlin.

Focus on the flow builder and collection process.

Practice

1. What is the primary purpose of a Flow in Kotlin?

easy

A. To represent a stream of data that can emit multiple values over time

B. To store a single immutable value

C. To create a user interface layout

D. To handle exceptions synchronously

Solution

1. Step 1: Understand what Flow represents

   A Flow is designed to emit multiple values asynchronously over time, like a stream of data.

2. Step 2: Compare options with Flow's purpose

   Only To represent a stream of data that can emit multiple values over time correctly describes Flow as a stream of data emitting multiple values over time.

3. Final Answer:

   To represent a stream of data that can emit multiple values over time -> Option A

4. Quick Check:

   Flow = stream of data [OK]

Hint: Flow emits multiple values over time, not just one [OK]

Common Mistakes:

• Confusing Flow with a single value container like LiveData
• Thinking Flow is for UI layout
• Assuming Flow handles exceptions synchronously

2. Which of the following is the correct way to create a Flow that emits numbers 1 to 3?

easy

A. val flow = flow { emit(1); emit(2); emit(3) }

B. val flow = flowOf(1, 2, 3)

C. All of the above

D. val flow = listOf(1, 2, 3).asFlow()

Solution

1. Step 1: Review Flow creation methods

   Kotlin provides multiple ways to create flows: using flow builder with emit, flowOf, and converting collections with asFlow().

2. Step 2: Check each option

   val flow = flow { emit(1); emit(2); emit(3) } uses flow builder with emit calls, B uses flowOf, and C converts a list to flow. All are valid ways.

3. Final Answer:

   All of the above -> Option C

4. Quick Check:

   Multiple ways to create Flow = D [OK]

Hint: Remember flow, flowOf, and asFlow() all create flows [OK]

Common Mistakes:

• Thinking only flow builder with emit works
• Confusing flowOf with a list
• Not knowing asFlow() converts collections

3. What will be printed by this code?

val flow = flow {
  emit(10)
  emit(20)
}

runBlocking {
  flow.collect { value -> println(value) }
}

medium

A. No output

B. 20 10

C. Only 10

D. 10 20

Solution

1. Step 1: Understand flow emission order

   The flow emits 10 first, then 20 in order.

2. Step 2: Collect prints emitted values in order

   Collect receives values in the same order they are emitted, so prints 10 then 20.

3. Final Answer:

   10 20 -> Option D

4. Quick Check:

   Flow emits in order = 10 20 [OK]

Hint: Flow emits values in order unless otherwise specified [OK]

Common Mistakes:

• Assuming values print in reverse order
• Expecting only first value to print
• Forgetting to run collect inside coroutine

4. Identify the error in this code snippet:

val flow = flow {
  emit(1)
  emit(2)
}

flow.collect { println(it) }

medium

A. Missing coroutine scope to call collect

B. emit() cannot be called inside flow builder

C. collect should be replaced with collectLatest

D. No error, code is correct

Solution

1. Step 1: Check how collect is called

   collect is a suspend function and must be called inside a coroutine or suspend function.

2. Step 2: Analyze code context

   Here, collect is called directly without coroutine scope like runBlocking or launch, causing error.

3. Final Answer:

   Missing coroutine scope to call collect -> Option A

4. Quick Check:

   collect needs coroutine scope [OK]

Hint: Always call collect inside coroutine scope [OK]

Common Mistakes:

• Calling collect outside coroutine
• Thinking emit() is invalid inside flow
• Confusing collect with collectLatest necessity

5. You want to create a Flow that emits numbers 1 to 5 but only collect even numbers. Which code correctly achieves this?

hard

A. val flow = flow {
for (i in 1..5) emit(i)
}

runBlocking {
flow.collect { if (it % 2 == 0) println(it) }
}

B. All of the above

C. val flow = flow {
for (i in 1..5) if (i % 2 == 0) emit(i)
}

runBlocking {
flow.collect { println(it) }
}

D. val flow = flowOf(1, 2, 3, 4, 5)

runBlocking {
flow.filter { it % 2 == 0 }.collect { println(it) }
}

Solution

1. Step 1: Analyze each code snippet

   val flow = flow { for (i in 1..5) emit(i) } runBlocking { flow.collect { if (it % 2 == 0) println(it) } } emits all numbers and filters during collect; C emits only even numbers directly; D uses filter operator before collect.

2. Step 2: Confirm all produce only even numbers on collect

   All options print only even numbers 2 and 4 when collected.

3. Final Answer:

   All of the above -> Option B

4. Quick Check:

   Multiple ways to filter Flow = B [OK]

Hint: Filter before or during collect both work [OK]

Common Mistakes:

• Thinking filter must be before collect only
• Not realizing emit can be conditional
• Forgetting to run collect inside coroutine