StateFlow and SharedFlow in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a StateFlow with an initial value of 0.

Android Kotlin

val _counter = MutableStateFlow([1])
val counter: StateFlow<Int> = _counter

Drag options to blanks, or click blank then click option'

A-1

Bnull

C"0"

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using null as initial value causes errors.

Using a string "0" instead of integer 0.

StateFlow requires an initial value. Here, 0 is the correct integer initial value.

2fill in blank

medium

Complete the code to emit a new value 5 to a MutableSharedFlow.

Android Kotlin

val events = MutableSharedFlow<Int>()
suspend fun sendEvent() {
  events.[1](5)
}

Drag options to blanks, or click blank then click option'

Asubscribe

Bcollect

Cemit

Dlaunch

Attempts:

3 left

💡 Hint

Common Mistakes

Using collect instead of emit.

Trying to call emit outside a suspend function.

To send a value to a SharedFlow, use the emit function inside a suspend function.

3fill in blank

hard

Fix the error in the code to collect values from a StateFlow named stateFlow inside a coroutine.

Android Kotlin

lifecycleScope.launch {
  stateFlow.[1] { value ->
    println(value)
  }
}

Drag options to blanks, or click blank then click option'

Acollect

Bemit

Csend

Dsubscribe

Attempts:

3 left

💡 Hint

Common Mistakes

Using emit instead of collect.

Trying to collect outside a coroutine.

To receive values from a StateFlow, use the collect function inside a coroutine.

4fill in blank

hard

Fill both blanks to create a SharedFlow that replays the last 3 emitted values and has a buffer capacity of 5.

Android Kotlin

val sharedFlow = MutableSharedFlow<Int>(replay = [1], extraBufferCapacity = [2])

Drag options to blanks, or click blank then click option'

A3

B5

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing replay with buffer capacity.

Setting replay or buffer to zero when not intended.

The replay parameter sets how many past values are replayed to new collectors. extraBufferCapacity sets the buffer size for emitted values.

5fill in blank

hard

Fill all three blanks to create a StateFlow that updates its value and exposes it as an immutable StateFlow.

Android Kotlin

private val _state = MutableStateFlow([1])
val state: StateFlow<Int> = _state.[2]

fun updateValue(newValue: Int) {
  _state.[3] = newValue
}

Drag options to blanks, or click blank then click option'

A0

BasStateFlow()

Cvalue

Demit

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to emit on StateFlow instead of setting value.

Exposing MutableStateFlow directly without asStateFlow.

Initialize MutableStateFlow with 0. Use asStateFlow() to expose immutable StateFlow. Update value by setting value property.

Practice

1. What is the main difference between StateFlow and SharedFlow in Kotlin?

easy

A. Both StateFlow and SharedFlow hold the latest value.

B. StateFlow holds the latest value, SharedFlow does not hold any value.

C. SharedFlow holds the latest value, StateFlow does not hold any value.

D. Neither StateFlow nor SharedFlow hold any value.

Solution

1. Step 1: Understand StateFlow behavior

   StateFlow always holds the latest emitted value and replays it to new collectors.

2. Step 2: Understand SharedFlow behavior

   SharedFlow emits events but does not hold or replay the latest value by default.

3. Final Answer:

   StateFlow holds the latest value, SharedFlow does not hold any value. -> Option B

4. Quick Check:

   StateFlow holds state, SharedFlow emits events [OK]

Hint: Remember: StateFlow = state holder, SharedFlow = event emitter [OK]

Common Mistakes:

• Thinking SharedFlow holds the latest value like StateFlow
• Confusing event emission with state holding
• Assuming both behave the same way

2. Which of the following is the correct way to create a StateFlow with an initial value of 0?

easy

A. val stateFlow = StateFlow<Int>()

B. val stateFlow = MutableSharedFlow(0)

C. val stateFlow = MutableStateFlow(0)

D. val stateFlow = SharedFlow(0)

Solution

1. Step 1: Identify correct constructor for StateFlow

   MutableStateFlow requires an initial value and is created with MutableStateFlow(initialValue).

2. Step 2: Check other options

   MutableSharedFlow does not take an initial value; StateFlow is an interface and cannot be instantiated directly.

3. Final Answer:

   val stateFlow = MutableStateFlow(0) -> Option C

4. Quick Check:

   MutableStateFlow needs initial value [OK]

Hint: StateFlow needs initial value via MutableStateFlow constructor [OK]

Common Mistakes:

• Trying to instantiate StateFlow interface directly
• Using SharedFlow constructor for StateFlow
• Passing initial value to MutableSharedFlow

3. Given the code below, what will be printed?

val stateFlow = MutableStateFlow(10)

stateFlow.value = 20

println(stateFlow.value)

medium

A. Compilation error

B. 10

C. null

D. 20

Solution

1. Step 1: Understand MutableStateFlow value assignment

   Assigning stateFlow.value = 20 updates the current value held by the StateFlow.

2. Step 2: Print the current value

   println(stateFlow.value) prints the latest value, which is 20.

3. Final Answer:

   20 -> Option D

4. Quick Check:

   StateFlow.value reflects latest assigned value [OK]

Hint: StateFlow.value always shows latest assigned value [OK]

Common Mistakes:

• Thinking initial value 10 will print instead of updated 20
• Confusing StateFlow with SharedFlow behavior
• Expecting null or error due to reassignment

4. What is wrong with this code snippet using SharedFlow?

val sharedFlow = MutableSharedFlow<Int>()

sharedFlow.value = 5

medium

A. SharedFlow does not have a value property to assign.

B. You must initialize SharedFlow with an initial value.

C. You cannot create a MutableSharedFlow without parameters.

D. The code is correct and will compile.

Solution

1. Step 1: Check SharedFlow properties

   SharedFlow does not hold state and has no value property to assign.

2. Step 2: Understand how to emit values

   Values are emitted using emit() or tryEmit() functions, not by assigning value.

3. Final Answer:

   SharedFlow does not have a value property to assign. -> Option A

4. Quick Check:

   SharedFlow emits events, no value property [OK]

Hint: Use emit() to send values in SharedFlow, not value assignment [OK]

Common Mistakes:

• Trying to assign value property on SharedFlow
• Confusing SharedFlow with StateFlow
• Assuming SharedFlow holds state like StateFlow

5. You want to create a SharedFlow that replays the last 3 emitted values to new collectors. Which configuration is correct?

hard

A. val sharedFlow = MutableSharedFlow<Int>(replay = 3)

B. val sharedFlow = MutableSharedFlow<Int>(replay = 0)

C. val sharedFlow = MutableStateFlow<Int>(3)

D. val sharedFlow = MutableSharedFlow<Int>(bufferSize = 3)

Solution

1. Step 1: Understand replay parameter in SharedFlow

   The replay parameter defines how many past values are replayed to new collectors.

2. Step 2: Check options for replaying last 3 values

   Setting replay = 3 in MutableSharedFlow correctly configures this behavior.

3. Final Answer:

   val sharedFlow = MutableSharedFlow<Int>(replay = 3) -> Option A

4. Quick Check:

   Replay count sets how many past events are replayed [OK]

Hint: Use replay parameter to replay past events in SharedFlow [OK]

Common Mistakes:

• Confusing bufferSize with replay
• Using StateFlow instead of SharedFlow for replay
• Setting replay to 0 expecting replay behavior