StateFlow and SharedFlow in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Difference between StateFlow and SharedFlow

Which statement correctly describes the main difference between StateFlow and SharedFlow in Kotlin?

A SharedFlow always holds the latest value and replays it to new collectors, while StateFlow does not hold any value and only emits new values.

B StateFlow always holds the latest value and replays it to new collectors, while SharedFlow does not hold any value by default and can replay a configurable number of values.

C StateFlow and SharedFlow are identical; both always replay all emitted values to new collectors.

D StateFlow is used only for one-time events, while SharedFlow is used for state management.

💡 Hint

Think about which flow remembers the last value and which one can replay multiple values.

StateFlow always holds the latest value and emits it immediately to new collectors. SharedFlow does not hold a value by default but can be configured to replay a set number of past values.

❓ ui_behavior

intermediate

StateFlow UI update behavior

Given a StateFlow that emits numbers, what will happen in the UI if a collector subscribes after the StateFlow has emitted values 1, 2, and 3?

A The collector receives all previous values 1, 2, and 3 in order, then new values.

B The collector receives only the first emitted value 1 and ignores the rest.

C The collector receives no values until a new value is emitted after subscription.

D The collector immediately receives the latest value 3 and then any new values emitted after subscription.

💡 Hint

Remember what StateFlow holds and replays to new subscribers.

StateFlow always holds the latest value and emits it immediately to new collectors. It does not replay all past values, only the latest one.

❓ lifecycle

advanced

SharedFlow and lifecycle-aware collection

You have a SharedFlow that emits UI events like navigation commands. What is the best way to collect this SharedFlow in an Android Activity to avoid missing events when the Activity is recreated?

A Collect the SharedFlow in onCreate() using lifecycleScope.launchWhenStarted to ensure events are received only when the Activity is visible.

B Collect the SharedFlow in onDestroy() to catch all events before the Activity is destroyed.

C Collect the SharedFlow in onCreate() without lifecycleScope to avoid missing events during recreation.

D Collect the SharedFlow in onResume() without lifecycle awareness to get all events immediately.

💡 Hint

Think about lifecycle-aware collection and when UI events should be handled.

Using lifecycleScope.launchWhenStarted ensures the SharedFlow is collected only when the Activity is visible, preventing missed events and avoiding leaks.

📝 Syntax

advanced

Creating a StateFlow with initial value

Which Kotlin code snippet correctly creates a MutableStateFlow with an initial value of 0?

A val stateFlow = MutableStateFlow.create(0)

B val stateFlow = MutableStateFlow<Int>()

C val stateFlow = MutableStateFlow(0)

D val stateFlow = MutableStateFlow.value(0)

💡 Hint

Check the constructor usage for MutableStateFlow.

MutableStateFlow requires an initial value passed directly to its constructor.

🔧 Debug

expert

Why does this SharedFlow not emit to new subscribers?

Consider this SharedFlow declaration:

val sharedFlow = MutableSharedFlow()

New subscribers do not receive any previously emitted values. Why?

A Because MutableSharedFlow by default has replay = 0, so it does not replay past values to new subscribers.

B Because MutableSharedFlow requires an initial value to replay to new subscribers.

C Because SharedFlow only emits values when collected before emission.

D Because MutableSharedFlow buffers all values and drops new subscribers.

💡 Hint

Check the default replay parameter of MutableSharedFlow.

MutableSharedFlow by default has replay set to 0, so it does not replay any past values to new subscribers.