Room with Flow for reactive data in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when you collect a Flow from Room DAO?

You have a Room DAO function returning Flow<List<User>>. What is the behavior when you collect this Flow in your UI?

Android Kotlin

interface UserDao {
  @Query("SELECT * FROM users")
  fun getAllUsers(): Flow<List<User>>
}

A The Flow emits null values when the database is empty.

B The Flow emits data only once and then completes.

C You must manually refresh the Flow to get new data.

D The UI receives updates automatically whenever the users table changes.

💡 Hint

Think about how Flow works with Room to provide live updates.

Room integrates with Kotlin Flow to emit new data whenever the database table changes, so the UI updates automatically.

❓ lifecycle

intermediate

How to safely collect a Room Flow in an Android Activity?

You want to collect a Flow from Room in an Activity to update UI. Which lifecycle-aware approach is best?

A Use lifecycleScope.launchWhenStarted { dao.getAllUsers().collect { ... } }

B Use runBlocking { dao.getAllUsers().collect { ... } } in onCreate.

C Call dao.getAllUsers().collect() directly on the main thread.

D Use GlobalScope.launch { dao.getAllUsers().collect { ... } }

💡 Hint

Consider Android lifecycle and coroutine scopes.

Using lifecycleScope.launchWhenStarted ensures collection happens only when Activity is started and cancels automatically.

📝 Syntax

advanced

What is the correct DAO function signature for a Flow query?

Choose the correct Kotlin function signature for a Room DAO that returns a Flow of a list of entities named Item.

A fun getItems(): Flow<Item>

B fun getItems(): List<Flow<Item>>

C fun getItems(): Flow<List<Item>>

D fun getItems(): LiveData<Flow<List<Item>>>

💡 Hint

Think about the type that represents a stream of lists.

The correct signature returns a Flow emitting lists of Item objects, not a list of Flows or other types.

🔧 Debug

advanced

Why does collecting a Room Flow not emit updates?

You collect a Flow from Room DAO but the UI never updates after initial data. What is a likely cause?

Android Kotlin

val usersFlow = userDao.getAllUsers()
lifecycleScope.launchWhenStarted {
  usersFlow.collect { users ->
    adapter.submitList(users)
  }
}

A The database table was not updated, so no new emissions occur.

B You forgot to call collectLatest instead of collect.

C Room requires manual refresh calls to emit new data.

D The Flow must be converted to LiveData to emit updates.

💡 Hint

Consider what triggers Flow emissions from Room.

Room emits new Flow data only when the underlying table changes. If no changes happen, no new emissions occur.

🧠 Conceptual

expert

How does Room with Flow handle backpressure and cancellation?

When collecting a Flow from Room, how does Room handle backpressure and cancellation to avoid resource leaks?

A Room buffers all database changes and emits them all even if the collector is slow.

B Room cancels the query and stops emitting when the collector cancels the Flow collection.

C Room requires manual cancellation of queries to avoid leaks.

D Room converts Flow to LiveData internally to handle backpressure.

💡 Hint

Think about how Flow and Room cooperate with coroutine cancellation.

Room integrates with Kotlin Flow to cancel queries automatically when the collector cancels, preventing leaks and handling backpressure.