Side effects (LaunchedEffect, SideEffect) in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when LaunchedEffect key changes?

Consider this Compose snippet:

var count by remember { mutableStateOf(0) }
LaunchedEffect(count) {
  println("Count is $count")
}

What happens when count changes?

Android Kotlin

var count by remember { mutableStateOf(0) }
LaunchedEffect(count) {
  println("Count is $count")
}

A The block runs only once when the composable is first composed.

B The block inside LaunchedEffect restarts and prints the new count value.

C The block never runs because count is mutable.

D The block runs repeatedly in an infinite loop.

💡 Hint

LaunchedEffect runs its block when the key changes.

LaunchedEffect watches the key parameter. When count changes, it cancels the previous coroutine and launches a new one, printing the updated value.

❓ lifecycle

intermediate

When does SideEffect run in Compose?

Given this Compose code:

SideEffect {
  println("SideEffect executed")
}

When is this block executed?

Android Kotlin

SideEffect {
  println("SideEffect executed")
}

A After every successful recomposition of the composable.

B Only once when the composable is first composed.

C Before the composable is composed.

D Only when the composable is disposed.

💡 Hint

Think about when SideEffect is designed to run in the Compose lifecycle.

SideEffect runs after every recomposition, allowing side effects that need to happen after UI updates.

🔧 Debug

advanced

Why does this LaunchedEffect cause repeated prints?

Look at this code:

var text by remember { mutableStateOf("") }
LaunchedEffect(Unit) {
  while(true) {
    delay(1000)
    println("Tick")
  }
}
Button(onClick = { text += "a" }) {
  Text(text)
}

What is the problem with this LaunchedEffect?

Android Kotlin

var text by remember { mutableStateOf("") }
LaunchedEffect(Unit) {
  while(true) {
    delay(1000)
    println("Tick")
  }
}
Button(onClick = { text += "a" }) {
  Text(text)
}

A The LaunchedEffect never runs because Unit is not a valid key.

B The LaunchedEffect restarts on every recomposition causing multiple "Tick" prints per second.

C The LaunchedEffect runs only once and prints "Tick" every second as expected.

D The Button click causes the LaunchedEffect to cancel immediately.

💡 Hint

Consider what key Unit means for LaunchedEffect.

Using Unit as key means LaunchedEffect runs once when first composed and does not restart on recompositions, so it prints "Tick" every second as intended.

🧠 Conceptual

advanced

Difference between LaunchedEffect and SideEffect?

Which statement correctly describes the difference between LaunchedEffect and SideEffect in Jetpack Compose?

A LaunchedEffect runs suspend functions and can launch coroutines; SideEffect runs synchronously after recomposition.

B LaunchedEffect runs only once; SideEffect runs only on disposal.

C Both run suspend functions but LaunchedEffect cancels on key change; SideEffect does not.

D SideEffect launches coroutines; LaunchedEffect runs synchronously after recomposition.

💡 Hint

Think about coroutine support and timing of execution.

LaunchedEffect is designed to launch coroutines and supports suspend functions. SideEffect runs synchronously after every recomposition and cannot run suspend functions.

📝 Syntax

expert

What error occurs with this LaunchedEffect usage?

Analyze this code snippet:

LaunchedEffect {
  println("Hello")
}

What error will this cause?

Android Kotlin

LaunchedEffect {
  println("Hello")
}

A Type error: LaunchedEffect expects a suspend lambda.

B Runtime error: LaunchedEffect block cannot print to console.

C Syntax error: Missing key parameter in LaunchedEffect call.

D No error: This code runs fine and prints "Hello" once.

💡 Hint

LaunchedEffect(vararg keys: Any?, block: suspend CoroutineScope.() -> Unit); vararg keys can be omitted.

No syntax error: keys is vararg Any? and can be empty/omitted. The block runs once when the composable enters the composition, similar to LaunchedEffect(Unit).