Side effects (LaunchedEffect, SideEffect) in Android Kotlin

Introduction

Side effects let your app do things outside of just showing UI, like loading data or logging. They help your app react to changes safely.

When you want to load data from the internet after the screen appears.

When you need to save user actions to a log or analytics service.

When you want to show a message once after a button is clicked.

When you want to run code only once when a screen is first shown.

When you want to update something outside the UI based on state changes.

Syntax

Android Kotlin

LaunchedEffect(key) {
    // suspend functions or side effect code here
}

SideEffect {
    // code that runs after every successful recomposition
}

LaunchedEffect runs suspend functions and cancels/restarts if the key changes.

SideEffect runs after every UI update but does not support suspend functions.

Examples

This runs once when the screen appears because the key is Unit (constant).

Android Kotlin

LaunchedEffect(Unit) {
    println("Screen appeared")
}

This reloads user data whenever userId changes.

Android Kotlin

LaunchedEffect(userId) {
    loadUserData(userId)
}

This prints a message after every UI update.

Android Kotlin

SideEffect {
    println("UI updated")
}

Sample App

This app shows a button with a click count. When you click, count changes.

LaunchedEffect prints a message only when count changes.

SideEffect prints a message every time the UI updates.

Android Kotlin

import androidx.compose.runtime.*
import androidx.compose.material3.*
import androidx.compose.ui.window.singleWindowApplication

fun main() = singleWindowApplication {
    var count by remember { mutableStateOf(0) }

    LaunchedEffect(count) {
        println("Count changed to $count")
    }

    SideEffect {
        println("UI recomposed with count = $count")
    }

    Button(onClick = { count++ }) {
        Text("Clicked $count times")
    }
}

Important Notes

Use LaunchedEffect for suspend functions like network calls.

SideEffect is good for quick updates like logging or updating external variables.

Keys in LaunchedEffect control when the effect restarts.

Summary

Side effects let your app do work outside just showing UI.

LaunchedEffect runs suspend code and restarts on key changes.

SideEffect runs after every UI update for quick side tasks.

Practice

1. What is the main purpose of LaunchedEffect in Jetpack Compose?

easy

A. To run suspend functions when a key changes

B. To immediately update UI elements

C. To create a new composable function

D. To handle user input events

Solution

1. Step 1: Understand LaunchedEffect role

   LaunchedEffect is designed to run suspend functions in response to changes in keys.

2. Step 2: Differentiate from UI updates

   It does not directly update UI or handle input but manages side effects like network calls.

3. Final Answer:

   To run suspend functions when a key changes -> Option A

4. Quick Check:

   LaunchedEffect runs suspend code on key change = D [OK]

Hint: LaunchedEffect runs suspend code on key changes [OK]

Common Mistakes:

• Confusing LaunchedEffect with UI rendering
• Thinking it handles user input directly
• Assuming it runs only once without key

2. Which of the following is the correct syntax to use SideEffect in a composable?

easy

A. SideEffect.launch { /* suspend code */ }

B. SideEffect { /* code to run after every recomposition */ }

C. SideEffect() { /* code to run once */ }

D. SideEffect.runOnUiThread { /* UI update code */ }

Solution

1. Step 1: Recall SideEffect usage

   SideEffect is a composable function that takes a lambda to run after every successful recomposition.

2. Step 2: Check syntax correctness

   SideEffect { /* code to run after every recomposition */ } matches the correct syntax without parentheses or suspend keywords.

3. Final Answer:

   SideEffect { /* code to run after every recomposition */ } -> Option B

4. Quick Check:

   SideEffect uses lambda syntax without parentheses = C [OK]

Hint: SideEffect uses simple lambda syntax without parentheses [OK]

Common Mistakes:

• Adding parentheses after SideEffect
• Trying to launch suspend code inside SideEffect
• Using SideEffect as a coroutine builder

3. What will be the output behavior of this code snippet?

var count = 0
LaunchedEffect(Unit) {
  count += 1
}
SideEffect {
  println("Count: $count")
}

medium

A. Prints "Count: 0" repeatedly on every recomposition

B. Prints "Count: 1" repeatedly on every recomposition

C. Prints "Count: 1" once after first composition

D. Prints "Count: 0" once and never again

Solution

1. Step 1: Analyze LaunchedEffect behavior

   LaunchedEffect(Unit) runs once on first composition and increments count to 1.

2. Step 2: Analyze SideEffect behavior

   SideEffect runs after every recomposition and prints current count value. However, since count is a regular variable and not a state, recompositions do not trigger updates to count or cause recompositions to reflect the incremented value. The printed value remains 0 repeatedly.

3. Final Answer:

   Prints "Count: 0" repeatedly on every recomposition -> Option A

4. Quick Check:

   Regular variable not triggering recomposition; SideEffect prints initial value repeatedly = B [OK]

Hint: Regular variables don't trigger recomposition; SideEffect prints initial value repeatedly [OK]

Common Mistakes:

• Assuming SideEffect runs only once
• Thinking count increments on every recomposition
• Confusing LaunchedEffect and SideEffect timing

4. Identify the error in this code using LaunchedEffect:

@Composable
fun MyScreen() {
  LaunchedEffect {
    delay(1000)
    println("Hello")
  }
}

medium

A. Cannot call delay inside LaunchedEffect

B. LaunchedEffect must be called outside composable

C. println cannot be used inside composable

D. Missing key parameter in LaunchedEffect call

Solution

1. Step 1: Check LaunchedEffect syntax

   LaunchedEffect requires at least one key parameter to track changes and restart the coroutine.

2. Step 2: Confirm other parts are valid

   Calling delay and println inside LaunchedEffect is allowed; it runs suspend code.

3. Final Answer:

   Missing key parameter in LaunchedEffect call -> Option D

4. Quick Check:

   LaunchedEffect needs key parameter = B [OK]

Hint: LaunchedEffect always needs a key parameter [OK]

Common Mistakes:

• Omitting key parameter in LaunchedEffect
• Thinking delay is disallowed inside LaunchedEffect
• Believing println can't be used in composables

5. You want to fetch data from the network only once when a screen appears and log a message after every UI update. Which combination correctly uses LaunchedEffect and SideEffect?

hard

A. Use LaunchedEffect without key for fetching and SideEffect(Unit) for logging

B. Use SideEffect(Unit) for fetching and LaunchedEffect { println("UI updated") } for logging

C. Use LaunchedEffect(Unit) for fetching and SideEffect { println("UI updated") } for logging

D. Use SideEffect with key for fetching and LaunchedEffect(Unit) for logging

Solution

1. Step 1: Identify correct usage for fetching once

   LaunchedEffect with Unit key runs once on first composition, ideal for fetching data.

2. Step 2: Identify correct usage for logging after UI updates

   SideEffect runs after every recomposition, perfect for logging UI updates.

3. Final Answer:

   Use LaunchedEffect(Unit) for fetching and SideEffect { println("UI updated") } for logging -> Option C

4. Quick Check:

   LaunchedEffect for one-time suspend; SideEffect for every UI update = A [OK]

Hint: LaunchedEffect for one-time tasks; SideEffect for every UI update [OK]

Common Mistakes:

• Using SideEffect for suspend fetching
• Omitting key in LaunchedEffect
• Confusing which runs once vs every recomposition