Side effects (LaunchedEffect, SideEffect) in Android Kotlin - Build, Publish & Deploy

Build & Publish - Side effects (LaunchedEffect, SideEffect)

Performance Impact

Using LaunchedEffect and SideEffect in Jetpack Compose helps manage side effects safely and efficiently. Proper use ensures UI updates happen without blocking the main thread, maintaining smooth 60fps animations. However, careless or repeated side effects can cause unnecessary recompositions or background work, increasing CPU and battery use.

Optimization Tips

To keep your app running smoothly at 60fps, launch side effects only when needed by providing correct keys to LaunchedEffect. Avoid launching heavy work inside SideEffect as it runs on the main thread. Use remember to cache values and prevent repeated side effect calls. Cancel or clean up coroutines properly to avoid memory leaks.

App Size and Startup Time

Using LaunchedEffect and SideEffect does not significantly affect your app's bundle size. They are part of Jetpack Compose runtime, which is already included. However, launching many side effects at startup can delay UI readiness, increasing perceived startup time. Keep side effects minimal and defer non-urgent work.

iOS vs Android Differences

On Android, LaunchedEffect and SideEffect are part of Jetpack Compose and run on Kotlin coroutines and the main thread respectively. iOS apps built with SwiftUI use different patterns like onAppear and task modifiers for side effects. Android requires careful coroutine management; iOS uses Combine or async/await. Both platforms emphasize avoiding blocking the main UI thread.

Store Review Guidelines

Ensure side effects do not cause excessive background work or battery drain, as Google Play and Apple App Store review battery usage and app responsiveness. Avoid launching network calls or heavy processing without user consent or visible UI feedback. Follow platform guidelines for background tasks and permissions to pass store reviews smoothly.

Self-Check Question

Your app takes 5 seconds to load this screen. What's likely wrong?

Answer: You might be launching heavy or blocking side effects directly in SideEffect or LaunchedEffect without proper keys or cancellation. This causes the UI to wait for these tasks, delaying rendering. Optimize by moving heavy work off the main thread and launching side effects only when necessary.

Key Result

Proper use of LaunchedEffect and SideEffect ensures smooth UI updates without blocking the main thread, maintaining 60fps performance and efficient battery use. Avoid heavy work in side effects and manage coroutine lifecycles to optimize app startup and responsiveness.

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