Mastering Launched Effect in Jetpack Compose: What, Why & How

atif rehman — Fri, 01 Aug 2025 06:23:07 +0000

Jetpack Compose has transformed the way we build UI in Android — but if you're not using LaunchedEffect properly, you're probably writing buggy or inefficient code.

Let’s break it down: What is LaunchedEffect, why is it important, and how do you use it correctly?

What is LaunchedEffect?

LaunchedEffect is a side-effect handler in Jetpack Compose. It lets you launch coroutines tied to the lifecycle of the composable, safely.

It only runs when its key(s) change, or when it first enters the composition.

kotlin LaunchedEffect(Unit) { // This runs once when the composable enters composition delay(1000) println("Effect triggered") }

Why Use LaunchedEffect?

You use LaunchedEffect for things like:

Fetching data when a screen loads
Triggering animations
Listening to a Flow
Running one-time setup logic

It’s lifecycle-aware

Automatically cancels if the composable leaves the UI
Safer than launching coroutines in init or onCreate

Don’t use LaunchedEffect for long-lived state
It's designed for one-off jobs like setup or reaction, not holding ongoing state.

Use remember or rememberCoroutineScope() if you need a long-lived coroutine scope within Compose.

Keys Matter!
The key parameter determines when the effect should re-run.

LaunchedEffect(Unit) { ... } // Runs once
LaunchedEffect(userId) { ... } // Runs when userId changes
LaunchedEffect(key1, key2) { ... } // Runs when either changes
Don’t use Unit if you want the effect to re-trigger — pass a meaningful key!

Final Tips

Use LaunchedEffect for one-shot tasks: data fetching, collecting flows, starting animations.
Always give it a key if you want it to react to changes.
Avoid logic inside Composables that causes uncontrolled recomposition.

DEV Community: atif rehman

Mastering Launched Effect in Jetpack Compose: What, Why & How