Derived state in Android Kotlin

Introduction

Derived state helps you create new values based on existing data without storing extra copies. It keeps your app simple and efficient.

When you want to show a value that depends on other values, like a total price from item prices.

When you want to update UI automatically when some data changes.

When you want to avoid repeating calculations in multiple places.

When you want to keep your data consistent and avoid bugs.

When you want to improve app performance by computing values only when needed.

Syntax

Android Kotlin

val derivedValue = originalValue.map { it * 2 }

val means the value won't change directly.

map creates a new value based on the original.

Examples

This creates a new list with each number doubled.

Android Kotlin

val numbers = listOf(1, 2, 3)
val doubled = numbers.map { it * 2 }

Derived greeting string from the name variable.

Android Kotlin

val name = "John"
val greeting = "Hello, $name!"

Derived boolean state based on age.

Android Kotlin

val isAdult = age >= 18

Sample App

This app shows a count and its double. The double is a derived state that updates automatically when count changes.

Android Kotlin

import androidx.compose.runtime.*
import androidx.compose.material3.*
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.foundation.layout.*
import androidx.compose.ui.unit.dp

@Composable
fun DerivedStateExample() {
  var count by remember { mutableStateOf(0) }
  val doubleCount by remember { derivedStateOf { count * 2 } }

  Column(modifier = Modifier.padding(16.dp)) {
    Text(text = "Count: $count")
    Text(text = "Double Count (derived): $doubleCount")
    Spacer(modifier = Modifier.height(8.dp))
    Button(onClick = { count++ }) {
      Text("Increase Count")
    }
  }
}

@Preview
@Composable
fun PreviewDerivedStateExample() {
  DerivedStateExample()
}

Important Notes

Derived state helps avoid unnecessary recomputations in UI.

Use derivedStateOf in Compose to create efficient derived values.

Always base derived state on existing state to keep data consistent.

Summary

Derived state creates new values from existing data without extra storage.

It keeps UI updated automatically when data changes.

Use it to write cleaner and more efficient code.