remember and mutableStateOf in Android Kotlin

Introduction

We use remember and mutableStateOf to keep track of values that can change in our app's screen. This helps the app update the screen when something changes.

When you want to save a user's input in a text box and show it on the screen.

When you want to toggle a button's color when it is clicked.

When you want to count how many times a button is pressed and show the count.

When you want to remember if a checkbox is checked or not.

Syntax

Android Kotlin

val variableName = remember { mutableStateOf(initialValue) }

remember keeps the value during recompositions (screen redraws).

mutableStateOf creates a value that can change and notify the screen to update.

Examples

This creates a number count starting at 0 that can change and update the screen.

Android Kotlin

val count = remember { mutableStateOf(0) }

This creates a text name starting empty that can change when the user types.

Android Kotlin

val name = remember { mutableStateOf("") }

This creates a true/false value isChecked starting as false.

Android Kotlin

val isChecked = remember { mutableStateOf(false) }

Sample App

This app shows a number that starts at 0. When you press the button, the number goes up by 1. The screen updates automatically because we use remember and mutableStateOf.

Android Kotlin

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

@Composable
fun CounterApp() {
  val count = remember { mutableStateOf(0) }

  Column(modifier = Modifier.padding(16.dp)) {
    Text(text = "You clicked: ${count.value} times")
    Spacer(modifier = Modifier.height(8.dp))
    Button(onClick = { count.value += 1 }) {
      Text("Click me")
    }
  }
}

@Preview
@Composable
fun PreviewCounterApp() {
  CounterApp()
}

Important Notes

Always use remember with mutableStateOf inside a @Composable function to keep state across recompositions.

Change the value by updating variable.value, not by reassigning the whole variable.

Using these helps your app respond to user actions smoothly.

Summary

remember keeps values during screen redraws.

mutableStateOf creates a value that can change and update the UI.

Use them together to make interactive and dynamic screens.

Practice

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

easy

A. To keep a value during recompositions so it doesn't reset

B. To create a mutable state that updates the UI

C. To launch a coroutine for background tasks

D. To define a new composable function

Solution

1. Step 1: Understand what recomposition means

   Recomposition happens when the UI redraws, and values can reset unless preserved.

2. Step 2: Identify the role of remember

   remember keeps a value alive during recompositions to avoid resetting.

3. Final Answer:

   To keep a value during recompositions so it doesn't reset -> Option A

4. Quick Check:

   remember = keeps value during redraws [OK]

Hint: remember keeps values stable across UI redraws [OK]

Common Mistakes:

• Confusing remember with mutableStateOf
• Thinking remember creates mutable state
• Assuming remember launches background tasks

2. Which of the following is the correct syntax to create a mutable state variable named count initialized to 0 using remember and mutableStateOf?

easy

A. val count = remember { mutableStateOf(0) }

B. var count = mutableStateOf(remember(0))

C. val count = mutableStateOf(remember { 0 })

D. var count = remember.mutableStateOf(0)

Solution

1. Step 1: Recall the correct order of functions

   remember wraps mutableStateOf to keep state across recompositions.

2. Step 2: Check syntax correctness

   val count = remember { mutableStateOf(0) } correctly uses val count = remember { mutableStateOf(0) }. Others misuse function calls or syntax.

3. Final Answer:

   val count = remember { mutableStateOf(0) } -> Option A

4. Quick Check:

   remember wraps mutableStateOf syntax [OK]

Hint: remember { mutableStateOf(value) } is the correct pattern [OK]

Common Mistakes:

• Swapping order of remember and mutableStateOf
• Using var instead of val incorrectly
• Calling remember or mutableStateOf as properties

3. Given the code snippet:

var count = remember { mutableStateOf(0) }
count.value += 1
println(count.value)

What will be printed after this code runs once?

medium

A. 0

B. Compilation error

C. null

D. 1

Solution

1. Step 1: Understand initial value of count

   count is a mutable state holding 0 initially.

2. Step 2: Increment the value and print

   count.value += 1 increases value to 1, so printing count.value outputs 1.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   mutableStateOf value increments correctly [OK]

Hint: mutableStateOf value accessed with .value [OK]

Common Mistakes:

• Printing count instead of count.value
• Forgetting to use .value to access state
• Assuming count is a simple Int

4. What is wrong with this code snippet?

var count = mutableStateOf(0)
count = remember { mutableStateOf(1) }

medium

A. Cannot assign remember result to a var

B. Using var instead of val for state causes errors

C. Remember must be called inside a composable function

D. mutableStateOf cannot be reassigned

Solution

1. Step 1: Identify where remember can be used

   remember can only be called inside a composable function context.

2. Step 2: Analyze the snippet context

   The snippet shows remember called outside composable, which is invalid.

3. Final Answer:

   Remember must be called inside a composable function -> Option C

4. Quick Check:

   remember requires composable context [OK]

Hint: remember only works inside @Composable functions [OK]

Common Mistakes:

• Calling remember outside composable functions
• Confusing var and val usage
• Thinking mutableStateOf cannot be reassigned

5. You want to create a counter button that increments a number each time it is clicked and keeps the count even if the screen recomposes. Which code snippet correctly implements this behavior?

hard

A. @Composable
fun Counter() {
val count = mutableStateOf(0)
Button(onClick = { count.value++ }) {
Text("Count: ${count.value}")
}
}

B. @Composable
fun Counter() {
var count by remember { mutableStateOf(0) }
Button(onClick = { count++ }) {
Text("Count: $count")
}
}

C. @Composable
fun Counter() {
var count = 0
Button(onClick = { count++ }) {
Text("Count: $count")
}
}

D. @Composable
fun Counter() {
val count = remember { 0 }
Button(onClick = { count++ }) {
Text("Count: $count")
}
}

Solution

1. Step 1: Ensure state is remembered and mutable

   Using remember { mutableStateOf(0) } with var count by delegates state correctly.

2. Step 2: Check button click updates state and UI

   Incrementing count++ updates state and triggers UI recomposition.

3. Step 3: Analyze other options

   @Composable fun Counter() { val count = mutableStateOf(0) Button(onClick = { count.value++ }) { Text("Count: ${count.value}") } } lacks remember, so state resets on recomposition. @Composable fun Counter() { var count = 0 Button(onClick = { count++ }) { Text("Count: $count") } } uses simple var without state, so UI won't update. @Composable fun Counter() { val count = remember { 0 } Button(onClick = { count++ }) { Text("Count: $count") } } remembers an Int, not mutable state, so increment fails.

4. Final Answer:

   @Composable fun Counter() { var count by remember { mutableStateOf(0) } Button(onClick = { count++ }) { Text("Count: $count") } } -> Option B

5. Quick Check:

   remember + mutableStateOf + var by delegate = correct counter [OK]

Hint: Use remember { mutableStateOf() } with var by for stateful UI [OK]

Common Mistakes:

• Not using remember causes state reset on recomposition
• Using simple var without state prevents UI updates
• Forgetting to use mutableStateOf for mutable state