remember and mutableStateOf in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a mutable state variable that remembers its value across recompositions.

Android Kotlin

val count = [1] { mutableStateOf(0) }

Drag options to blanks, or click blank then click option'

Aremember

BmutableStateOf

CmutableState

DstateOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using mutableStateOf without remember causes state to reset on recomposition.

Confusing remember with mutableStateOf.

The remember function is used to keep the state across recompositions in Compose.

2fill in blank

medium

Complete the code to declare a mutable state integer variable initialized to 0.

Android Kotlin

var count by [1] { mutableStateOf(0) }

Drag options to blanks, or click blank then click option'

Aremember

BmutableStateOf

CstateOf

DmutableState

Attempts:

3 left

💡 Hint

Common Mistakes

Using mutableStateOf alone without remember.

Using stateOf which is immutable.

Using remember with mutableStateOf allows the variable to be mutable and remembered.

3fill in blank

hard

Fix the error in the code to properly remember a mutable state string initialized to "Hello".

Android Kotlin

val text = remember { [1]("Hello") }

Drag options to blanks, or click blank then click option'

AstateOf

Bremember

CmutableStateOf

DmutableState

Attempts:

3 left

💡 Hint

Common Mistakes

Using stateOf instead of mutableStateOf.

Using remember inside itself.

mutableStateOf creates a mutable state holder, which remember keeps across recompositions.

4fill in blank

hard

Fill both blanks to create a mutable state variable that remembers a Boolean value initialized to false.

Android Kotlin

var isChecked by [1] { [2](false) }

Drag options to blanks, or click blank then click option'

Aremember

BmutableStateOf

CstateOf

DmutableState

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping remember and mutableStateOf.

Using stateOf which is immutable.

remember keeps the state across recompositions, and mutableStateOf creates the mutable state.

5fill in blank

hard

Fill all three blanks to create a mutable state variable that remembers a String initialized to "Compose" and is mutable.

Android Kotlin

var title by [1] { [2]("[3]") }

Drag options to blanks, or click blank then click option'

Aremember

BmutableStateOf

CCompose

DstateOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using stateOf instead of mutableStateOf.

Forgetting to use remember.

Putting the initial value outside the quotes.

The variable title is remembered with remember, made mutable with mutableStateOf, and initialized to "Compose".

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