MutableState

Learn reactive state with mutableStateOf.

What is MutableState?

mutableStateOf is the reactive state creator of Compose. When the value changes, bound composables redraw automatically.

Example: val state = mutableStateOf(0). You read and write state.value. Writing makes Compose redraw every composable that observes this state. No manual subscription is needed; Compose tracks it automatically. Used with remember { mutableStateOf(0) }, the state is both retained and reactive. The stateInfo model in this lesson shows the concept.

Reading and writing state

You access a MutableState value through .value. Reading makes the composable an observer; writing triggers a redraw.

Example: val s = remember { mutableStateOf(0) }, then Text("${'$'}{s.value}"). Writing s.value = 5 makes Compose redraw the Text. This is unidirectional data flow: state flows down, events flow up. Compose manages this flow automatically; no manual update.

Exercise Write stateInfo

Define fun stateInfo(): String. It should return "MutableState". In main, write println(stateInfo()).

Show example solution

fun stateInfo(): String = "MutableState"
fun main() {
    println(stateInfo())
}

Test yourself

Which creates reactive state?
```
The reactive state creator is mutableStateOf.
```
- val x = 0
- var y = 0
- const z = 0
- val s = mutableStateOf(0)
Show answer

val s = mutableStateOf(0) — mutableStateOf produces reactive state.

Complete the mutableStateOf call

val state = ____

Show answer

val state = mutableStateOf(0)

Match the MutableState parts

mutableStateOf

State creator
state.value

Value access
recomposition

Auto redraw
remember

Retention

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