Android Nomad - #35 State

Comparison of remember, rememberSaveable, and derivedStateOf in Jetpack Compose.

1. remember

remember is used to preserve state across recompositions.

• Purpose: Stores a mutable state in the Composition.
• Lifecycle: Value is retained as long as the composable remains in the Composition.
• Use case: For storing simple state that doesn't need to survive configuration changes or process death.

Example:

var count by remember { mutableStateOf(0) }

2. rememberSaveable

rememberSaveable is similar to remember, but it also preserves state across configuration changes and process death.

• Purpose: Stores a mutable state that survives configuration changes and process death.
• Lifecycle: Value is retained even if the app is killed and restored.
• Use case: For important UI state that should be restored if the user leaves and comes back to the app.

Example:

var text by rememberSaveable { mutableStateOf("") }

3. derivedStateOf

derivedStateOf is used to create a state that's derived from other state objects.

• Purpose: Computes a value based on other state objects and caches the result.
• Lifecycle: Recalculated when any of the state objects it depends on changes.
• Use case: For values that are expensive to compute or need to be cached.

Example:

val sortedList by remember { derivedStateOf { originalList.sorted() } }

Key Differences

1. Persistence:

- remember: Persists across recompositions.

- rememberSaveable: Persists across recompositions and configuration changes.

- derivedStateOf: Recalculates when dependencies change.

2. Usage:

- remember and rememberSaveable are used for storing state.

- derivedStateOf is used for computing derived state based on other state objects.

3. Performance:

- remember is lightweight.

- rememberSaveable has some overhead due to saving/restoring state.

- derivedStateOf can improve performance by caching complex calculations.

4. Scope:

- remember and rememberSaveable are typically used at the composable function level.

- derivedStateOf is often used within a remember block or in a ViewModel.

When to Use Each

- Use remember for simple, local UI state that doesn't need to survive configuration changes.

- Use rememberSaveable for important UI state that should persist across configuration changes and process death.

- Use derivedStateOf when you have a value that depends on other state and you want to optimize recomposition or cache expensive calculations.