Sealed and state
- Kotlin course
- Intermediate
- Lesson 27/116
- 5 min
Learn state management with sealed classes.
State management
Sealed classes are ideal for state management. A screen or operation can have mutually exclusive states like loading, success or error. Each state carries its own data.
Example: sealed class UiState; object Loading : UiState(); data class Success(val data: String) : UiState(); data class Error(val message: String) : UiState(). A UiState instance is in only one state at a time. when updates the UI per state; all states are checked, so no branch is forgotten. This simplifies complex state logic.
Loading, Success, Error
A UI state is usually in one of three states: Loading, Success (carries data) and Error (carries a message).
Example: object Loading : UiState(); data class Success(val data: String) : UiState(); data class Error(val message: String) : UiState(). Loading is a single state (object); Success and Error are data classes because they carry data. when produces a different UI per state: loading spinner, data list, error message. This pattern is common in Android and UI apps.
Practice Render with the UiState sealed class
Define a sealed class named UiState. Add three subtypes: object Loading : UiState(), data class Success(val data: String) : UiState() and data class Error(val message: String) : UiState(). Write a function render(state: UiState): String whose body uses when (state) to return "loading" for Loading, state.data for Success and state.message for Error. In main create a Success("records") instance and print the result of render.
Starter code
// Define the UiState sealed class
// object Loading, data class Success and Error subtypes
// render function: branch with when
// instantiate Success in main, print renderSample solution
sealed class UiState
object Loading : UiState()
data class Success(val data: String) : UiState()
data class Error(val message: String) : UiState()
fun render(state: UiState): String {
return when (state) {
is Loading -> "loading"
is Success -> state.data
is Error -> state.message
}
}
fun main() {
val s: UiState = Success("records")
println(render(s))
}Expected output
records
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the sealed state definition
sealed ____ UiState
object Loading : UiState()
data class Success(val data: String) : UiState()sealed class UiState
object Loading : UiState()
data class Success(val data: String) : UiState()Explanation: sealed class declares the closed UiState hierarchy.
2 Match the state pattern
| Term | Meaning |
|---|---|
object Loading |
Singleton state |
data class Success |
State with data |
data class Error |
Error state |
when (state) |
Branch by state |
3 Why use a sealed class for state management?
- Because a sealed class is fast at runtime
- Because all states fit in one variable
- Because a sealed class is a database
- Because the compiler checks all states and no branch is forgotten Correct answer
Explanation: With a sealed class, when is exhaustive; no state branch is forgotten.