Android Fundamentals

Kotlin essentials

• Null safety is in the type system. String? and String are different types, and the compiler will hold you to it.
• Data classes give you equals, hashCode, copy and destructuring.
• Sealed classes model a closed set of states — perfect for UI state.

sealed interface SkillUiState {
    data object Loading : SkillUiState
    data class Ready(val skills: List<Skill>) : SkillUiState
    data class Failed(val message: String) : SkillUiState
}

The lifecycle that actually bites

Rotation, dark mode, language change and low memory can all destroy and recreate your Activity. Anything held only in the Activity is gone.

• Survives recreation: ViewModel.
• Survives process death: SavedStateHandle, or persisted storage.
• Survives nothing: a property on the Activity.

Compose and recomposition

Compose re-runs composable functions when the state they read changes. This means:

• A composable must be side-effect free — it can run many times.
• Read state as late as possible, so less gets recomposed.
• Hoist state: a composable that takes a value and a callback is testable and reusable; one that owns its own state is neither.

@Composable
fun LevelChip(level: Int, modifier: Modifier = Modifier) {
    Text(
        text = "L$level",
        style = MaterialTheme.typography.labelSmall,
        modifier = modifier
            .background(MaterialTheme.colorScheme.surfaceVariant, CircleShape)
            .padding(horizontal = 8.dp, vertical = 2.dp),
    )
}

Coroutines

• Structured concurrency: work is scoped, and cancelling the scope cancels the work. Use viewModelScope.
• Flow is a stream. StateFlow always has a current value — that is what a screen wants.
• Never block the main thread. Use withContext(Dispatchers.IO) for disk and network.