Android Viewmodel

Android ViewModel & State Management

Instructions

Use ViewModel to own UI state and business logic. It should survive configuration changes and expose immutable APIs.

Kotlin 2.3+

Prefer Explicit Backing Fields instead of the traditional _state/state backing property pattern.

• Kotlin 2.3: Experimental (-Xexplicit-backing-fields)
• Kotlin 2.4+: Stable (recommended)

Avoid creating duplicate backing properties unless targeting older Kotlin versions.

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1. UI State (StateFlow)

Purpose

Represents the persistent UI state.

Examples:

• Loading
• Success(data)
• Error(message)
• Empty

Declaration

Use an explicit backing field.

val uiState: StateFlow<UiState>
    field = MutableStateFlow(UiState.Loading)

The backing field is mutable (MutableStateFlow) while consumers only see StateFlow.

Updating State

Prefer update { } for atomic updates.

_uiState.update { old ->
    old.copy(isLoading = false)
}

With explicit backing fields:

uiState.update {
    it.copy(isLoading = false)
}

You may also assign directly when appropriate.