write-viewmodel

Development

Use when writing a new ViewModel or modifying an existing one in the Alkaa project. Triggers on tasks like "add a ViewModel", "create VM for X screen", "implement state handling", or "connect use case to UI".

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Write ViewModel

Overview

ViewModels in Alkaa bridge domain use cases and Compose UI via cold flows and sealed state classes. They contain no business logic — only orchestration, mapping, and scope management.

Structure

Sealed State Class — data object for stateless states, data class for states with data, ImmutableList for list payloads → see references/CODE_PATTERNS.md
Flow Exposure — Cold Flow<ViewState> by default; hot StateFlow only when multiple collectors share the same stream → see references/CODE_PATTERNS.md
Use Case Injection — Inject use cases as constructor parameters; never repositories or other ViewModels → see references/CODE_PATTERNS.md
Coroutine Scope — applicationScope for DB mutations that survive ViewModel destruction; viewModelScope for UI-bound work → see references/CODE_PATTERNS.md
Mappers — Dedicated mapper class injected as constructor param; never map inline → see references/CODE_PATTERNS.md

For unit test setup and fake patterns → see references/CODE_PATTERNS.md and the write-unit-tests skill.

Scope Rules

Situation	Scope
DB insert / update / delete	`applicationScope.launch { }`
Network call that should persist	`applicationScope.launch { }`
Debounced UI updates	`viewModelScope`
Short-lived UI-bound work	`viewModelScope`

Common Mistakes

Mistake	Fix
Calling repository directly	Wrap in a use case first
Exposing `StateFlow` for a screen-scoped stream	Return cold `Flow<ViewState>`
Mapping models inline in ViewModel	Create a `XxxMapper` class
Using `viewModelScope` for DB writes	Use `applicationScope.launch { }`
Depending on another ViewModel	Extract shared logic into a use case
Testing with mocks	Write a `XxxFake` that implements the use case interface

Related Skills

Fake patterns and test structure → use write-unit-tests skill
Composable and screen wiring → use write-composable skill