# View Models

## Definition

A **View Model** represents a **fully prepared UI state**.

It answers the question:

> “What does the UI need in order to render this screen without thinking?”

View Models are **UI-owned** classes.
They do not represent business truth and do not enforce domain rules.

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## Responsibilities

A View Model MAY:

- accept an API DTO as input
- derive UI-specific fields
- combine or reshape data for rendering
- perform formatting (dates, numbers, labels)
- handle localization and presentation logic
- use Display Objects for reusable UI concerns

A View Model MUST:

- be fully usable by the UI without further computation
- expose only data and UI-oriented helpers
- be created in a consistent, explicit way

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## Restrictions

A View Model MUST NOT:

- contain business logic
- validate domain rules
- enforce permissions or authorization
- contain domain entities or value objects
- perform side effects
- be sent back to the server

If a View Model decides whether something is *allowed* or *correct*,
that logic belongs in the Core, not here.

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## Ownership & Placement

- View Models belong to the **frontend**
- They live close to the UI, not in shared or core layers
- They are not shared with the backend

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## Creation Rules

- View Models are created from API DTOs
- DTOs never reach pages/components; map DTO → ViewModel in website services
- UI components must never construct View Models themselves
- Construction happens in services or presentation layers
- The UI only consumes View Models, never DTOs

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## Testing

View Models SHOULD be tested when they contain:

- formatting logic
- localization behavior
- non-trivial derived fields

View Models do NOT need tests if they only expose data without logic.

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## Summary

- View Models describe **UI state**
- They are **presentation-focused**, not business-focused
- They reduce complexity in components
- They form a stable contract for the UI