viewmodels

ViewModel is a software design pattern component in the MVVM architecture that acts as an intermediary between the user interface and the domain or model layer. It encapsulates presentation logic and state, exposing data, properties, and commands that the view binds to. The viewmodel does not reference the view directly, enabling testability and separation of concerns; the view binds to the viewmodel, typically through data binding and observation.

In MVVM, the Model represents the data and business rules; the View is the user interface. The ViewModel presents data in a form convenient for display and interaction, and handles user actions by translating them into updates to the model or services.

Responsibilities include exposing data as observable properties, transforming data for display, handling user input via commands or actions, performing validation, and coordinating navigation or service calls. It may manage state across the view's lifecycle and coordinate with models or backend services.

Common techniques include data binding, property change notification (such as INotifyPropertyChanged in .NET or observable patterns in other environments), commands, and asynchronous data loading. Implementations vary by framework, with examples in WPF and Knockout, and Android’s architecture components; some environments use components that fulfill similar roles rather than a strict viewmodel.

Benefits include improved testability, maintainability, and a clear separation of concerns; viewmodels can be unit tested without a view. Drawbacks include potential boilerplate, added complexity, and the risk of placing nontrivial logic in the viewmodel.

Relation to other patterns: MVVM relies on data binding to synchronize the view and the viewmodel, contrasting with MVC where controllers manage input. Design goals emphasize a thin view, a reusable viewmodel, and decoupled models.