ViewModel

ViewModel is a software architectural pattern used in the Model–View–ViewModel (MVVM) design. It provides an abstraction of the user interface that exposes data and actions in a way that is independent of presentation details. The ViewModel retrieves and prepares data for presentation, handles view-related logic, and mediates between the Model (data and domain logic) and the View (user interface). The goal is to separate concerns so that UI code can evolve without altering business logic and vice versa, improving testability and maintainability.

In MVVM, the View binds to properties and commands exposed by the ViewModel. The ViewModel exposes observable data and actions that the View can react to, usually without the View needing to know how the data is produced. The ViewModel should avoid direct references to UI elements, focusing instead on state and behavior that support rendering. Implementations commonly use data binding, observable patterns, or reactive streams to propagate changes from the ViewModel to the View and from user input back to the ViewModel.

Common platform realizations include .NET and WPF where the ViewModel implements property change notifications (such as INotifyPropertyChanged) and collections for dynamic data; Android's architecture components use ViewModel alongside LiveData and coroutines; iOS and web frameworks also employ MVVM concepts, with Vue.js often described as applying a ViewModel layer between data and templates.

The ViewModel complements other patterns such as Model-View-Controller by concentrating on presentation logic and state management rather than rendering. Benefits include improved testability, better separation of concerns, and easier maintenance, while potential drawbacks include added boilerplate and the risk of overloading the ViewModel with responsibilities if not carefully scoped.