Suggest an editImprove this articleRefine the answer for “What does the ViewModel do in MVVM?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)The **ViewModel** in the **MVVM (Model-View-ViewModel)** architecture is the central layer that connects the **data (Model)** and the **interface (View)**. It manages all the **presentation logic**: it holds the screen state, handles user actions, and updates data, while **knowing nothing about the interface itself**. **Key point:** the ViewModel is the "brain" of the interface: it knows *what* must be shown, but not *how* it looks.Shown above the full answer for quick recall.Answer (EN)ImageThe **ViewModel** in the **MVVM (Model-View-ViewModel)** architecture is the central layer that connects the **data (Model)** and the **interface (View)**. It manages all the **presentation logic**: it holds the screen state, handles user actions, and updates data, while **knowing nothing about the interface itself**. --- ### 1. The main role of the ViewModel > The ViewModel is the "brain" of the interface. > It knows **what** must be shown, but not **how** it looks. --- ### 2. Main responsibilities #### 1. **Holds the screen state** Contains all the data needed to display the View: text, lists, loading flags, errors, and so on. When this data changes, the View updates automatically through **data binding**. *Example:* `isLoading = true` → the View shows a loading indicator, `userName = "Maria"` → the text on the screen changes without extra code. --- #### 2. **Handles user actions** On clicks, swipes, or data input, the ViewModel receives an event from the View and decides what to do: - calls **Model** methods, - updates the state, - notifies the View that the data has changed. *Example:* The user clicked the "Save" button → `ViewModel.saveUser()` calls `Model.save()`, and after it completes, updates `saveStatus = "success"`. --- #### 3. **Organizes the connection between the View and the Model** The ViewModel is a mediator that turns "raw" data from the model into a format convenient for display: - formats dates, numbers, currency; - filters or sorts lists; - combines data from different sources. *Example:* `user.birthDate` from the model is turned into `"25 years old"` for the View. --- #### 4. **Knows nothing about the View** The ViewModel has no references to UI components (buttons, fields, interface elements). It just provides data and commands, and the View "subscribes" to them. This makes it possible to: - test the ViewModel separately from the interface, - use the same ViewModel with different Views (for example, a screen and a widget). --- ### 3. How it works (layer interaction) ```javascript User → View ↔ ViewModel ↔ Model ``` 1. The user interacts with the View (clicks a button). 2. The View calls a command on the ViewModel (`onLoginClicked()`). 3. The ViewModel calls Model methods (`auth.login()`) and gets a result. 4. It updates its properties (`isLoading`, `userInfo`). 5. The View updates automatically through *binding*. --- ### 4. Example in pseudocode ```kotlin class UserViewModel { val userName = ObservableField<String>() val isLoading = ObservableField<Boolean>() fun loadUser() { isLoading.set(true) userRepository.getUser { user -> userName.set(user.name) isLoading.set(false) } } } ``` The View is simply "subscribed" to `userName` and `isLoading`, and it automatically updates the interface when they change. --- **Summary:** > The **ViewModel** is the connecting link between the interface and the data. > It manages state, logic, and commands, > provides reactive updates of the View, > and makes it possible to test the logic without launching the interface.For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.