Hello Dev, Here is the list of the most frequently asked questions of view-model in interviews with answers. 

As we know view model is a part of Android MVVM architecture click here to know more about the view model in Android. Let’s dive into the view model questions directly.

1:- What is ViewModel in Android, and why is it used?

ViewModel is an Android Architecture Component used for storing and managing UI-related data in a lifecycle-aware manner. It’s designed to survive configuration changes (like screen rotations) and prevent data loss.

2:- Explain the difference between ViewModel and savedInstanceState for retaining UI data during configuration changes.

ViewModel retains data across configuration changes, while savedInstanceState is used to save and restore transient UI state data (e.g., EditText contents). ViewModel is more suitable for preserving complex data and business logic.

3:- How is a ViewModel different from a Fragment or Activity in terms of lifecycle?

 ViewModel has a lifecycle that’s not tied to any particular UI component (Activity or Fragment). It survives configuration changes and can be shared between multiple UI components.

4:- What is the purpose of the ViewModelProvider and ViewModelProvider.Factory in Android?

ViewModelProvider is used to create or retrieve a ViewModel instance, while ViewModelProvider.Factory is responsible for creating ViewModel instances with the necessary dependencies.

5:- What is the recommended way to create and access a ViewModel in an Android component like an Activity or Fragment?

You should use ViewModelProvider to create and access ViewModels. This ensures that ViewModels are retained correctly across configuration changes.

6:- How can you share data between different Fragments using ViewModel in Android?

You can use a shared ViewModel created at the activity level. Both Fragments can access the same ViewModel instance using ViewModelProvider.

7. Explain the purpose of the onCleared method in ViewModel.

The onCleared method is called when the ViewModel is no longer used and is being destroyed. It’s a good place to clean up resources or perform any necessary cleanup operations.

8. What are the benefits of using ViewModel with LiveData in Android app development?

ViewModel and LiveData work well together to manage UI-related data. LiveData in ViewModel allows for real-time updates to UI components, ensuring they always reflect the latest data.

9:- Can you use ViewModel without LiveData in Android? If so, how?

Yes, you can use ViewModel without LiveData by exposing simple data properties and methods for UI components to observe. However, LiveData is recommended for more seamless data synchronization.

10:- What is the relationship between ViewModel and Dagger (dependency injection) in Android development?

ViewModels can be injected with dependencies using Dagger. This ensures that the ViewModel has access to the required dependencies for data retrieval or manipulation.

11:- What is the purpose of the ViewModelStore in Android, and how does it relate to ViewModels?

The ViewModelStore is used to retain and manage ViewModel instances. It is associated with the lifecycle of an Android component (e.g., Activity or Fragment) and ensures that ViewModels are not recreated unnecessarily.

A ViewModelStore can be considered as a container that stores the ViewModels in a HashMap. Where the key is the string value and the value is the ViewModel being saved(ViewModelProvider uses a concatenation of the string_key + ViewModel class canonical name).

12:- How can you handle complex data transformations or business logic in a ViewModel?

You can encapsulate complex data transformations and business logic within the ViewModel, ensuring that the UI components remain focused on presentation and remain agnostic to the data processing.

13:- What are the potential memory leak issues associated with ViewModels, and how can you prevent them?

Memory leaks can occur if you don’t clear references to ViewModels correctly. To prevent this, avoid holding references to long-lived UI components in ViewModels and use onCleared for cleanup.

14:- Can you use multiple ViewModels in a single Android component (e.g., Activity or Fragment)? If so, when might you do this?

Yes, you can use multiple ViewModels in a single Android component. This might be necessary when different parts of the UI have distinct data and behavior requirements.

15:- Explain the role of the ViewModelProvider.Factory and how you can customize it for dependency injection.

The ViewModelProvider.Factory is responsible for creating ViewModel instances. You can customize it to inject dependencies or perform any other necessary setup when creating ViewModels.

16. How can you handle ViewModel persistence across the entire application’s lifecycle when using Android’s ViewModel architecture component?

To ensure ViewModel persistence across the entire application’s lifecycle, you can create a custom ViewModelProvider.Factory that manages ViewModel instances outside the scope of individual components, such as activities or fragments.

17:- When a device is rotated from portrait to landscape or vice versa, activity is destroyed and a new instance is created, how is this ViewModel instance still the same?

Every Activity and fragment has a ViewModelStore, which makes them a ViewModelStoreOwner. Whenever an activity is first started, the getViewModelStore() method is called and a ViewModelStore instance is created if it is null.

If you go ahead and rotate the screen, onRetainNonConfigurationInstance() is called and viewModelStore instance is saved into NonConfigurationInstance for use after configuration changes.

When the app finishes rotating, getViewModelStore() method is called and an old instance of ViewModelStore is provided to the activity from the NonConfigurationInstance object. This is how ViewModel survives configuration changes.

18:- ViewModel store is linked to activity/fragment and when in the process of rotation the current instance is destroyed and the new instance is created then how this ViewModelStore object is still the same? 

If an activity is being created for the first time then always new ViewModel store objects will be created. So It is NonConfigurationInstance object that is passed from old destroyed activity to newly created activity when rotations happen. It contains all the non-configuration related information including the ViewModel store which contains the ViewModel of the old activity object.

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