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ViewModel – it is a model of the view. The purpose of the ViewModel is to apply any business logic to the Model before exposing it to the View for consumption. This way the View is free of business logic.

In the Android application CryptoOracle the data is persisted in SQLite database. The reasons are two:
The application can still work and show persisted data when there is no internet connection.

Room has great integration with LiveData. Results from SQL queries can be easily wrapped in LiveData container. This way LiveData can be observed in the UI. When the data is changed in the database, all active observers get notified and update the corresponding views.

The web layer is the top layer in the architecture. It is responsible for processing user queries and returning back responses to the user. These operations happen in the controllers.

Repository is a class which purpose is to provide a clean API for accessing data. What that means is that the Repository can gather data from different data sources(different REST APIs, cache, local database storage) and it provides this data to the rest of the app.

The View component in the MVVM architecture are the UI controllers – Activities and Fragments. They should be as free as possible from any business logic. Their role is to present the data, not to transform it.

LiveData is an observable data holder class. It is also lifecycle-aware, which means that it respects the lifecycle of the other app components, such as activities, fragments and services. It notifies only active observers, represented by the Observer class.

As we said in the previous article Model is the data in our application. These are classes representing objects that we persist in our database or that we get from network calls to services.

Writing application code before thinking about an architecture is a really bad idea. Android programming does not make a difference. Everybody who starts creating Android apps begins doing so without following a specific architecture.