Boilerplate code for implementing MVVM in Android using Jetpack libraries, coroutines, dependency injection and local persistance

Overview

MVVM Foundation

This projects aims to speed up development of Android apps by providing a solid base to extend

Libraries

  • Jetpack Fragment
  • Material3 : for UI components and theming.
  • ViewPager2 : for ViewPager UI component. Used in sliding tabs.
  • EasyBinding : Abstract classes for Activities and Fragments that use viewBinding.
  • Hilt : dependency injection.
  • Room : local SQLite database.
  • RxJava3 : reactive programming in Android (so far not used, but good to have for the future).

Architecture

├── BaseApplication.kt
├── constants
│   └── NavigationConstants.kt
├── di_modules
│   ├── DatabaseModule.kt
│   └── DispatcherModule.kt
├── MainActivity.kt
├── model
│   ├── database
│   │   ├── AppDatabase.kt
│   │   ├── entities
│   │   │   └── TimeRecord.kt
│   │   └── TimeRecordsDao.kt
│   └── repository
│       └── TimeRecordsRepository.kt
├── view
│   ├── ButtonControlsFragment.kt
│   ├── FeedFragment.kt
│   └── navigation
│       └── ViewPagerAdapter.kt
└── viewmodel
    └── ButtonControlsViewModel.kt
  • BaseApplication is the entry point to the app. Annotated with @HiltAndroidApp to use dependency injection.
  • constants contains constants.
  • di_modules contains dependency injection providers (When Hilt is unable to figure out a dependency to inject, create a module to explicitly define how to provide the dependency).
  • model contains data related logic and infrastructure.
    • database contains classes related to database.
      • entities are data classes used by the database.
    • repoitory contains repositories (used to abstract database operations).
  • view contains Activities, Fragments and related UI classes (adapters, viewholders, etc).
  • viewmodel contains viewModels.

How does this work

Consider we need to show a list of TimeRecords (provided entity) in FeedFragment

  1. TimeRecordsDao defines a method for retrieving the list from the database TimeRecordsDao.getAll() : List<TimeRecord> by executing an annotated SQL query.
  2. TimeRecordsRepository defines a method TimeRecordsRepository.getAllRecords(): List<TimeRecord> that calls the Dao method with a Dispatcher Context (to avoid blocking the UI thread).
  3. ButtonControlsViewModel creates a coroutine ButtonControlsViewModel.getAllRecords() that calls the repository method within a coroutineScope and posts results to timeRecordsLiveData.
  4. FeedFragment observes ButtonControlsViewModel.timeRecordsLiveData and reacts to changes in the data, updating the UI.
  5. ButtonControlsFragment calls ButtonControlsViewModel.getAllRecords() to start the flow

Takeaways

  • The Dao is an interface that provides abstract suspend functions to communicate (execute queries) with the Database. Methods do not have a body, the annotation generates the code under the hood.
  • The Repository knows nothing about the Database, it only knows what the injected Dao can do and defines the injected Dispatcher to use.
  • The ViewModel knows nothing about the Dao, it only knows the methods that the injected Repository can provide and creates coroutines.
  • The ViewModel knows nothing about the View either, it uses callbacks to interact with Activities/Fragments and liveData to provide data to the observers (Activities/Fragments).
  • The View only knows about the ViewModel, it has absolutely no idea regarding the Database or the Repository.
  • This separation of concerns allows us to change specific components (for example, the Dispatcher injected to the Repository or the Repository injected to the ViewModel) with ease.
  • Sharing ViewModels across Fragments/Activities allows sharing state and methods (we call getDataAllRecords() in ButtonControlsFragment and observe the state changes results on FeedFragment)
  • Dependency injection (di) allows to change/update dependencies with less code and less memory allocation. For instance, we need the same instance of ButtonControlsViewModel for FeedFragment and ButtonControlsFragment, so we inject the ViewModel in the activity scope byActivityViewModels() to make the same instance available for both fragments. Without Hilt sharing a ViewModel in this case would require to make it a Singleton (and implement the singleton logic)
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