Breaking Up with Big ViewModels

This is a sample Android app that demonstrates how to split long ViewModels into smaller ones, using the Mediator pattern, and tie them to smaller and distinct parts of the screen.

🎥 Check out our droidcon talk!

For more information, watch our droidcon talk "Breaking Up with Big ViewModels — Without Breaking Your Architecture"

What You'll Learn

• Break a large ViewModel into smaller, independently testable ViewModels, each tied to a smaller component of the screen.
• Enable the smaller, screen-scoped ViewModels to reactively communicate with each other through a shared Mediator.
• Use Hilt or Koin to scope the Mediator to the screen’s lifecycle, automatically disposing it when the user exits that screen.

Architecture

View Layer

• AppNavHost - Compose Navigation setup
• GridScreen - Main gallery screen displaying media items
  • Grid - Shows the grid of media items
  • TopBar - Shows selection count and overflow menu
  • BottomBar - Action buttons (favorite, share, star, delete)

ViewModel Layer

• GridViewModel - Handles the grid state (the list of media items)
• TopBarViewModel - Manages top bar state (selection count)
• BottomBarViewModel - Controls bottom bar visibility and actions
• GridMediator - Holds the common UI state for the ViewModels to observe

UseCase Layer

• GetMediaUseCase - Fetches media data from repository

Repository Layer

• MediaRepository - Abstracts data access

Data Source Layer

• MediaDataSource / MediaDataSourceImpl - Handles data fetching from assets