DATA_FLOW.md

SpendWise App Data Flow

MVVM Architecture with Room

The app follows the Model-View-ViewModel (MVVM) architecture pattern to separate UI logic from data management.

Data Flow Steps (UI → Database)

1. User Action: The user enters an amount, category, date, and note in the AddExpenseFragment and clicks "Save".
2. Fragment: The AddExpenseFragment collects the input data and calls mainViewModel.insertExpense(expense).
3. ViewModel: The MainViewModel (which is viewModelScope aware) launches a coroutine and calls repository.insertExpense(expense).
4. Repository: The ExpenseRepository acts as a clean API for data access. It calls expenseDao.insertExpense(expense).
5. DAO (Data Access Object): The ExpenseDao executes the actual SQL INSERT query into the Room Database.
6. Database: The AppDatabase (Room) stores the data in the SQLite file on the device.

Data Flow Steps (Database → UI)

1. Observation: In HomeFragment, we observe viewModel.allExpenses (a LiveData<List<Expense>>).
2. ViewModel: MainViewModel exposes allExpenses which comes directly from repository.allExpenses.
3. Repository: ExpenseRepository gets this LiveData from expenseDao.getAllExpenses().
4. DAO & Room: Room automatically triggers the LiveData update whenever the underlying database table changes (e.g., after the insert above).
5. UI Update: When the LiveData emits a new list, the Observer in HomeFragment receives it and calls adapter.submitList(expenses).
6. RecyclerView: The ExpenseAdapter calculates the diff and efficiently updates the list on the screen.

Summary of Components

• View (UI): Activity, Fragments, XML Layouts. Displays data and captures user input.
• ViewModel: MainViewModel. Holds UI state and business logic. Survives configuration changes.
• Model (Data):
  • Repository: ExpenseRepository. Mediator between ViewModel and Data Sources.
  • Room Database: AppDatabase, ExpenseDao, CategoryDao, Entities. The single source of truth.