What is the unidirectional data flow (UDF) pattern?

Short answer

An architectural pattern in which state flows down toward the UI, and events and user interactions flow up toward the ViewModel.

Explanation

UDF guarantees a single source of truth. The UI does not modify variables directly; it fires an intent/event (such as OnItemClicked). The ViewModel receives this event, executes the required business logic through repositories, and then produces a new immutable state that the UI picks up and redraws itself.

Code example

// 1. State flows down
val uiState: StateFlow<NewsUiState>

// 2. Events flow up
fun onEvent(event: NewsUiEvent) {
    when (event) {
        is NewsUiEvent.Refresh -> fetchNews()
        is NewsUiEvent.Bookmark -> saveArticle(event.articleId)
    }
}

Common mistakes

• Modifying state from multiple places inside the UI instead of making the ViewModel solely responsible for emitting states.

Follow-up questions

• What is the architectural difference between MVVM and MVI in Android?

Sources

• Android Developers — Architecture guide / UI Layer