Why state

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Goal

Understand why a hard-coded list is not enough for a real app, and what unidirectional data flow solves. After this page you will have the vocabulary to follow the ViewModel and StateFlow pages.

Prerequisites

• One-screen list app (beginner exit artifact)

The problem with hard-coded data

In the beginner tier you built a screen backed by a plain Kotlin val:

val library = listOf(
    Book("The Pragmatic Programmer", "Hunt & Thomas"),
    // …
)

That list never changes while the app runs. Three things break the moment real data enters the picture.

Loading takes time. Network calls, database queries, and file reads complete milliseconds to seconds after the screen first draws. There is no slot in a plain val to represent "not ready yet" — the screen either shows stale data or crashes on a null value.

Operations can fail. The network can be down. The database can reject a query. A hard-coded list sidesteps this entirely. Real code must show an error state and let the user retry.

Data can change. A user action (pull-to-refresh, a background sync, a real-time update) might replace the list entirely. Compose redraws only when state changes, so you need state — not a plain value — to drive the UI.

Unidirectional data flow

Android's answer is a single direction: data flows down, events flow up.

Repository  →  ViewModel  →  UiState  →  Composable
                   ↑                           |
                   └──────── user event ───────┘

• The repository owns the source of truth (network, database, or in-memory stub).
• The ViewModel fetches from the repository, maps the result into a UiState, and exposes it as a StateFlow.
• The composable collects the flow and renders whichever state is current.
• User events (button taps, retry clicks) call back into the ViewModel, which triggers a new fetch.

Each arrow in that diagram is the subject of one page in this tier. The pages build on each other; the exit artifact is on page 07.

Next → 02 UiState and ViewModel