What is the difference between local state and global state in a mobile application, and when should you use each?

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Understanding the distinction between local and global state is foundational for building well-structured mobile applications. Each serves a different purpose, and choosing correctly has a significant impact on maintainability and performance.

Local state refers to data that is relevant only to a single widget or component. Examples include whether a dropdown is open, the current text in an input field, animation progress, or form validation errors. This state does not need to be shared with other parts of the application. In Flutter, you manage local state with StatefulWidget and setState. In React Native, you use the useState hook or component-level state. Local state is simple to reason about because changes are contained within the component that owns them.

Global state, on the other hand, is data that multiple parts of the application need to access or modify. Common examples include the authenticated user profile, shopping cart contents, theme preferences, notification counts, or data fetched from an API that feeds several screens. Global state typically lives in a centralized store or is provided through dependency injection so that any widget in the tree can read or update it.

You should use local state when the data is ephemeral and self-contained. If toggling a switch only affects the current screen's UI, there is no reason to put it in a global store. Elevating everything to global state leads to unnecessary complexity, increased boilerplate, and harder-to-trace bugs. A good rule of thumb is to start local and only promote state to global when you find yourself needing to pass it through multiple layers of the widget tree or when two unrelated screens both depend on the same data.

Global state becomes necessary when the data has a lifecycle that extends beyond a single screen, when it needs to survive navigation changes, or when multiple features read and write the same values. Authentication tokens are a classic example: the login screen writes them, the API layer reads them, and the profile screen displays user information derived from them.

In practice, most applications use a combination of both. A screen might pull user data from a global store while managing its own scroll position locally. The key is to keep the boundary clear. Avoid leaking implementation details of one screen into global state just because it is convenient. Instead, design your state layers intentionally: ephemeral UI state stays local, shared business data goes global, and server-fetched data may live in a caching layer like React Query or Riverpod's async providers.

Getting this boundary right early in a project prevents the common problem of a bloated global store that becomes difficult to refactor as the application grows.