Onentry

Onentry, often written as onEntry or entry, is a term used in software design to describe actions that are executed when an entity enters a particular state, region, or context. It is commonly employed as a naming convention for callbacks or methods that are invoked automatically by an engine, framework, or state machine when the relevant transition occurs.

In finite state machines and statecharts, entry actions specify what should happen immediately after the system enters a state. Typical purposes include initializing data, starting a process, resetting timers, or updating outputs. Entry actions are paired with exit actions, which run when leaving the state, helping to structure side effects around transitions.

In user interface and application frameworks, on-entry hooks can run when a view, screen, or component becomes active or is navigated to. This allows developers to prepare the new context, fetch data, or configure UI elements as part of the transition into the active state. Variants exist across languages and frameworks, and the exact mechanism—callback, method, or lifecycle hook—depends on the platform.

Implementation considerations include ensuring that entry actions are efficient and, where appropriate, idempotent to avoid repeated work on re-entry. Designers may separate entry actions from other initialization to improve readability and testability, and to manage sequencing with other transition effects.

See also entry action, onExit, state machine, and event-driven programming.