terminationHandler

TerminationHandler is a callback mechanism used in process management APIs to execute code when a launched external process ends. It is most commonly associated with the Foundation Process class on Apple platforms, where a terminationHandler property can be set to run a closure when the process terminates.

In practice, terminationHandler is a closure of the form (Process) -> Void and is assigned to a Process instance before or after launch. When the external process finishes, the runtime invokes the closure, passing the Process object as an argument. Through this callback, a program can inspect the process terminationStatus, perform cleanup, or trigger follow-up actions such as notifying users or starting another task.

Usage considerations include thread context and timing. The terminationHandler may execute on a background thread, and it should be safe to access shared resources or update UI only through appropriate synchronization mechanisms (for example, dispatching to the main queue if UI work is required). It is also important to retain the Process instance for as long as the process is expected to run; if the object is deallocated, the terminationHandler will not be invoked.

Common use cases include asynchronous workflow orchestration, logging and auditing of external tasks, and coordinating multiple subprocesses. If a terminationHandler is not used, developers may rely on synchronous wait mechanisms or periodic polling of terminationStatus, but the handler provides a convenient, event-driven alternative.

Related concepts include the Process class, terminationStatus, and, in some cases, terminationReason (where available). Termination handling is part of a broader pattern of lifecycle callbacks used to manage external processes within an application.