messagepump

Message pump, also called a message loop, is a core control flow construct in many GUI toolkits. It is a loop that runs on a thread and retrieves, translates, and dispatches messages or events from the system or other parts of the program. The pump coordinates input handling, painting, timers, and other asynchronous activities, ensuring that the user interface remains responsive.

In the Windows API, the typical pump repeatedly calls GetMessage to retrieve the next message from the thread's queue, then TranslateMessage and DispatchMessage to deliver it to the window procedure. The loop ends when GetMessage returns zero, indicating a WM_QUIT. Variants using PeekMessage allow non-blocking checks and enable idle work or background tasks.

Other GUI frameworks implement analogous event loops. For example, Cocoa uses run loops (NSRunLoop) to schedule input and timers; Qt and GTK run their own event loops as part of the application. While the term message pump is particularly common in Windows documentation, the underlying concept is common across platforms: a single thread processes events and dispatches them to handlers.

Because the loop runs on the UI thread, long-running work should be offloaded to background threads or scheduled via asynchronous tasks. Blocking the pump prevents the application from processing input or painting, leading to an unresponsive UI. Developers may insert idle-time callbacks, timer events, or modal loops to support specific interaction patterns.

Historically named from Windows programming, the message pump remains a foundational pattern for event-driven GUI programming and can be extended to handle I/O readiness and custom events through the framework’s dispatch mechanisms.