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An event handler is code invoked to respond to a particular software event, such as a key press, resize or completed operation. The event's data identifies what happened; the event handler interprets it using the application's current state. Qt dispatches an event object to its target and then to an appropriate event-specific method.^[ref]

For example, a resize event handler can recompute a plot's displayed layout from the new dimensions. A later paint software event draws that layout. Keeping each response short lets an event loop handle other work promptly. When delivery is delayed, the application's state may already have changed, so the original occurrence and its eventual handling must be distinguished.

An event loop repeatedly obtains pending software events and dispatches them to their event handlers, waiting when no work is available. This lets one thread service many intermittent activities without a separate continuously running loop for each. Qt's application event loop dispatches timer, network and other software events.^[ref]

Suppose a button starts a calculation that takes ten seconds. Running that calculation synchronously inside its event handler prevents the same event loop from repainting or processing further clicks during those ten seconds. Dividing the work into short steps or delegating it to another thread lets the event handler return. An event loop coordinates readiness and dispatch; it does not itself provide parallel computing for long computations.

A graphical user interface lets a user inspect and manipulate a program through visual controls such as windows, buttons, menus and text fields. A GUI widget combines a displayed representation with interaction behavior. Input events select a control or action, while application state determines what is drawn next.

For example, pressing a button can initiate a calculation and later display its result. The Event handler should not have to embed every computational detail; separating the underlying model from its view makes other interfaces and tests possible. Qt Widgets provides visual elements, layout management and event handling as separate cooperating mechanisms.^[ref] A visible control is therefore an interaction interface, not an authorization boundary or proof that the requested action succeeded.

A GUI widget is an interactive or display component of a graphical user interface, such as a button, text field or list. It combines presentation, relevant state and responses to software events. Widgets can contain other widgets; a layout manager arranges them using their size requirements.^[ref]

For example, a numeric-entry GUI widget stores editable text, draws it and reacts to keystrokes. The text 1e can be a legitimate intermediate editing state even when the completed value must be numeric. Separating editing behavior from the application's data model makes that distinction manageable. A disabled button conveys interface state; server-side authorization still has to decide whether the underlying action is permitted.

Scala Swing provides Scala wrappers for Java Swing graphical user interfaces. Its notification mechanism implements an observer pattern: a publisher emits software events, and a reactor subscribes with listenTo(publisher). Adding a handler to reactions supplies a partial function that matches the software events the reactor handles. Subscription and handling are distinct: one determines which publisher is heard, and the other determines what to do with a matching notification.^[ref]

For example, a button publishes a button-click notification, and a subscribed display component handles it by updating a label. Another publisher's notifications need a subscription of their own. Calling deafTo(publisher) ends the subscription to that publisher without requiring removal of every handler.^[ref]

A software event represents an occurrence that a program can observe and handle, such as a key press, timer expiry or completed operation. It may carry data identifying what happened, where and to which object. An event handler performs the relevant response; an event loop commonly dispatches pending events to their destinations.

In Qt, an event object is delivered to a target object whose event method dispatches to a type-specific handler. Mouse, keyboard, resize and timer events carry different information.^[ref] A queued event can be handled after the original occurrence, so event time and handling time differ. Reentrant or asynchronous handlers must also account for changes in application state between those moments.