targetaction

Targetaction, often written as target-action or target-action pattern, is a user interface programming pattern in which a control or view (the sender) is configured with a target object and an action, typically a method or selector, to be invoked in response to a user event such as a click or tap. The pattern enables the control to delegate behavior to another object without embedding the logic inside the control itself.

In practice, the control stores a reference to the target and a representation of the action. When the event occurs, the control sends a message to the target, invoking the action. Depending on language, the action may receive the control (the sender) as a parameter, or a more general event object; many frameworks support optional sender parameters.

Origins and platforms: The mechanism originated in NeXTSTEP and early Apple Cocoa frameworks; in Cocoa and Cocoa Touch it is implemented via selectors and runtime messaging. In Interface Builder, developers can connect controls to actions. Some other platforms implement similar concepts under different names, such as event listeners or delegates.

Variants and usage: A single action method can be shared by multiple controls; a single control can be bound to different targets programmatically. The pattern supports dynamic binding and runtime wiring, enabling changes in behavior without subclassing the control.

Advantages and caveats: It promotes decoupling of UI from behavior and simplifies reuse, but it relies on proper lifecycle management to avoid sending messages to deallocated targets; in languages with weak references, ARC semantics help; In Swift, actions are often defined with @objc and #selector.