CLIM's windowing substrate provides an input architecture and standard functionality for notifying clients of input that is distributed to their sheets. Input includes such events as the pointer entering and exiting sheets, pointer motion (whose granularity is defined by performance limitations), and pointer button and keyboard events. At this level, input is represented as event objects.
Sheets either participate fully in the input protocol or are mute for input. If any functions in the input protocol are called on a sheet that is mute for input, the sheet-is-mute-for-input error will be signaled.
In addition to handling input event, a sheet is also responsible for providing other input services, such as controlling the pointer's appearance, and polling for current pointer and keyboard state.
Input is processed on a per-port basis.
The event-processing mechanism has three main tasks when it receives an event. First, it must determine to which client the event is addressed; this process is called distributing . Typically, the client is a sheet, but there are other special-purpose clients to which events can also be dispatched. Next, it formats the event into a standard format, and finally it dispatches the event to the client. A client may then either handle the event synchronously, or it may queue it for later handling by another process.
Input events can be broadly categorized into pointer events and keyboard events . By default, pointer events are dispatched to the lowest sheet in the hierarchy whose region contains the location of the pointer. Keyboard events are dispatched to the port's keyboard input focus; the accessor port-keyboard-input-focus contains the event client that receives the port's keyboard events.
Common Lisp Interface Manager 2.0 User's Guide - 7 Aug 2017