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The Filter Manager provides applications with the facility to register pre and post-filters against WIMP tasks?. Filters are useful when a task needs to intercept events that may or may not be under the control of the task.
Filters do not need to be applied to a specific task as they can be assigned to respond to all tasks. Each filter is a routine that must have well defined entry and exit conditions.
A Pre-filters pre-filter are is so-called because it is assigned to be called before when a task calls the Wimp (e.g. viaWimp_Poll . ). With They a are pre-filter, called before control passes to the event Wimp, mask allowing the task filter passes to adjust the parameters of the call or override it completely.Wimp_Poll can also be changed.
A Post-filters post-filter are is so-called because it is assigned to be called just when before the Wimp returns passes control of execution back to a task. The This reason-code allows and the data-block filter returned to by monitor or adjust the result of a call that the task had made, or perform additional processing such as rendering an image over the top of a window.Wimp_Poll can be modified if required before they are passed back to a task.