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There are two types of user interface events—notify and filter.

Notify events are an indication that a user action has already occurred, such as when the user has changed the value of a control. Use notify events to respond to an event after it has occurred and LabVIEW has processed it. You can configure any number of Event structures to respond to the same notify event on a specific object. When the event occurs, LabVIEW sends a copy of the event to each Event structure configured to handle the event in parallel.

Filter events inform you that the user has performed an action before LabVIEW processes it, which allows you to customize how the program responds to interactions with the user interface. Use filter events to participate in the handling of the event, possibly overriding the default behavior for the event. In an Event structure case for a filter event, you can validate or change the event data before LabVIEW finishes processing it, or you can discard the event entirely to prevent the change from affecting the VI. For example, you can configure an Event structure to discard the Panel Close? event, preventing the user from interactively closing the front panel of the VI. Filter events have names that end with a question mark, such as Panel Close?, to help you distinguish them from notify events. Most filter events have an associated notify event of the same name, but without the question mark, which LabVIEW generates after the filter event if no event case discarded the event.

As with notify events, you can configure any number of Event structures to respond to the same filter event on a specific object. However, LabVIEW sends filter events sequentially to each Event structure configured for the event. The order in which LabVIEW sends the event to each Event structure depends on the order in which the events were registered. Each Event structure must complete its event case for the event before LabVIEW can notify the next Event structure. If an Event structure case changes any of the event data, LabVIEW passes the changed data to subsequent Event structures in the chain. If an Event structure in the chain discards the event, LabVIEW does not pass the event to any Event structures remaining in the chain. LabVIEW completes processing the user action which triggered the event only after all configured Event structures handle the event without discarding it.

Note  National Instruments recommends you use filter events only when you want to take part in the handling of the user action, either by discarding the event or by modifying the event data. If you only want to know that the user performed a particular action, use notify events.

Event structure cases that handle filter events have an Event Filter Node. You can change the event data by wiring new values to these terminals. If you do not wire a data item, that item remains unchanged. You can completely discard an event by wiring a TRUE value to the Discard? terminal.

Note  A single case in the Event structure cannot handle both notify and filter events. A case can handle multiple notify events but can handle multiple filter events only if the event data items are identical for all events.