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An event is an asynchronous notification that something has occurred. Events can originate from the user interface, external I/O, or other parts of the program. User interface events include mouse clicks, key presses, and so on. External I/O events include hardware timers or triggers that signal when data acquisition completes or when an error condition occurs. Other types of events can be generated programmatically and used to communicate with different parts of the program. LabVIEW supports user interface and programmatically generated events but does not support external I/O events.

In an event-driven program, events that occur in the system directly influence the execution flow. In contrast, a procedural program executes in a predetermined, sequential order. Event-driven programs usually include a loop that waits for an event to occur, executes code to respond to the event, and reiterates to wait for the next event. How the program responds to each event depends on the code written for that specific event. The order in which an event-driven program executes depends on which events occur and on the order in which they occur. Some sections of the program might execute frequently because the events they handle occur frequently, and other sections of the program might not execute at all because the events never occur.

When you use events in LabVIEW, consider the caveats and recommendations for doing so.

Why Use Events?

Use user interface events in LabVIEW to synchronize user actions on the front panel with block diagram execution. Events allow you to execute a specific event-handling case each time a user performs a specific action. Without events, the block diagram must poll the state of front panel objects in a loop, checking to see if any change has occurred. Polling the front panel requires a significant amount of CPU time and can fail to detect changes if they occur too quickly. By using events to respond to specific user actions, you eliminate the need to poll the front panel to determine which actions the user performed. Instead, LabVIEW actively notifies the block diagram each time an interaction you specified occurs. Using events reduces the CPU requirements of the program, simplifies the block diagram code, and guarantees that the block diagram can respond to all interactions the user makes.

Use programmatically generated events to communicate among different parts of the program that have no dataflow dependency. Programmatically generated events have many of the same advantages as user interface events and can share the same event handling code, making it easy to implement advanced architectures, such as queued state machines using events.