inside events

8/2/2019 Inside Events

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National Instruments Confidential

Inside LabVIEW:Event-driven Programming


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Overview

What is event-driven programming

How to use events in LabVIEW

Caveats and common mistakes to avoid


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What Is Event-driven Programming?

Order of execution is influenced by events on the userinterface

Code executes in response to change in state of system

No need to poll for change in state Event loops versus call-backs


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Introducing... the Event Structure

What is it? New structure node with multiple sub-diagrams, like a Case

Structure

How does it work?

Goes to sleep until an event occurs on the panel

Automatically executes the right event case (sub-diagram) forthe event

Structure completes when it finishes handling exactly oneevent; does not implicitly loop


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Event Structure Basic Usage

Normally used in a WhileLoop

Sleeps and handles exactlyone event per iteration

Event Data available interminals on structures leftborder

Can right-click on event data

and show only the terminalsyou need


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Event Structure Configuration

Event Structure is configured via dialog by right-clicking its border and using Edit Events

LabVIEW 6.1 events are divided into three classes:

VI: Mouse Down/Move/Up, Key Down/Repeat/Up,Menu Selection (App/User), Panel Closing

Control: Mouse Down/Up, Key Down/Repeat/Up,

Value Changed

Application: Application Exiting


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Notify vs. Filter Events

Events are divided into two categories: Filterand Notify Notify Events are after-the-fact notification that something

happened

Filter Events allow you to change the events data as it

happens, or discard the event entirely Key events, Mouse events, Menu events, and the front panel window

closing event are all filter events

Filter Event Examples:

Make string controls that map upper- to lower-case Bring up a confirmation dialog when the user tries to close the panel



Parts of an Event Structure

Event Selector Label identifies currently shown event case Timeout - value in milliseconds to wait for events;

default value is1 (indefinite)

Event Data Node - data unique to each event case

Event Filter Node - allows you to changeor discard event data for a Filter event

before the event is completely processed

Defaulted Tunnels - new kind of tunnel

which allows unwired cases to providedefault values



Simple Event Demos

Event tracking of front panel controls Event Tracking.vi (LabVIEW 6.1 shipped example)

Confirmation dialog on closing of VI's front panel Event Conditional Stop.vi (LabVIEW 6.1 shipped example)



Event Registration and Panel Locking

Events are registered when a VI with an event structure onits diagram becomes reserved

When an registered event occurs, it is queued until the event

structure(s) configured for it executes

It is not possible to miss events or process them out of order

By default, front panel is locked until the event is handled

You can turn off locking, but only for Notify Events

Events are unregistered when the VI becomes idle



Limitations of Events in LV 6.1

Static registration only; cant register on the fly Events are generated onlyby direct user interaction

No events on device I/O, etc.

Not possible to programmatically fake events in 6.1(without cheating)



Caveats and Common Mistakes

Value Changed events and Latched Booleans Only reading the front panel terminal will reset the Boolean

Stop Booleans and the event-handling loop

Dont mix event-handling and polling in the same loop

Programmatic changes DO NOT generate events

Static events generated for entire duration of VI

Make sure Event Structures are always in loops to handle events

An event-handling loop is not a state machine



Events and UI Design Patterns

Sophisticated UIs which use a state machine orqueued message handler design can be easily

retrofitted to use events

Simply replace the idle case in your state machinewhich does polling with an event structure configured

to detect the same UI changes



Who Can Best Take Advantage?

Events may benefit any application that: polls for value changes on multiple controls

hides the window close box

uses the menu primitives

uses keyboard.llb

needs to clean up or request confirmation when LabVIEWshuts down

Events work with Remote Panels, with either a fullLabVIEW client or the web browser plug-in



Resources

Shipping Examples(examples/general/uievents.llb)

User manual and on-line reference

LabVIEW Technical Resource article (http://ltrpub.com)A Powerful New Tool for UI Programming (Vol. 9, Issue 4)

Articles on NI Dev-Zone (http://zone.ni.com - search forevents and event-driven)

See http://www.ni.com/infoand enter code niweek02 for on-line access to these slides and further information
http://ltrpub.com/http://zone.ni.com/http://www.ni.com/infohttp://www.ni.com/infohttp://zone.ni.com/http://ltrpub.com/


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