Dale Roberts

Introduction to Visual ProgrammingIntroduction to Visual Programming

Dale Roberts, LecturerComputer Science, IUPUIE-mail: droberts@cs.iupui.edu

Department of Computer and Information Science,School of Science, IUPUI

Fall 2003

Dale Roberts

Event Driven Processing

Application Level ProgrammingApplication Level Programming

Uses widget sets and style guidelines to create Uses widget sets and style guidelines to create application GUIs. Examples: Motif, MFC, Qt, GTK, application GUIs. Examples: Motif, MFC, Qt, GTK,

etc.etc.

Graphical Systems ProgrammerGraphical Systems Programmer

Person who implements the widget sests and Person who implements the widget sests and windowing systems abovewindowing systems above

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Programming ParadigmsProgramming Paradigms

By now, you have been exposed to two By now, you have been exposed to two programming paradigms:programming paradigms:

Functional ProgrammingFunctional Programming

Object-oriented ProgrammingObject-oriented Programming

Can you explain their differences in terms of:Can you explain their differences in terms of:How to decompose a problem?How to decompose a problem?

How to express control?How to express control?

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Functional DecompositionFunctional Decomposition

How do to decompose a problem?How do to decompose a problem?Top-down functional decomposition approach.Top-down functional decomposition approach.

Step-wise refinementStep-wise refinement

Successive versions of algorithms written in pseudo-codeSuccessive versions of algorithms written in pseudo-code

Problem is divided into smaller subproblemsProblem is divided into smaller subproblems

Each “decomposition” is a function call.Each “decomposition” is a function call.

Keep decomposing until functions are close enough to Keep decomposing until functions are close enough to target language to code.target language to code.

How do you express control?How do you express control?Each step is a function call.Each step is a function call.

Flow of control statements are used within functionsFlow of control statements are used within functions

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Object-Oriented DesignObject-Oriented Design

How do to decompose a problem?How do to decompose a problem?Bottom-up building block approach.Bottom-up building block approach.

Identify objects needed to solve the problem.Identify objects needed to solve the problem.

Create an Abstract Data Type implemented with objectsCreate an Abstract Data Type implemented with objects

Identify how values are represented in private data members.Identify how values are represented in private data members.

Define operations that act upon values as public member functions.Define operations that act upon values as public member functions.

Encapsulate implementation details within the class.Encapsulate implementation details within the class.

How do you express control?How do you express control?Use UML to define relationships between classes.Use UML to define relationships between classes.

Flow-of-control between classes is relatively unstructured.Flow-of-control between classes is relatively unstructured.

Must still define a client that uses classes to solve a problem.Must still define a client that uses classes to solve a problem.

Client is usually responsible for user-interaction, not classes.Client is usually responsible for user-interaction, not classes.

Stratifies application into presentation, processing, and data layers.Stratifies application into presentation, processing, and data layers.

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Event-driven ProgrammingEvent-driven Programming

Event-driven programming is the standard Event-driven programming is the standard approach to creating graphical user interfaces approach to creating graphical user interfaces (GUIs)(GUIs)

An event-driven program is object-orientedAn event-driven program is object-orientedNot a new programming “paradigm”Not a new programming “paradigm”

Object-oriented programming was originally development to Object-oriented programming was originally development to implement graphical objects within GUI operating systemsimplement graphical objects within GUI operating systems

However, top-level control is expressed differentlyHowever, top-level control is expressed differentlyThe user is the top-level loopThe user is the top-level loop

Think of Word, or a game programThink of Word, or a game program

Every action in your program is a reaction to the userEvery action in your program is a reaction to the userDecompose program in terms of “what will I do if the user does…”Decompose program in terms of “what will I do if the user does…”

User inaction may trigger background actions (e.g. games)User inaction may trigger background actions (e.g. games)

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Detecting Asynchronous EventsDetecting Asynchronous Events

PollingPollingRepeatedly read input devices in an infinite loopRepeatedly read input devices in an infinite loop

Interrupt-drivenInterrupt-drivenHardware-triggered context-switching in response to user-Hardware-triggered context-switching in response to user-events events

Event-drivenEvent-drivenExplicit event waitingExplicit event waitingCall-back functions (i.e. signals) in response to user Call-back functions (i.e. signals) in response to user eventsevents

Qt uses event-driven processing.

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Method #1 PollingMethod #1 Polling

Interaction is governed by a simple loop:Loop forever:

{

read input

respond to input

}

What limitations does this have?

Does it have any advantages?

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Method #2 Interrupt-driven ProcessingMethod #2 Interrupt-driven Processing

1. Enable device, then

2. proceed with “background” processing until an interrupt is received, at which point

3. Save state (context-switch)

4. Respond to input

5. Restore state and go to step #2.

What advantages/disadvantages does this have?

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Method #3: Event-driven ProcessingMethod #3: Event-driven Processing

Interaction is once again governed by a loop:

Loop forever:

{

if (event) then

respond

else

do (one unit of) background

processing or

go to sleep (for one unit)

}

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Event-driven Processing (cont)Event-driven Processing (cont)

All major GUI packages (Motif, MGC, Qt, GTK, All major GUI packages (Motif, MGC, Qt, GTK, Java AWT, …) are event driven.Java AWT, …) are event driven.

Why?Why?More portable than interrupt-driven.More portable than interrupt-driven.At the expense of what?At the expense of what?

More efficient than pollingMore efficient than pollingmost do have polling commandsmost do have polling commands

Can rely on operating system to do time-slicingCan rely on operating system to do time-slicingcontext-switching is very hardware/operating system specific.context-switching is very hardware/operating system specific.

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Events / SignalsEvents / Signals

Any event-driven graphics package has devices that can signal events

In old standards, this was limited to hardware devicesIn newer packages (e.g. Qt), any widget can signal events; the (hardware) mouse is the same as a (software) slider or button.

Generally, the event tells youWhich device/widget signaled the eventSome “measure” giving the new state

E.g., whether a mouse button was depressed or released

Warning: old systems tend to use the term “events”while newer systems may call them signals (e.g. Qt)

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Call-back Functions / SlotsCall-back Functions / Slots

A call-back function is an application-specific function called in response to an event

In Qt, these are called “slots”, but this term is unique to Qt

Generally, the “measure” of the event is passed as an argument

The main loop of a GUI program is:Wait for an event

Call the associated call-back function

Return to the top of the loop

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GUI API Event LoopsGUI API Event Loops

Loop forever:

{

if (input) then {

find out which application receives the event;

invoke the callback function;

}

else

select one application with a

background callback function;

invoke background callback;

}

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Pick Correlation

The process of selecting which window (or application) an event belongs to is called pick correlation

Pick correlation is usually object-oriented:Every window knows where its children are

children send parents a message when they move, etc.

The top-level window assigns events to children

Child processes mayhandle the event through a call-back; or

ask their children “is this yours”?

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Hiding the Main Loop

Modern widget packages (like Qt) hide the main loop from the programmer.

Programmers declare signalsi.e. what events to respond to

Programmers define slotsi.e. how to respond to events

Programmers connect signals to slots

The main loop (wait for signal / call slot / loop) is part of the widget package

Programmers call the main loop, but can’t alter it, other than through signals and slots

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Example: A Main Loop in Qt

int main (int argc, char* argv[])int main (int argc, char* argv[])

{{

QApplication app( argc, argv);QApplication app( argc, argv);

GUI main( &app, &state)GUI main( &app, &state)

app.setMainWidget( &main);app.setMainWidget( &main);

main.Show();main.Show();

return app.exec();return app.exec();

}} Where’s the program? GUI is a widget. The GUI class defines signals and slots, and the GUI’s constructor connection them…

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What (really) are widgets?What (really) are widgets?

The objects in an object-oriented GUI are called The objects in an object-oriented GUI are called widgetswidgets..Every widget:Every widget:

Knows its location (for pick correlation)Knows its location (for pick correlation)Knows whether of not its visibleKnows whether of not its visibleKnows how to resize itselfKnows how to resize itselfKnows how to redraw itselfKnows how to redraw itselfKnows its “children” widgets (if it’s a container)Knows its “children” widgets (if it’s a container)Has call-back functions (slots) for handling events Has call-back functions (slots) for handling events (signals)(signals)

Every window is a widgetEvery window is a widgetNot all widgets are windowsNot all widgets are windows

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Examples of widgetsExamples of widgets

Text editing windows (canvases)Text editing windows (canvases)

Push buttonsPush buttons

MenusMenus

SlidersSliders

Radio buttonsRadio buttons

LED displaysLED displays

BordersBorders

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Building a GUIBuilding a GUI

Every application has a top-level widgetEvery application has a top-level widgetIn Qt, the top-level widget is called QApplicationIn Qt, the top-level widget is called QApplication

QApplication implements the main signal/slot loopQApplication implements the main signal/slot loop

QApplication is a widget but not a windowQApplication is a widget but not a window

QApplication in turn has a single top-level windowQApplication in turn has a single top-level window

Inside is a hierarchy of lesser widgets:Inside is a hierarchy of lesser widgets:frames, etc., for grouping and position widgetsframes, etc., for grouping and position widgets

low-level widgets:buttons, pop-up menus, etc.low-level widgets:buttons, pop-up menus, etc.

Call-back functions (signals) are attached to implement Call-back functions (signals) are attached to implement responded to user actionsresponded to user actions

Events are passed by the OS to the application’s Events are passed by the OS to the application’s top-level widget.top-level widget.

Dale Roberts

AcknowledgementsAcknowledgementsSome of the slides were originally written by J. Ross Beveridge, updated by Dale Roberts.