Dale Roberts Introduction to Visual Programming Dale Roberts, Lecturer Computer Science, IUPUI Department of Computer and Information Science, School of Science, IUPUI Fall 2003

Dale Roberts Event Driven Processing Application Level Programming Uses widget sets and style guidelines to create application GUIs. Examples: Motif, MFC, Qt, GTK, etc. Graphical Systems Programmer Person who implements the widget sests and windowing systems above

Dale Roberts Programming Paradigms By now, you have been exposed to two programming paradigms: Functional Programming Object-oriented Programming Can you explain their differences in terms of: How to decompose a problem? How to express control?

Dale Roberts Functional Decomposition How do to decompose a problem? Top-down functional decomposition approach. Step-wise refinement Successive versions of algorithms written in pseudo-code Problem is divided into smaller subproblems Each “decomposition” is a function call. Keep decomposing until functions are close enough to target language to code. How do you express control? Each step is a function call. Flow of control statements are used within functions

Dale Roberts Object-Oriented Design How do to decompose a problem? Bottom-up building block approach. Identify objects needed to solve the problem. Create an Abstract Data Type implemented with objects Identify how values are represented in private data members. Define operations that act upon values as public member functions. Encapsulate implementation details within the class. How do you express control? Use UML to define relationships between classes. Flow-of-control between classes is relatively unstructured. Must still define a client that uses classes to solve a problem. Client is usually responsible for user-interaction, not classes. Stratifies application into presentation, processing, and data layers.

Dale Roberts Event-driven Programming Event-driven programming is the standard approach to creating graphical user interfaces (GUIs) An event-driven program is object-oriented Not a new programming “paradigm” Object-oriented programming was originally development to implement graphical objects within GUI operating systems However, top-level control is expressed differently The user is the top-level loop Think of Word, or a game program Every action in your program is a reaction to the user Decompose program in terms of “what will I do if the user does…” User inaction may trigger background actions (e.g. games)

Dale Roberts Detecting Asynchronous Events Polling Repeatedly read input devices in an infinite loop Interrupt-driven Hardware-triggered context-switching in response to user- events Event-driven Explicit event waiting Call-back functions (i.e. signals) in response to user events Qt uses event-driven processing.

Dale Roberts Method #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?

Dale Roberts Method #2 Interrupt-driven Processing Enable device, then proceed with “background” processing until an interrupt is received, at which point Save state (context-switch) Respond to input Restore state and go to step #2. What advantages/disadvantages does this have?

Dale Roberts Method #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) }

Dale Roberts Event-driven Processing (cont) All major GUI packages (Motif, MGC, Qt, GTK, Java AWT, …) are event driven. Why? More portable than interrupt-driven. At the expense of what? More efficient than polling most do have polling commands Can rely on operating system to do time-slicing context-switching is very hardware/operating system specific.

Dale Roberts Events / Signals Any event-driven graphics package has devices that can signal events In old standards, this was limited to hardware devices In 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 you Which device/widget signaled the event Some “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)

Dale Roberts Call-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

Dale Roberts GUI 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; }

Dale Roberts 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 may handle the event through a call-back; or ask their children “is this yours”?

Dale Roberts Hiding the Main Loop Modern widget packages (like Qt) hide the main loop from the programmer. Programmers declare signals i.e. what events to respond to Programmers define slots i.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

Dale Roberts Example: A Main Loop in Qt 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…

Dale Roberts What (really) are widgets? The objects in an object-oriented GUI are called widgets. Every widget: Knows its location (for pick correlation) Knows whether of not its visible Knows how to resize itself Knows how to redraw itself Knows its “children” widgets (if it’s a container) Has call-back functions (slots) for handling events (signals) Every window is a widget Not all widgets are windows

Dale Roberts Examples of widgets Text editing windows (canvases) Push buttons MenusSliders Radio buttons LED displays Borders

Dale Roberts Building a GUI Every application has a top-level widget In Qt, the top-level widget is called QApplication QApplication implements the main signal/slot loop QApplication is a widget but not a window QApplication in turn has a single top-level window Inside is a hierarchy of lesser widgets: frames, etc., for grouping and position widgets low-level widgets:buttons, pop-up menus, etc. Call-back functions (signals) are attached to implement responded to user actions Events are passed by the OS to the application’s top-level widget.

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