Introduction to Qt

1. Department of Computer and Information Science,School of Science, IUPUI Introduction to Qt Dale Roberts, Lecturer Computer Science, IUPUI E-mail: droberts@cs.iupui.edu

2. The Qt Story • The Qt toolkit is a multi-platform C++ GUI toolkit (class library) that has been developed over a 6 year period. • The company Troll Tech AS was founded in 1994 to secure future development of Qt. • On May 20, 1995, Qt was made available under commercial and non-commercial GNU licenses. The non-commercial license grants any developer the right to use Qt to develop software for the free software community. • It was ten months before the first commercial license was purchased. The European Space Agency purchased the second. • Around 1997, Qt was chosen as the code basis for the KDE linux desktop environment. • Qt 3.0 was released in 2001 with Windows, Unix, Linux, Embedded Linux, and Mac OS X libraries.

3. Why GUI Toolkits? • Unix GUIs are typically based on the X Window System. • Primitive X11 functions let you draw primitive graphics like line and rectangles, set foreground and background colors, and (most importantly) interact with the user and send events back to the server. • The functions are network transparent, meaning that the server can display graphic on the local workstation or across the world. This is the same model as telnet. • Programming graphical objects like buttons, scrollbars, dialog boxes, and toolbars is very difficult using pure X11. • GUI Toolkits facilitate GUI programming under Unix. • Motif is both a GUI toolkit and popular GUI standard. Dalheimer argues that it is does not support type safety and is awkward compared to Qt. However, Qt does support the Motif look-and-feel as well as Windows.

4. Why Cross-Platform GUI toolkits • Increases target market. • May provides the same look-and-feel across platform. Reduces training and documentation costs.

5. Stategies for Implementing Cross-Platform GUIs • API Layering – Mapping one API to many others • Example – wxWindows – Win32 API on top of Motif or Xt API under Unix. • Advantages – easy to write, 100% compatible native look and feel. • Disadvantages – • slower • problems mapping to vastly different API architectures • Lowest common denominator – i.e. no pop-up help anywhere • Objects required a C++ wrapper to work with them in C++

6. Stategies for Implementing Cross-Platform GUIs • API Emulation – Implement a full API emulation (programming gaps where APIs to not directly map) • Examples – MainWin and Wind/U – Win32 API on top of Motif or Xt API under Unix. • Advantages – Running on native platform requires no emulation, and is therefore fast. Provides full emulation. I.e. you can run Word under Unix. • Disadvantages – • slower on emulated platforms • towers of layers: MFC on Win32 on Motif on Xt on X11 • toolkit harder to emulate • Problems with undocumented features • No direct support for C++. APIs still needs a C++ wrapper.

7. Stategies for Implementing Cross-Platform GUIs • GUI Emulation – Complete API written on top of primitive graphics (i.e. supported by all GUI OS). Each widget is drawn by the toolkit. • Examples – Qt (C++) and GTK (C) • Advantages – • Faster because toolkit layer talks directly to OS. • Easy to change display style from within application – Motif style under Windows, or Windows style under Unix. • Widgets are implemented as C++ classes and can be inherited from directly in client code. (This is the reason Qt is chosen instead of GTK) • Disadvantages – • Does not support porting of existing programs (i.e. no Word under Unix). • More work to create implementations for every OS release for every target platform. This means Qt support lags behind OS releases. Impacts not only Qt but any widgets created by user • Emulation of look-and-feel is not 100% exact.

8. Qt Assistant • All documentation is available through the trolltech web site. • Qt Assistant is a Qt help browser that runs under Windows. • It has with search and indexing features that make it quicker and easier than the web.

9. A Simple Example/* HelloWorld.cpp */ 1 #include <qapplication.h> 2 #include <qlabel.h> 3 4 int main(int argc, char **argv) { 5 6 QApplication myapp(argc, argv); 7 8 Qlabel *mylabel = new Qlabel(“Hello World”, 0); 9 mylabel->resize(100, 200); 10 11 myapp.setMainWidget(mylabel); 12 mylabel->show(); 13 return myapp.exec(); 14 }

10. Line-by-line 1 #include <qapplication.h> 2 #include <qlabel.h> Always #include any Q types referenced in code. 6 QApplication myapp(argc, argv); Creates an object to manage application-wide resources. Passes argc and argv because Qt supports a few command line arguments of its own. • Qlabel *mylabel = new Qlabel(“Hello World”, 0); Creates a QLabel widget on the heap. A widgets is any visual element in a user interface. Widgets can contain other widgets. For example a window may contain a QMenuBar,0 QToolBar, QStatusBar, and other widgets. The 0 parameters says that that the label is a stand-alone window, is not inside another window.

11. Line-by-line 9 mylabel->resize(100, 200); Invokes the resize() member function. 11 myapp.setMainWidget(mylabel); Make the label the main application widget. This means that closing the label windows closes the application. 12 mylabel->show(); Invoke the show() member function to make the label visible. All widgets are created invisible so that their properties can be manipulated without flickering. For example, you would show a widget and then change its size and color. You would change the size and color first, and then show the widget. 13 return myapp.exec(); Passes control of the application to Qt. At this point the application goes into “event-driven” mode. It will just sit there until the user does something to create an even. This is the same concept as Word. Word starts and waits for the user to do something.

12. Events • Signals: emit events • declare as signals, otherwise normal member functions • You don't implement them. Rather, you send them with the (new) keyword emit • E.g. emit(sliderChanged(5)) • Slots: receive and handle events • Normal member functions declared as slots • Connect: must connect signals to slots • QObject::connect( mymenu, SIGNAL(activated(int)), myobject, SLOT(slotDoMenuFunction(int)) ); • moc: meta object compiler (preprocessor) converts these new keywords to real C++

13. Widgets • Base class for all UI widgets • Properties • width, height, backgroundColor, font, mouseTracking, backgroundPixmap, etc. • Slots • repaint, show, hide, move, setGeometry, setMainWidget, etc. • Signals: • mouseMoveEvent, keyPressEvent, resizeEvent, paintEvent, enterEvent, leaveEvent, etc.

14. Other Features of Qt • The Qt Paint Engine • QPainter is highly optimized and contains several caching mechanisms to speed up drawing. Under X11, it caches GCs (graphics contexts), which often make it faster than native X11 programs. • QPainter contains all the functionality one would expect from a professional 2D graphics library. The coordinate system of a QPainter can be transformed using the standard 2D transformations (translate, scale, rotate and shear). • Qt supports Open GL for 3D graphics. • Qt also contains a set of general purpose classes and a number of collection-classes to ease the development of multi-platform applications. • Qt has platform independent support for the operating system dependent functions, such as time/date, files/directories and TCP/IP sockets.

15. Acknowledgements • Plantinga, Harry. Calvin College • Trolltech Tutorials. • pages.cpsc.ucalgary.ca/ ~saul/hci_topics/topics/history.html