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Custom widgets are what make your application different from the rest. The special task that your application will perform is often handled through a special widget. Having said this, I recommend that you pick standard widgets whenever possible because it can be difficult for the users of your application to learn how to use your special widget. Designing widgets that fit into the Qt way of writing applications is not hard. First, you need to find a widget to inherit from the starting point. If there is no given starting point, you have to start from the QWidget class. After you have picked a suitable starting point, you must decide which events you want to pay attention to. This helps you decide which event handling functions to override. The event handlers can be considered your interface with users. When you have decided on your interface, you need to tend to the rest of the application, including setters, getters, signals, and slots (as well as setting up size policies and creating size hints). Make sure to think through usage scenarios other than the current one to make your widget reusable. An investment in time when writing a widget can help you in future projects because you can save having to reinvent the wheel time after time. After having discussed all these software development issues, I must emphasize the most important aspect of your widgets: usability. Try thinking as a user and make sure to test your design on real users before putting it in your production software.

When writing form-based applications, you will occasionally find that users will press Refresh immediately after submitting a form. This has the effect of repeating whatever actions were performed the first time the form was submitted, but this might not always be the behavior your users expect. If your form was submitted with a POST, most browsers will display a message to the user asking them whether they want to resubmit the data (see Figure 6-3). This will not happen with a GET, so POST is preferable to GET in those circumstances.

ll painting in Qt is performed through the QPainter class in one way or another. Widgets, pictures, delegates everything uses the same mechanism. There is actually one exception to the rule (to use OpenGL directly), but you ll start with the QPainter class.

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Triple DES provides existing DES users with an easy upgrade path to start using Triple DES Once the system is tested for reliability and stability with Triple DES, you can then increase security by using three different keys Backward compatibility was built into Triple DES partially to support many systems that used microchips implementing regular DES in hardware These microchips needed to be replaced with microchips that ran Triple DES The design of Triple DES allowed the DES chips to be replaced with Triple DES chips with minimal system disruption However, the backward compatibility in Triple DES is also useful for softwarebased implementations Using Triple DES (with three different 56-bit keys) allows you to achieve a higher level of security than with just one 56-bit key.

Figure 6-3. The dialog box displayed by Firefox when you click Refresh on a page where POST data has been submitted Of course, the best way to solve this issue is to structure your code in such a way that if the user refreshes the page, the data isn t resubmitted. Here s one way of achieving this with an HTTP redirect: # in the controller def form(self): return render('/simpleform.html')

Using Qt you can draw on almost anything: widgets, pictures, pixmaps, images, printers, OpenGL areas, and so on. The common base class of all these drawables is the QPaintDevice class. Since a widget is a paint device, you can easily create a QPainter for drawing onto the widget; simply pass this as argument to the constructor, as shown in Listing 7-1. Listing 7-1. Pass this as argument to the QPainter constructor from a paint event handler to set everything up. void CircleBar::paintEvent( QPaintEvent *event ) { ... QPainter p( this ); ... } To set up a painter for another paint device, just pass a pointer to it to the painter constructor. Listing 7-2 shows how a painter for a pixmap is set up. The pixmap that is 200 pixels wide and 100 pixels high is created. The painter for drawing on the pixmap is then created, and a pen and a brush are set up. Pens are used to draw the boundary of whatever shape you are drawing. Brushes are used to fill the interior of the shape. Before continuing, you need to know what a pixmap is and how it is different from an image or a picture. There are three major classes for representing graphics in Qt: QPixmap is optimized for being shown onscreen, QImage is optimized for loading and saving images, and QPicture records painter commands and makes it possible to replay them later.

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