c# barcode image generation library Subclassing QWidget in Visual C#

Create ECC200 in Visual C# Subclassing QWidget

To begin, your custom widget needs to implement QWidget and include the Q_OBJECT declaration. Listing 5 1 shows a trivial example:

Listing 5 1. A trivial widget class MyWidget : public QWidget { Q_OBJECT public: explicit MyWidget (QWidget *parent = 0) : QWidget(parent) {} protected: void paintEvent(QPaintEvent *) { QPainter painter(this); painter.setPen(Qt::blue); painter.setFont(QFont("Arial", 18)); painter.drawText(rect(), Qt::AlignCenter, "Hello world"); } private: Q_DISABLE_COPY(MyWidget) };

This widget simply paints the message Hello World at its origin. It does, however, demonstrate the basic requirements you must fulfill to provide your own widget: Your custom widget must inherit QWidget. Like any QObject-based class, your widget must include the Q_OBJECT declaration in its class definition. Your custom widget implements its functionality by overriding parent methods in QWidget. As you ll see in the section Handling Incoming Events later in the chapter, many of these methods are event handlers for specific Qt-based events passed to your widget. Instances of widgets are best thought of as unique objects, and thus can t be copied. To prevent the compiler from including default copy constructors for your widget, use the Q_DISABLE_COPY macro in private declarations in your class to ensure that the copy constructors for your widget remain private.

Previously you ve learned about Qt s system of layouts, which let you specify how widgets arrange themselves in a container. A key part of the layout system is how widgets indicate their size preference and policies to the layout manager. They communicate this using two methods: sizeHint and sizePolicy. The sizeHint method provides a hint to the layout manager how big the widget would like to be. It returns a QSize, which has width and height member functions to provide the dimensions, along with an isValid function that indicates whether the size is valid or not. (It also has convenience methods for performing arithmetic on sizes, including scaling a size to fit within a predetermined width and height.) How the layout system interprets the sizeHint depends on the sizePolicy, which can have the following values for each of the horizontal and vertical axes: When the value is QSizePolicy::Fixed, the sizeHint-returned value is the only acceptable alternative, so the widget can never grow or shrink. When the value is QSizePolicy::Minimum, the sizeHint-returned value is minimal and sufficient. The widget can be expanded, but there is no advantage to it being larger. When the value is QSizePolicy::Maximum, the sizeHint-returned value is a maximum, and the widget can be shrunk if other widgets require the space. When the value is QSizePolicy::Preferred, the sizeHint-returned value is best, but the widget can be shrunk or expanded and still be useful.

When the value is QSizePolicy::Expanding, the sizeHint-returned value is a sensible size, but the widget should get as much space as possible. When the value is QSizePolicy::MinimumExpanding, the sizeHintreturned value is minimal and sufficient. The widget can make use of extra space, so it should get as much space as possible. When the value is QSizePolicy::Ignored, the sizeHint-returned value is ignored and the widget be made as large as possible. The widget returns these values when the layout invokes sizePolicy, which returns a QSizePolicy instance. Of course, you can override a specific widget s desired horizontal or vertical size policy by calling setSizePolicy. But in general when creating your own widget, it s easier to override sizePolicy altogether. Expanding our previous example, Listing 5 2 shows what we might write, with the methods you ve already seen elided for brevity.

Listing 5 2. Handling size preferences in a custom widget class MyWidget : public QWidget { Q_OBJECT public: explicit MyWidget (QWidget *parent = 0) QSize sizeHint() { return QSize(80, 60); } QSizePolicy sizePolicy() { return QSizePolicy(QSizePolicy::MinimumExpanding, QSizePolicy::MinimumExpanding); } protected: void paintEvent(QPaintEvent *event) private: Q_DISABLE_COPY(MyWidget) };

A widget with these methods would start small, filling a rectangle 80 pixels wide and 60 pixels tall, and grow to fit whatever space the layout could provide.