IconEditor::IconEditor(QWidget *parent, const char *name) The implementation file begins with #include directives and the IconEditor's constructor: The three private variables hold the values of the three properties.
IconEditor reimplements three protected functions from QWidget and has a few private functions and variables. Void setImagePixel(const QPoint &pos, bool opaque) Void drawImagePixel(QPainter *painter, int i, int j) Void mousePressEvent(QMouseEvent *event) The Q_OBJECT macro is necessary for classes that define properties. Properties may be of any type supported by QVariant. When we make use of the widget in Qt Designer, custom properties appear in Qt Designer's property editor below the properties inherited from QWidget. For example, the penColor property is of type QColor and can be read and written using the penColor() and setPenColor() functions. Each property has a type, a "read" function, and a "write" function. The IconEditor class uses the Q_PROPERTY() macro to declare three custom properties: penColor, iconImage, and zoomFactor. Void setPenColor(const QColor &newColor) IconEditor(QWidget *parent = 0, const char *name = 0) Q_PROPERTY(int zoomFactor READ zoomFactor WRITE setZoomFactor) Q_PROPERTY(QImage iconImage READ iconImage WRITE setIconImage) Q_PROPERTY(QColor penColor READ penColor WRITE setPenColor) Let's begin by reviewing the header file. The IconEditor is a widget that could be used in an icon editing program.
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To demonstrate how to write a custom widget using this approach, we will create the IconEditor widget shown in Figure 5.2.
If they didn't exist in Qt, it would still be possible to create them ourselves using the public functions provided by QWidget in a totally platform-independent manner. Qt's built-in widgets, like QLabel, QPushButton, and QTable, are implemented this way. This approach gives us complete freedom to define and control both the appearance and the behavior of our widget. This is achieved by subclassing QWidget and reimplementing a few event handlers to paint the widget and to respond to mouse clicks. When none of Qt's widgets are suitable for the task at hand, and when there's no way to combine or adapt existing widgets to obtain the desired result, we can still create the widget we want.
Even so, we could just as easily have subclassed QHBox and created the QSpinBox and QSlider in the subclass's constructor. That's the approach we used in Chapter 1 to create the Age application, with a QHBox, a QSpinBox, and a QSlider. If the widget has no signals and slots of its own and doesn't reimplement any virtual functions, it is even possible to simply assemble the widget by aggregating existing widgets without a subclass. Whichever approach is taken, the resulting class inherits directly from QWidget. Naturally, this can also be done entirely in code. ui.h file or in a subclass) to provide the desired behavior.
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Set up the signals and slots connections and add any necessary code (either in a.Add the necessary widgets to the form, then lay them out.Create a new form using the "Widget" template.Custom widgets that are built by composing existing widgets can usually be developed in Qt Designer: Most custom widgets are simply a combination of existing widgets, whether they are built-in Qt widgets or other custom widgets such as HexSpinBox.
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