QStyleOption Class

Detailed Description

QStyleOption and its subclasses contain all the information that QStyle functions need to draw a graphical element.

For performance reasons, there are few member functions and the access to the member variables is direct (i.e., using the . or -> operator). This makes the structures straightforward to use and emphasizes that these are simply parameters used by the style functions.

The caller of a QStyle function usually creates QStyleOption objects on the stack. This combined with Qt's extensive use of implicit sharing for types such as QString, QPalette, and QColor ensures that no memory allocation needlessly takes place.

The following code snippet shows how to use a specific QStyleOption subclass to paint a push button:

 void MyPushButton::paintEvent(QPaintEvent *)
 {
     QStyleOptionButton option;
     option.initFrom(this);
     option.state = isDown() ? QStyle::State_Sunken : QStyle::State_Raised;
     if (isDefault())
         option.features |= QStyleOptionButton::DefaultButton;
     option.text = text();
     option.icon = icon();

     QPainter painter(this);
     style()->drawControl(QStyle::CE_PushButton, &option, &painter, this);
 }

In our example, the control is a QStyle::CE_PushButton, and according to the QStyle::drawControl() documentation the corresponding class is QStyleOptionButton.

When reimplementing QStyle functions that take a QStyleOption parameter, you often need to cast the QStyleOption to a subclass. For safety, you can use qstyleoption_cast() to ensure that the pointer type is correct. For example:

 void MyStyle::drawPrimitive(PrimitiveElement element,
                             const QStyleOption *option,
                             QPainter *painter,
                             const QWidget *widget)
 {
     if (element == PE_FrameFocusRect) {
         const QStyleOptionFocusRect *focusRectOption =
                 qstyleoption_cast<const QStyleOptionFocusRect *>(option);
         if (focusRectOption) {
             // ...
         }
     }
     // ...
 }

The qstyleoption_cast() function will return 0 if the object to which option points is not of the correct type.