Using Custom Widgets with Qt Designer

Promoting Widgets

If some forms must be designed, but certain custom widgets are unavailble to the designer, we can substitute similar widgets to represent the missing widgets. For example, we might represent instances of a custom push button class, MyPushButton, with instances of QPushButton and promote these to MyPushButton so that uic generates suitable code for this missing class.

When choosing a widget to use as a placeholder, it is useful to compare the API of the missing widget with those of standard Qt widgets. For specialized widgets that subclass standard classes, the obvious choice of placeholder is the base class of the custom widget; for example, QSlider might be used for specialized QSlider subclasses.

For specialized widgets that do not share a common API with standard Qt widgets, it is worth considering adapting a custom widget for use in Qt Designer. If this is not possible then QWidget is the obvious choice for a placeholder widget since it is the lowest common denominator for all widgets.

To add a placeholder, select an object of a suitable base class and choose Promote to ... from the form's context menu. After entering the class name and header file in the lower part of the dialog, choose Add. The placeholder class will now appear along with the base class in the upper list. Click the Promote button to accept this choice.

Now, when the form's context menu is opened over objects of the base class, the placeholder class will appear in the Promote to submenu, allowing for convenient promotion of objects to that class.

A promoted widget can be reverted to its base class by choosing Demote to from the form's context menu.