CopperSpice API  1.7.2
Using Custom Widgets

The Designer application can display custom widgets through its extensible plugin mechanism, allowing the range of designable widgets to be extended by the user and third parties.

Handling Custom Widgets

Although the Designer supports all of the standard CopperSpice widgets, and can be configured to handle widgets supplied in third party libraries, some specialized widgets may not be available as standard for a number of reasons:

  • Custom widgets may not be available at the time the user interface is being designed.
  • Custom widgets may be platform-specific, and designers may be developing the user interface on a different platform to end users.
  • The source code for a custom widget is not available, or the user interface designers are unable to use the widget for non-technical reasons.

In the above situations, it is still possible to design forms with the aim of using custom widgets in the application. To achieve this, we can use the widget promotion feature of Qt Designer.

In all other cases, where the source code to the custom widgets is available, we can adapt the custom widget for use with Qt Designer.

Promoting Widgets

If some forms must be designed, but certain custom widgets are unavailable 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 CopperSpice 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 CopperSpice 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.

User Defined Custom Widgets

Custom widgets can be adapted for use with Qt Designer, giving designers the opportunity to configure the user interface using the actual widgets that will be used in an application rather than placeholder widgets. The process of creating a custom widget plugin is described in Creating Custom Widgets for Qt Designer.

To use a plugin created in this way, it is necessary to ensure the plugin is located on a path that Qt Designer searches for plugins. Generally, plugins stored in $QTDIR/plugins/designer will be loaded when Qt Designer starts. Further more information refer to Building and Installing Plugins and Creating Plugins.