Implementing a GTK+ custom widget

The next step is to implement a custom widget, which is not extending from a currently existing widget, but rather creates a widget from scratch. We will need to do this if we really can't find any similar widgets from GTK+ standard widgets, which can do what we require from the widget that we want to implement.

We can only do this with Vala; unfortunately, as the version of Seed used during the writing of this book can't handle the overriding of a function properly when called from within a class.

Our example does not really reflect the real life situation just discussed because you can easily search a widget which can be readily used, instead of implementing it from scratch. However, the example is provided to simply show the effort involved and how to do it.

Imagine that we need a widget which fulfills the following requirements:

• The widget should be be able to draw a decoration, such as a rectangle, inside its area
• When we click the widget with a mouse, the color changes, indicating that it is being pressed
• When we release the mouse, the color goes back to the normal state's color
• When we click the widget with a mouse, there would be a signal telling that the widget is activated, and when we release the mouse, another signal would be triggered telling that it is now deactivated