# 3. Hierarchical composition of a window

Interfaces in GtkAda are built in layers, as in Motif. For instance, a typical dialog is basically a Gtk_Window, that in turn contains a Gtk_Box, itself divided into two boxes and a Gtk_Separator, and so on.

Altough this may seem more complicated than setting absolute positions for children, this is the simplest way to automatically handle the resizing of windows. Each container that creates a layer knows how it should behave when it is resized, and how it should move its children. Thus almost everything is handled automatically, and you don’t have to do anything to support resizing.

If you really insist on moving the children to a specific position, look at the Gtk_Fixed widget and its demo in `testgtk/`. But you really should not use this container, since you will then have to do everything by hand.

All the containers are demonstrated in `testgtk/`, in the GtkAda distribution. This should help you understand all the parameters associated with the containers. It is very important to master these containers, since using the appropriate containers will make building interfaces a lot easier.

If you look at the widget hierarchy (Widgets Hierarchy), you can see that a Gtk_Window inherits from Gtk_Bin, and thus can have only one child. In most cases, the child of a Gtk_Window will thus be a Gtk_Box, which can have any number of children.

Some widgets in GtkAda itself are built using this strategy, from the very basic Gtk_Button to the more advanced Gtk_File_Selection.

For example, by default a Gtk_Button contains a Gtk_Label, which displays the text of the button (like ‘OK’ or ‘Cancel’).

However, it is easy to put a pixmap in a button instead. When you create the button, do not specify any label. Thus, no child will be added, and you can give it your own. See `testgtk/create_pixmap.adb` for an example on how to do that.