SizeGroup provides a mechanism for grouping a number of widgets together so they all request the same amount of space.
This is typically useful when you want a column of widgets to have the same size, but you can’t use a Grid widget.
In detail, the size requested for each widget in a SizeGroup is the maximum of the sizes that would have been requested for each widget in the size group if they were not in the size group. The mode of the size group (see set_mode) determines whether this applies to the horizontal size, the vertical size, or both sizes.
Note that size groups only affect the amount of space requested, not the size that the widgets finally receive. If you want the widgets in a SizeGroup to actually be the same size, you need to pack them in such a way that they get the size they request and not more. For example, if you are packing your widgets into a table, you would not include the gtk_fill flag.
SizeGroup objects are referenced by each widget in the size group, so once you have added all widgets to a SizeGroup, you can drop the initial reference to the size group with unref. If the widgets in the size group are subsequently destroyed, then they will be removed from the size group and drop their references on the size group; when all widgets have been removed, the size group will be freed.
Widgets can be part of multiple size groups; GTK+ will compute the horizontal size of a widget from the horizontal requisition of all widgets that can be reached from the widget by a chain of size groups of type gtk_size_group_horizontal or gtk_size_group_both, and the vertical size from the vertical requisition of all widgets that can be reached from the widget by a chain of size groups of type gtk_size_group_vertical or gtk_size_group_both.
Note that only non-contextual sizes of every widget are ever consulted by size groups (since size groups have no knowledge of what size a widget will be allocated in one dimension, it cannot derive how much height a widget will receive for a given width). When grouping widgets that trade height for width in mode gtk_size_group_vertical or gtk_size_group_both : the height for the minimum width will be the requested height for all widgets in the group. The same is of course true when horizontally grouping width for height widgets.
Widgets that trade height-for-width should set a reasonably large minimum width by way of width_chars for instance. Widgets with static sizes as well as widgets that grow (such as ellipsizing text) need no such considerations.
GtkSizeGroup as GtkBuildable
Size groups can be specified in a UI definition by placing an `<object>` element with `class="GtkSizeGroup"` somewhere in the UI definition. The widgets that belong to the size group are specified by a `<widgets>` element that may contain multiple `<widget>` elements, one for each member of the size group. The ”name” attribute gives the id of the widget.
An example of a UI definition fragment with GtkSizeGroup: