A Gtk+ user interface is constructed by nesting widgets inside widgets. Container widgets are the inner nodes in the resulting tree of widgets: they contain other widgets. So, for example, you might have a Window containing a Frame containing a Label. If you wanted an image instead of a textual label inside the frame, you might replace the Label widget with a Image widget.

There are two major kinds of container widgets in Gtk+. Both are subclasses of the abstract Container base class.

The first type of container widget has a single child widget and derives from Bin. These containers are decorators, which add some kind of functionality to the child. For example, a Button makes its child into a clickable button; a Frame draws a frame around its child and a Window places its child widget inside a top-level window.

The second type of container can have more than one child; its purpose is to manage layout. This means that these containers assign sizes and positions to their children. For example, a HBox arranges its children in a horizontal row, and a Table arranges the widgets it contains in a two-dimensional grid.

To fulfill its task, a layout container must negotiate the size requirements with its parent and its children. This negotiation is carried out in two phases, size requisition and size allocation.

The size requisition of a widget is it's desired width and height. This is represented by a Requisition.

How a widget determines its desired size depends on the widget. A Label, for example, requests enough space to display all its text. Container widgets generally base their size request on the requisitions of their children.

The size requisition phase of the widget layout process operates top-down. It starts at a top-level widget, typically a Window. The top-level widget asks its child for its size requisition by calling widgetSizeRequest. To determine its requisition, the child asks its own children for their requisitions and so on. Finally, the top-level widget will get a requisition back from its child.

When the top-level widget has determined how much space its child would like to have, the second phase of the size negotiation, size allocation, begins. Depending on its configuration (see windowSetResizable), the top-level widget may be able to expand in order to satisfy the size request or it may have to ignore the size request and keep its fixed size. It then tells its child widget how much space it gets by calling widgetSizeAllocate. The child widget divides the space among its children and tells each child how much space it got, and so on. Under normal circumstances, a Window will always give its child the amount of space the child requested.

A child's size allocation is represented by an Allocation. This contains not only a width and height, but also a position (i.e. X and Y coordinates), so that containers can tell their children not only how much space they have gotten, but also where they are positioned inside the space available to the container.

Widgets are required to honor the size allocation they receive; a size request is only a request, and widgets must be able to cope with any size.

Container introduces child attributes - these are object attributes that are not specific to either the container or the contained widget, but rather to their relation. Typical examples of child attributes are the position or pack-type of a widget which is contained in a Box.

The Container class does not itself define any child attributes, they are defined (and documented) by the various Container subclasses.

Child attributes can be set or obtained in a similar way to ordinary attributes. So ordinary attributes are set like so:

 set object [ attr := value ]

Whereas child attributes take the child object as a parameter:

 set container [ attr child := value ]

And similarily for getting a child attribute's value:

 value <- get container (attr child)

 |  GObject
 |   +----Object
 |         +----Widget
 |               +----Container
 |                     +----Bin
 |                     +----Box
 |                     +----CList
 |                     +----Fixed
 |                     +----Paned
 |                     +----IconView
 |                     +----Layout
 |                     +----List
 |                     +----MenuShell
 |                     +----Notebook
 |                     +----Socket
 |                     +----Table
 |                     +----TextView
 |                     +----Toolbar
 |                     +----TreeView
 

Specify how resize events are handled.

Default value: ResizeParent

The width of the empty border outside the containers children.

Allowed values: <= (maxBound :: Int)

Default value: 0