10.1.1 Space composition

The space composition is accomplished by the generic function compose-space. When applied to a pane, compose-space returns an object of type space-requirement indicating the needs of the pane in terms of preferred size, minimum size and maximum size. The phase starts when compose-space is applied to the top-level pane of the application frame. That pane in turn may ask its children for their space requirements, and so on until the leaves are reached. When the top-level pane has computed its space requirments, it asks the system for that much space. A conforming window manager should respect the request (space wanted, min space, max space) and allocate a top-level window of an acceptable size. The space given by the system must then be distributed among the panes in the hierarchy space-allocation.

Each type of pane is responsible for a different method on compose-space. Leaf panes such as labelled gadgets may compute space requirements based on the size and the text-style of the label. Other panes such as the vbox layout pane compute the space as a combination of the space requirements of their children. The result of such a query (in the form of a space-requirement object) is stored in the pane for later use, and is only changed as a result of a call to note-space-requirement-changed.

Most composite panes can be given explicit values for the values of :width, :min-width, :max-width, :height, :min-height, and :max-height options. If such arguments are not given (effectively making these values nil), a general method is used, such as computing from children or, for leaf panes with no such reasonable default rule, a fixed value is given. If such arguments are given, their values are used instead. Notice that one of :height and :width might be given, applying the rule only in one of the dimensions.

Subsequent calls to compose-space with the same arguments are assumed to return the same space-requirement object, unless a call to note-space-requirement-changed has been called in between.