10.1.2 Space allocation

When allocate-space is called on a pane P, it must compare the space-requirement of the children of P to the available space, in order to distribute it in the most preferable way. In order to avoid a second recursive invokation of compose-space at this point, we store the result of the previous call to compose-space in each pane.

To handle this situtation and also explicitly given size options, we use an :around method on compose-space. The :around method will call the primary method only if necessary (i.e., (eq (slot-value pane 'space-requirement) nil)), and store the result of the call to the primary method in the space-requirement slot.

We then compute the space requirement of the pane as follows:

          (setf (space-requirement-width ...)  (or explicit-width
                (space-requirement-width request)) ...
                (space-requirement-max-width ...)  (or explicit-max-width
                explicit-width (space-requirement-max-width request)) ...)

When the call to the primary method is not necessary we simply return the stored value.

The spacer-pane is an exception to the rule indicated above. The explicit size you can give for this pane should represent the margin size. So its primary method should only call compose on the child. And the around method will compute the explicit sizes for it from the space requirement of the child and for the values given for the surrounding space.