88 CHAPTER 7. GRAPHICAL USER INTERFACE DESIGN IN SCHEME

Likewise we could use a  grid  in place of  table  in this example. Try it and

see how they differ.

There is one other major layout mechanism that is quite useful   the border

layout. This layout allows one to put five components on a page. They can go

in the north, south, east, west, and center. When the window is resized, the

center component grows (or shrinks) horizontally and vertically, while the other

components stay near the edges. For example, we could extend the previous

example, by putting the table in the center of a border layout with labels in the

four other positions:

(define w

(window "Hello"

(border

(center

(table 2 2

(label "red" red)

(label "green" green)

(label "yellow" yellow)

(label "blue" blue)

))

(north (label "north" white))

(south (label "south" yellow))

(east (label "east" green))

(west (label "west" red))

)))

(.pack w)

(.show w)

This example also shows how layouts can be  nested.  Notice that we have put

a table in the  center  of the border layout. Likewise, we could put a table or

a column in the east or west of the border. Or we could put a grid inside one

of the table entries.

Exercise 18 Build several GUIs with colored labels as above. Try to create

layouts that are nested several levels deep and see what happens when you expand

and contract the window.

7.8

Buttons

Buttons are similar to labels except that you can attach  actions  to them.

These actions are carried out when the button is pushed. We will discuss actions

in more detail below, but for now we give a simple example. In the following

example, pushing the buttons resizes the window as specified in the label:

(define w

(window "Hello"

(row 'none 'center white

(button "big" red