7.9. CHOICE COMPONENTS

89

(action (lambda(e) (.resize w 500 500))))

(button "small" green

(action (lambda(e) (.resize w 10 10))))

(button "just right" yellow

(action (lambda(e) (.pack w))))

(button "gone"

(action (lambda(e) (.hide w))))

)))

(.pack w)

(.show w)

Observe that we add an action to a button by adding a term of the following

form to its argument list:

(action (lambda(e) .....))

where the  ....  represents some scheme code that will be executed when the

button is pushed.

7.9

Choice components

The choice procedure produces a component that allows the user to choose

from a fixed set of choices. The following GUI lets the user make a choice of

size for the window:

(define w

(window "Hello"

(choice "big" "small" "just right" "gone"

(action (lambda(e)

(case (readstring (.getSource e))

(("big") (.resize win 500 500))

(("small") (.resize win 10 10))

(("just right") (.pack win))

(("gone") (.hide win))

))))))

(.pack w)

(.show w)

This example shows one way of writing an action that depends on the choice

taken by the user. The tricky part is the

(readstring (.getSource e))

The  e  in this case is the  event  that occurs when the user presses selects a

choice. The  .getSource  procedure determines the component that generated

that event (in this case, the choice), and the  readstring  reads the string la 

belling that choice. The rest of the action is a simple  case  expression that

selects the appropriate action based on the user's choice.