92 CHAPTER 7. GRAPHICAL USER INTERFACE DESIGN IN SCHEME

(define w (window "test"

(col

(label "enter a number and hit return to double

(t "x" (textfield "1" 20))

(button "double" (action (lambda(e)

(let ((val (readexpr (t "x"))))

(writeexpr (t "x") (* 2 val))))))))))

(.pack w) (.show w)

Figure 7.2: A simple example of tagging and actions

(t "abc" X)

to lookup the object whose name is  abc , you use the syntax

(t "abc")

For example, the program in Figure 7.2 shows the standard way of creating a

textfield with an associated action that reads and writes data onto the textfield

when a button is pushed.

7.14

Actions

One of the most important arguments to components is the  action  argument

which specifies what to do when the component is selected (e.g. when a button is

pushed or a menuitem selected, or a choice selected, or a textfield value entered).

The syntax for the action argument is:

(action (lambda (e) COMMANDS))

the  lambda  specifies that the  COMMANDS  are to be delayed until the

action is performed. The  e  represent the kind of event that triggered the

action. From the  e  variable we can get the time the action was triggered,

which component it came from, where the mouse was (for a mouse event), etc.

7.15

Reading and Writing on Components

The most common actions associated to simple GUIs involve reading some val 

ues entered or selected by the user, and using those values to compute some

quantity that is then written onto another component. Thus we need to have a

mechanism for reading and writing Scheme components. These procedures are

shown below:

  (readstring COMPONENT)   reads the text on a component as a string

  (readexpr COMPONENT)   reads the text on a component as a Scheme

term