7.2. OVERVIEW OF THE JLIB TOOLKITS

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and determines the precise minimum size of the window so that everything just

fits inside it. The (.show win) command makes the window appear.

Although this little program does quite a bit (requiring a paragraph to de 

cide), the code for the program is relatively concise. Each word that appears

in the program has a particular effect (except for the lambda(e) which we will

explain later). By replacing col with row we would get a horizontal layout, and

by replacing label with button our greeting would be able to take an action.

7.2

Overview of the JLIB toolkits

The key idea of the JLIB toolkit is to use the flexibility and expressiveness

of Scheme to create a GUI building library which allows complex GUIs to be

built by evaluating a simple and declarative expression whose structure closely

resembles the structure of the GUI itself.

The JLIB model is based on five fundamental concepts:

  COMPONENTS   there are a small number of ways to construct basic

components (buttons, windows, ...)

  LAYOUTS   there are a small number of ways to layout basic components

(row, col, table, grid, ...)

  ACTIONS   there is a simple mechanisms for associating an action to a

component

  PROPERTIES   there are easy ways for setting the font and color of

components

  TAGS   this is a mechanism for giving names to components while they

are being laid out.

Another key idea is that operations on all components should be as uniform

as possible. For example, there are procedures  readstring  and  writestring 

which allow one to read a  string  from a component, and write a string onto

a component. Thus  writestring  can change the string on a label, a button, a

textfield, a textarea. It can also change the title of a window or add an item to

a choice component. Likewise, readstring returns the label of a button, the text

in a textarea or textfield, the text of the currently selected item in a choice, the

title of a window, and the text of a label.

Similarly, JLIB is designed so that the component and layout constructors

require a few initial arguments of specified types, followed by many optional

arguments which may apper in any order and whose types are used to determine

their effect.

For example, a font object will change the font of the component, a color

object will change the background color, a Listener object will add an action

listener to the component, a string will generally become the label of the object.

We introduce each of these components in turn. As we go through these

examples you should try them out yourself in a Jscheme interpreter window.