3.7 System Structures: Using a Screen-Control Package

The Ada.Text_IO package provides operations for reading from the terminal keyboard and writing to the screen, but it provides no direct operations for controlling the screen in interesting ways, such as moving the cursor to a given row-column position before writing. Doing this requires an additional package which uses Ada.Text_IO to send control characters to the teminal; the control characters act as instructions to it instead of data it should display. Because this package, which we will call Screen, is not part of standard Ada, we provide it with this book. The details of just how this package operates are left until Chapter 7, but it is possible for you to use the package without understanding its innards.

A package consists of two files, the specification file and the body file. As discussed in Section 3.6, the specification gives the "contract with the user," or list of promised resources. The body delivers the actual source code for the procedures and functions promised by the specification. Because the input/output and Ada.Calendar packages are supplied in precompiled form by all Ada compilers, we have seen only their specifications; the source code for the bodies is not available to us. Other packages may be supplied to you in source-code form, with both the specification and body files provided. Screen is one of these packages.

Program 3.8 shows the specification for Screen.

Program 3.8
Specification for Screen Package

PACKAGE Screen IS
------------------------------------------------------------------------
--| Procedures for drawing pictures on ANSI Terminal Screen
--| Author: Michael B. Feldman, The George Washington University
--| Last Modified: July 1995
------------------------------------------------------------------------

  -- constants; the number of rows and columns on the terminal

  Screen_Depth : CONSTANT Integer := 24;
  Screen_Width : CONSTANT Integer := 80;

  -- subtypes giving the ranges of acceptable inputs 
  -- to the cursor-positioning operation

  SUBTYPE Depth IS Integer RANGE 1..Screen_Depth;
  SUBTYPE Width IS Integer RANGE 1..Screen_Width;

  PROCEDURE Beep;
  -- Pre:  None
  -- Post: Terminal makes its beep sound once

  PROCEDURE ClearScreen;
  -- Pre:  None
  -- Post: Terminal Screen is cleared

  PROCEDURE MoveCursor (Column : Width; Row : Depth);
  -- Pre:  Column and Row have been assigned in-range values
  -- Post: Cursor is moved to the given spot on the screen

END Screen;

This package provides two constants, Screen_Width and Screen_Depth, corresponding to the number of columns (usually 80) and rows (usually 24) on the screen. There are also two subtypes, Width and Depth, giving the ranges for valid cursor positions (1..Screen_Depth and 1..Screen_Width, respectively).

The package provides three procedures. The first two, Beep and ClearScreen, take no parameters: A procedure call statement

    Screen.Beep;

    Screen.ClearScreen;

    Screen.MoveCursor (Row => 10, Column => 22);
    Text_IO.Put (Item => '*');

    Screen.MoveCursor (Row => 5, Column => 10);
    Text_IO.Put (Item => "-----");

Note the style of comments documenting each of these procedures. These are called preconditions and postconditions and are used to describe each procedure's assumptions and behavior in an informal but structured way. We'll come back to this subject in more detail in Chapter 6; meanwhile, you can get used to reading this style of documenting our packages.

Program 3.9 gives the body file for this package. You might not understand exactly how the procedures work. Don't worry about this right now; we will return to it in Chapter 7.

Program 3.9
Body of Screen Package

WITH Ada.Text_IO;
WITH Ada.Integer_Text_IO;
PACKAGE BODY Screen IS
------------------------------------------------------------------------
--| Body of screen-handling package
--| Author: Michael B. Feldman, The George Washington University
--| Last Modified: July 1995            
------------------------------------------------------------------------

  PROCEDURE Beep IS
  BEGIN
    Ada.Text_IO.Put (Item => ASCII.BEL);
  END Beep;

  PROCEDURE ClearScreen IS
  BEGIN
    Ada.Text_IO.Put (Item => ASCII.ESC);
    Ada.Text_IO.Put (Item => "[2J");
  END ClearScreen;

  PROCEDURE MoveCursor (Column : Width; Row : Depth) IS
  BEGIN
    Ada.Text_IO.Put (Item => ASCII.ESC);
    Ada.Text_IO.Put ("[");
    Ada.Integer_Text_IO.Put (Item => Row, Width => 1);
    Ada.Text_IO.Put (Item => ';');
    Ada.Integer_Text_IO.Put (Item => Column, Width => 1);
    Ada.Text_IO.Put (Item => 'f');
  END MoveCursor;

END Screen;

Preparing to Use a Non-System Package

    WITH Screen;

    DELAY 0.1;

Program 3.10
Smiley: A Program that Uses Package Screen

WITH Ada.Text_IO;
WITH Screen;
PROCEDURE Smiley IS
------------------------------------------------------------------------
--| Draws a "smiley face" in the center of the terminal screen
--| Author: Michael B. Feldman, The George Washington University
--| Last Modified: July 1995
------------------------------------------------------------------------
BEGIN -- Smiley

  Screen.ClearScreen;
  Screen.Beep;
  DELAY 0.1;
  Screen.Beep;
  DELAY 0.1;
  Screen.Beep;
  DELAY 0.1;
  Screen.MoveCursor (Row => 7, Column => 34);
  Ada.Text_IO.Put (Item =>    "HAVE A NICE DAY!");
  Screen.MoveCursor (Row => 9, Column => 39);
  Ada.Text_IO.Put (Item =>     "_____");
  Screen.MoveCursor (Row => 10, Column => 37);
  Ada.Text_IO.Put (Item =>   "/       \");
  Screen.MoveCursor (Row => 11, Column => 36);
  Ada.Text_IO.Put (Item =>  "/         \");
  Screen.MoveCursor (Row => 12, Column => 35);
  Ada.Text_IO.Put (Item => "|           |");
  Screen.MoveCursor (Row => 13, Column => 35);
  Ada.Text_IO.Put (Item => "|   O   O   |");
  Screen.MoveCursor (Row => 14, Column => 36);
  Ada.Text_IO.Put (Item =>  "\    o    /");
  Screen.MoveCursor (Row => 15, Column => 37);
  Ada.Text_IO.Put (Item =>   "\ \___/ /");
  Screen.MoveCursor (Row => 16, Column => 38);
  Ada.Text_IO.Put (Item =>    "\     /");
  Screen.MoveCursor (Row => 17, Column => 39);
  Ada.Text_IO.Put (Item =>     "-----");
  Screen.MoveCursor (Row => 24, Column => 1);

END Smiley;

                             HAVE A NICE DAY!

                                  _____
                                /       \
                               /         \
                              |           |
                              |   O   O   |
                               \    o    /
                                \ \___/ /
                                 \     /
                                  -----

Copyright © 1996 by Addison-Wesley Publishing Company, Inc.