Object-oriented Software in Ada 95

22 A text user interface
This chapter defines an API (Application Program Interface) for use by an application program that reads
and writes textual information to and from windows on a VDU screen. The TUI (Text User interface) uses
the metaphor of non-overlapping windows. The application program is written using an event-driven
regime. In this way, call-back functions are written to implement services requested by a user of the
application.
The next chapter describes in detail the implementation of the TUI.
22.1 Specification
A TUI (Text User Interface) provides various types of non-overlapping windows on a VDU screen. The windows
provided are:
• A text window into which characters from the Ada type Character can be written.
• A dialog window that provides a mechanism for a user to enter character data. Associated with a dialog
window is a function that is called on completion of the user’s entered data.
• A menu window from which a user can select a menu item from a list of available options.
For example, a demonstration application that converts miles into kilometres uses the TUI to input and display
data as follows:
+----------------------------------+
| |
| Miles to kilometres |
| |
+----------------------------------+
#--------------------+ +----------------------------------+
|Dialog| Miles | |Distance in miles = 50.00 |
|--------------------| |Distance in Kms = 80.47 |
|50.0* | | |
+--------------------+ +----------------------------------+
Note:
A # in the top left-hand corner of a window signifies which window has the focus for input.
The interface for this program consists of three distinct windows:
• A text window which displays the title of the program
“Miles to kilometres”
• A dialog window that solicits input from the user. In this distance in miles to be converted into
kilometres.
• A text window to display of the results of the conversion.
The TUI interface for writing to windows is modelled on the procedure Put in the package Ada.Text_Io.
Associated with an instance of a Dialog window is a call-back function that implements the functionality of the
user interaction. This is often referred to as an event-driven metaphor.
The dialog window's call-back function is executed when a user has finished inputting data into the program.
For example, in the miles to kilometres program the dialog window's call-back function is executed when a user
presses return completing the entry of the miles to be converted into kilometres. The call-back function calculates
the conversion to kilometres and displays the answer in the result’s window.
© M A Smith - May not be reproduced without permission