HP 3000 Manuals HP 3000 Manuals
How HP ALLBASE/4GL Screens Work
HP ALLBASE/4GL screens use quite different screen handling techniques
than you may have used in more conventional programming environments. In
particular, HP ALLBASE/4GL screens provide extensive facilities for
automatic data formatting, data validation, and data movement.
All of the developer screens you use in developing the training
application are in fact HP ALLBASE/4GL screens. They demonstrate the
features you can include in your own applications.
HP ALLBASE/4GL screens and the automatic logic associated with them form
a fundamental part of any application you develop. When HP ALLBASE/4GL
displays a screen, the screen and logic associated with the screen
control the application. In particular, screens control all interaction
between your application and the user. The type of screen, and the way
the screen is called, determines where control resumes after the screen
processing is complete.
Screen Types
HP ALLBASE/4GL uses three types of screens. They are:
* Menus.
* Data screens.
* Windows.
Menus.
Menus have the following features:
* They present the user with a range of options in the application.
You have already been using menus to select options in the
developer application.
Frequently, an option on a menu calls a further menu. For
example, this occurs when you select the Dictionary option on the
developer main menu.
* They allow the use of the keyboard to select options. You have
already used the keyboard facilities to select options from the
developer menus. Exactly the same keyboard facilities are
available to the end users of your applications.
* No action occurs until the user selects and activates an option.
When HP ALLBASE/4GL displays a menu, all current activity in the
application (except background processing) is terminated
immediately. Nothing further happens until the user selects and
executes one of the options on the menu, or initiates an action
from a function key.
In general, HP ALLBASE/4GL displays a menu when an application
first starts and when the current process terminates.
Data Screens.
Data screens allow the user to enter data for the application and allow
the application to display information for the user. Data screens
contain input fields, display fields, and literals or titles. Data
screens can also have a scroll area for information display. This is
particularly useful if you have more information to display than can
readily fit on the screen.
Data screens have the following properties:
* They form the basis of all interactive data input and information
display for the application.
* They are the primary means of user data entry.
* They are the primary means of information display.
* They can usually perform most of the data formatting, data
validation, and data movement required by an application.
Windows.
A data screen can be overlaid with a window screen. A window screen can
present additional information to the user as well as accepting input
from the user.
A window uses the same types of screen items and can perform the same
data manipulation as a data screen. However, HP ALLBASE/4GL can only
display a window on an already displayed data screen. Menus cannot use
windows.
You have already seen and used a window on the file/SQL table definition
screen in the dictionary menu. You may want to go back to this screen
now to refresh your memory.
Function Keys
All screens can use a set of function keys. The function keys allow the
user to select various application options or system functions that may
be needed in the application. Besides the default function keys
available in HP ALLBASE/4GL, you can set up function keys that are
specific to your application.
System Items
All screens can contain system items. These are fields that display the
contents of various system or application defined fields. System items
can display the following information for the end user:
* The current date and time.
* Communication area fields.
* Master titles.
* Application titles.
* Numeric or alphanumeric constants.
* Variables or calculated items.
* Scratch-pad fields.
Displaying Screens
HP ALLBASE/4GL displays a screen when it receives a SCREEN command in
logic, or a D- action from a menu, a function key, or a decision table.
The SCREEN command is one of the logic commands that can be called in a
function or a process. After you have learned how to create the screens
for your application, you will learn how to use HP ALLBASE/4GL to create
a process that contains a SCREEN command.
Once HP ALLBASE/4GL displays a screen, the screen itself controls the
sequence of events occurring in the application. In general, the screen
remains in control until the user initiates an action that terminates the
screen. This occurs regardless of the type of screen.
For a menu, no further actions or processing occurs until the user
selects and activates a menu option.
For a data screen, the screen remains active until the user initiates an
appropriate action to terminate the screen, such as the "Commit Data"
action.
In general, interaction with the user is not possible if HP ALLBASE/4GL
is not displaying a screen.