This topic describes two techniques to isolate your user-interface code:
  • Use preprocessor directives
  • Use the VIEW-AS phrase

Use preprocessor directives

OpenEdge provides a language preprocessor that allows you to write applications that are easy to read, modify, and transport to other operating systems. The preprocessor is a component of the OpenEdge Compiler. Before the Compiler analyzes your source code and creates r-code, the preprocessor examines your source code and performs text substitutions.

You control the preprocessor by placing preprocessor directives throughout your source code. A preprocessor directive is a statement that begins with an ampersand (&) and is meaningful only to the preprocessor.

The preprocessor recognizes a built-in preprocessor constant that identifies the window system where a file is being compiled, called &WINDOW-SYSTEM. The possible values include "MS-WINnn" and "TTY." You can use this constant to direct the preprocessor to determine which code to run on each window system, as follows:

FORM
  fld1
  fld2
&IF "{&WINDOW-SYSTEM}" = "MS-WIN97" &THEN
  fld3 AT 20
&ELSE
  fld3 AT 15
&ENDIF
   WITH FRAME XYZ.

The SESSION system handle also has a WINDOW-SYSTEM attribute. Your application can use this to test the current window system while it is running:

IF SESSION:WINDOW-SYSTEM = "MS-WIN97" THEN
   .
   .
   .

Both the &WINDOW-SYSTEM preprocessor constant and the SESSION handle WINDOW-SYSTEM attribute perform the same basic function. The &WINDOW-SYSTEM preprocessor constant allows you to write code that is conditionally compiled, while the WINDOW-SYSTEM attribute is a run-time function.

The WINDOW-SYSTEM attribute evaluates as follows:

  • If Windows nn is running, and the Windows nn user interface is running, this attribute evaluates to MS-WINnn. Otherwise, if the Windows nn user interface is not running, it evaluates to MS-WINDOWS.
  • If the application is not running in a Windows environment, this attribute evaluates to TTY.

OpenEdge supports an override option that enables applications that require a WINDOW-SYSTEM attribute to return the value of MS-WINDOWS for all Microsoft operating systems. To establish this override capability, define the Window System key in the Startup Section of the current environment, which might be in the registry or an initialization file. If the Window System key is located, the WINDOW-SYSTEM attribute returns the value associated with the Window System key on all platforms.

You can also specify offsets and ROW and COLUMN specifications using preprocessor constants, then define the constants separately for different environments. OpenEdge allows you to specify fractional character units so that you can specify precise locations for objects in a graphical environment. In a character environment, the ROW and COLUMN values are truncated to integer values, as follows:

&IF "{&WINDOW-SYSTEM}" = "TTY" &THEN
&GLOBAL-DEFINE COL3 20
&ELSE
&GLOBAL-DEFINE COL3 15
&ENDIF
FORM
  fld1
  fld2
  fld3 AT {&COL3}
  WITH FRAME XYZ.

Use the VIEW-AS phrase

If possible, use the VIEW-AS phrase with the DEFINE VARIABLE statement instead of with screen I/O statements. The VIEW-AS phrase describes how a field or variable is represented on the screen. You can use the VIEW-AS phrase as a modifier to the DEFINE VARIABLE statement or as a modifier to a screen I/O statement, such as DISPLAY.

With the DEFINE VARIABLE statement, the VIEW-AS phrase specifies the default type of widget for the variable being defined. Use the VIEW-AS phrase when defining a variable so that code that manipulates the variable, such as an UPDATE statement, does not have to specify the representation. This helps you isolate your user-interface code.

For more information, see VIEW-AS phrase.