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ON KEY clauses can be included into the PROMPT statement to specify the behaviour of the keys pressed while the PROMPT statement is executed and the user input is performed. The keys can be assigned to form widgets and the actions will be triggered when such widget is pressed. However, if a key combination is not assigned to any widget, it can be activated with the help of the keyboard. One PROMPT statement can contain any number of the ON KEY clauses, they can be placed in any order.
ON KEY clause can use one or several of the following keys separated by commas enclosed in parentheses to specify the key to which the other statements in ON KEY clause refer:
|
ESC/ESCAPE |
ACCEPT |
|
NEXT/ NEXTPAGE |
PREVIOUS/PREVPAGE |
|
INSERT |
DELETE |
|
RETURN |
TAB |
|
INTERRUPT |
HELP |
|
LEFT |
RIGHT |
|
UP |
DOWN |
|
F1 – F256 |
CONTROL-char |
Any character can be used as char in the combination CONTROL-char except the following characters: A, D, H, I, J, K, L, M, R, and X. The key names can be entered either in lower case or in upper case letters.
Any 4GL or SQL statements can be used in the ON KEY clause. These statements are executed when the key specified in the corresponding ON KEY clause is pressed.
When an ON KEY clause within the PROMPT statement is activated by pressing the corresponding key, the following actions are performed:
The program control is passed to the corresponding ON KEY clause
The statements of the ON KEY clause are executed
The control is passed to the first statement that follows the END PROMPT keywords.
If the user presses the key specified in the ON KEY clause, this key is not typed in the PROMPT field. As the control does not return to the PROMPT statement after the ON KEY clause is executed, the value entered into the PROMPT field is assigned to the variable in the FOR clause. If the key is pressed before any value has been entered into the PROMPT field, the variable remains undetermined.
The value entered by the user can be changed by assigning a new value to the variable used in the FOR clause by means of the statements of an ON KEY clause.
PROMPT "Enter the date of order" ATTRIBUTE (REVERSE, GREEN) FOR ord_date
ATTRIBUTE (BLUE, CENTURY = "C")
ON KEY (F5)
LET ord_date = TODAY
END PROMPT
Some keys require special attention if used in the ON KEY clause:
|
Key |
Usage Features |
|
ESC/ESCAPE |
If you want to use this key in ON KEY clause, you must specify another key as the Accept key in the OPTIONS statement, because ESCAPE is the Accept key by default |
|
INTERRUPT |
The DEFER INTERRUPT statement must be executed in order that this key could be used in the ON KEY clause. On pressing the Interrupt key the corresponding ON KEY clause is executed, int_flag is set to non-zero value |
|
QUIT |
The DEFER QUIT statement must be executed in order that this key could be used in the ON KEY clause. On pressing the QUIT key the corresponding ON KEY clause is executed and int_flag is set to non-zero value |
|
CTRL-char (A, D, H, K, L, R, X) |
4GL reserves these keys for field editing and they should not be used in the ON KEY clauses |
|
CTRL-char (I, J, M) |
These key combinations by default mean TAB, NEWLINE and RETURN. If they are used in the ON KEY clause, they cannot perform their default functions while the OK KEY clause is functional. Thus, if you use these keys in an ON KEY clause, the period of time during which this clause is functional should be restricted. |
Some restrictions in key usage might be applied depending on your operational system. Many systems use such key combinations as CONTROL-C, CONTROL-Q, and CONTROL-S for the Interrupt, XON, and XOFF signals.