How to check variable if present or not ?

Question

I have a main form and put the :global.gdate, When I call the others form
I want to use the :global.gdate.
I wirte a trigger "when new form instance" for each form (except main form) so I can get :global.date.
But I can only running this program from main form, if I am running the form without the main form, the error message :global.gdate does not exists.
Question:
Is it posible to check if :global.gdate is exists or not ?
maybe somthing like :
IF get_variable(:global.gdate) IS NULL THEN --?????????
:global.gdate := sysdate;
END IF;

Answer 1

From Oracle documentation:
DEFAULT_VALUE built-in
Description
Copies an indicated value to an indicated variable if the variable's current value is NULL. If the variable's current value is not NULL, DEFAULT_VALUE does nothing. Therefore, for text items this built-in works identically to using the COPY built-in on a NULL item. If the variable is an undefined global variable, Form Builder creates the variable.
Syntax
PROCEDURE DEFAULT_VALUE
(value_string VARCHAR2,
variable_name VARCHAR2);
Example:
** Built-in:  DEFAULT_VALUE
** Example:   Make sure a Global variable is defined by
**           assigning some value to it with Default_Value
BEGIN
  ** Default the value of GLOBAL.Command_Indicator if it is
  ** NULL or does not exist.
  Default_Value('***','global.command_indicator');
  ** If the global variable equals the string we defaulted
  ** it to above, then it must have not existed before
  IF :Global.Command_Indicator = '***' THEN
    Message('You must call this screen from the Main Menu');
    RAISE Form_Trigger_Failure;
  END IF;
END;