[Cobol] How do I implement and use callbacks or event handlers in Cobol?

To implement and use callbacks or event handlers in Cobol, you would typically follow these steps:

  1. Define a procedure division for the callback/event handling logic.

    • This division contains the code that will be executed when the event occurs or when the callback is triggered.
    • It may include any necessary data manipulation or business logic.
  2. Declare a special register or a data item to hold a reference to the callback procedure.

    • This register or data item will store the address of the callback procedure.
  3. Set the value of the callback register to the address of the callback procedure.

    • This can be done by using the "SET ADDRESS OF" statement or by copying the address from another data item.
  4. Pass the callback register as a parameter to the procedure or program that will trigger the callback.

    • The triggering procedure or program will then call the callback procedure using the stored address.
  5. In the callback procedure, perform the necessary actions based on the event or condition that triggered it.

Here is an example of how these steps could be implemented in Cobol:

 1IDENTIFICATION DIVISION.
 2PROGRAM-ID. MAIN-PROGRAM.
 3
 4DATA DIVISION.
 5WORKING-STORAGE SECTION.
 677 CALLBACK-PROCEDURE POINTER.
 7PROCEDURE DIVISION.
 8MAIN-LOGIC.
 9    SET ADDRESS OF CALLBACK-PROCEDURE TO CALLBACK-PROCEDURE-POINTER
10    PERFORM TRIGGER-CALLBACK USING CALLBACK-PROCEDURE-POINTER
11    DISPLAY "Callback was triggered"
12    STOP RUN.
13
14TRIGGER-CALLBACK USING CALLBACK-PROCEDURE-POINTER.
15    CALL CALLBACK-PROCEDURE-POINTER
16    EXIT.
17
18CALLBACK-PROCEDURE.
19    DISPLAY "Callback procedure was executed"
20    EXIT.

In this example, the MAIN-PROGRAM sets the CALLBACK-PROCEDURE-POINTER to the address of the CALLBACK-PROCEDURE and then calls the TRIGGER-CALLBACK procedure, passing the pointer as a parameter. The TRIGGER-CALLBACK procedure then calls the callback procedure using the stored address, executing the logic defined in the CALLBACK-PROCEDURE.

Note that the exact syntax may vary slightly depending on the specific Cobol dialect or compiler you are using.