Event Loop Method

The event loop method enables sophisticated screen handling with tremendous flexibility and greatly reduced coding.

There are three components in an event loop:

• Loop control. This specifies the condition or conditions that terminates the loop.
• Execution of an event. One event occurs on each repetition of the loop, until the loop control conditions are met.
• Event testing. One or more checks by the application program to evaluate events. Rather than dictating each event, the application program responds to the current event.

Example of an RM/Panels Event Loop

PERFORM UNTIL F3-KEY-PRESSED    (Loop Control) 
     PERFORM RMP--EE-PANELNAME  (Execution) 
     EVALUATE RMP--LAST-FIELD
        WHEN "CUSTOMER-NUMBER"  (Event Test)
           PERFORM VALIDATE-CUSTOMER-NUMBER
        WHEN "CUSTOMER-STATE"   (Event Test)
           PERFORM VALIDATE-CUSTOMER-STATE 
     END-EVALUATE
END-PERFORM.

The event is input to a field/control on the screen, which is performed by the procedure RMP--EE-PANELNAME. This procedure executes the RM/Panels EXECUTE EVENT standard runtime function. The EXECUTE EVENT standard runtime function accomplishes the following:

• Checks the parameters passed to it to see what event to perform
• Performs the event (field/control input)
• Sets parameters to indicate what event was executed and what event should be executed next
• Returns control to the application program

Each time the EXECUTE EVENT standard runtime function is performed, only one event is executed, thus the application program can always intercept control after any event. It then has the ability to perform special processing or force a particular event to occur.

There are many advantages to using the event loop method:

1. Writing tedious screen handling code is eliminated.
2. Adding special field/control level logic is made easier.
3. Field/control level logic is insulated from changes in field/control entry order and position.
4. Size of screen handling code is dependent upon the amount of special processing to be done, rather than the number of fields/controls on the screen.
5. Adding new fields/controls does not affect the event loop unless the fields/controls require special processing.
6. Event loops can be nested, which allows multiple panels to be used on the screen. The following section describes the use of multiple panels and special considerations in nesting event loops.