ABAP/4 offers three types of modularization units:
Events
Subroutines
Function modules
A modularization unit is a like a shell into which you can put code. It allows you to segregate a group of lines of code from the rest, and then execute them at a specific time. The lines of code within a modularization unit act very much like a mini-program that can be called from another program.
An event is a tag that identifies a section of code. The section of code associated with an event begins with an event name and ends when the next event name is encountered. The event names are initialization, start-of-selection, and end-of-selection. Event names are reserved words. You cannot create new events-you can only use existing ones.
A driver program is one program that controls another (driven) program. SAP supplies driver programs with the R/3 system. You supply the driven program. When you start your program, a driver program always starts first, and then it calls the events in your program. To reiterate, a driver program starts first and then controls when your program gets control. This has always been the case for all of the programs you have written so far; you just have not been aware of it until now. To repeat once more, when you start your program, a driver program starts first and controls your program by calling events within it.
It triggers the initialization event, causing the code belonging to initialization to be executed. If you have not coded an initialization event in your program, the driver program skips this step.
It shows the selection screen for your program. (A selection screen is the screen that contains the input fields for your parameter statements.) If your program doesn't have a selection screen, it skips this step.
It triggers the start-of-selection event, causing the code belonging to that event to be executed. If you have not coded a start-of-selection event in your program, it skips this step.
It triggers the end-of-selection event in your program, executing all of the code belonging to it. If you haven't coded an end-of-selection, it skips this step.
It shows the output list to the user.
The order of execution for events is determined by the driver program, not by your program. Therefore, you can code the events in any order and the execution order will still be the same. The order of events in your program doesn't matter; they will always be triggered in the same sequence by the driver program.
There are eleven different events in ABAP/4, categorized in Table 17.1 according to how they are triggered.
Table 17.1 ABAP/4 Events
Category Events
<b>Driver initialization</b>
at selection-screen
start-of-selection
get
end-of-selection
<b>User</b>
at line-selection
at pfn
at user-command
<b>Program</b>
top-of-page
end-of-page
If you don't see any events in a program, always remember that one still exists-start-of-selection
if we use the statement <b>stop </b> immediately control leaves the current event and goes directly to the end-of-selection event. Executing stop within end-of-selection leaves the event. It doesn't cause an infinite loop.
Bluesky
