Event blocks in ABAP program

EVENT BLOCKS

An event is a tag that identifies a section of code.
The section of code associated with an event name and ends when a next name is encountered.

The ABAP Runtime system calls the event blocks in sequence designed fot generating and processing lists.

1)First INITIALIZATION event block is called.

2)Then a Selection Screen is sent to the Presentation Server.

3)After the user leaves the selection screen,START-OF-SELECTION is called.

4)If START-OF-SELECTION contains ABAP statements like WRITE,SKIP,ULINE, a list buffer is filled.

5)The list buffer is subsequently sent to the Presentation server as a OUTPUT LIST.

Some of the events are:
1. Initialization.
-> It is an event that you can use if you need to set a large number of default values.
-> This event block allows you to set default values that can only be determined at runtime
-> It takes place before the standard selection screen is displayed
-> Dynamic Allocation

2. At Selection-Screen.
-> This event is fired after the user-input on a selection screen has been processed,but while the selection screen is still active
-> User Input Validations
-> Passes input data from selection screen to ABAP program
-> Change selection screen
-> Process user input.

3. Start-of-Selection.
-> It takes place after the standard selection screen has been processed,before the data is read from logical database
-> After all data is read by the logical database
-> All Subroutine processing takes place here.


4.End-of-Selection.
-> This event is fired after all the selection is done from the database tables.
-> At this event generally all the data fetched is displayed on the list.

5.Top-of-Page.
-> It takes place when it encounters the top of the page.
-> Mostly to print the header data like company name,date,report name etc.

6. End-of-Page.
-> It takes place when it encounters the bottom of the page.
-> Mostly to print the footer data like no. of pages,time etc

7. At line Selection
-> This event is fired when a user clicks or selects a particular line on the list.
-> When the user triggers the predefined function code PICK

8. At PFn.
-> When the user triggers the predefined function code PFnn

9. At User-command.
-> When the user triggers a function code defined in the program

SAP has a inbuilt program called Driver Program which controls another program in the R/3 System.
DRIVER EVENTS:
Initialization.
At Selection-Screen.
Start-of-selection.
End-of Selection.
USER EVENTS:
At line-Selection
At PFn.
At User-Command.
PROGRAM EVENTS:
Top-of-Page.
End-of-Page.