Screen flow logic contains the procedural part of a screen. You create it in the flow logic editor, which is very similar to the ABAP Editor. The language used to program screen flow logic has a similar syntax to ABAP, but is not part of ABAP itself. It is sometimes referred to as screen language.

Unlike ABAP programs, screen flow logic contains no explicit data declarations. You define screen fields by placing elements on the screen mask.

The screen flow logic is like an ABAP program in that it serves as a container for processing blocks. There are four event blocks, each of which is introduced with the screen keyword PROCESS:

PROCESS BEFORE OUTPUT.
...

PROCESS AFTER INPUT.
...

PROCESS ON HELP-REQUEST.
...

PROCESS ON VALUE-REQUEST.
...

As in ABAP, the event block is introduced by the corresponding keyword statement, and it concludes either when the next block is introduced, or at the end of the program. The first two statements are created automatically by the Screen Painter when you create a new screen. The corresponding events are triggered by the runtime environment:

• PROCESS BEFORE OUTPUT (PBO) is automatically triggered after the PAI processing of the previous screen and before the current screen is displayed. You can program the PBO processing of the screen in this block. At the end of the PBO processing, the screen is displayed.
• PROCESS AFTER INPUT (PAI) is triggered when the user chooses a function on the screen. You can program the PAI processing of the screen in this block. At the end of the PAI processing, the system either calls the next screen or carries on processing at the point from which the screen was called.
• PROCESS ON HELP-REQUEST (POH) and PROCESS ON VALUE-REQUEST (POV) are triggered when the user requests field help (F1) or possible values help (F4) respectively. You can program the appropriate coding in the corresponding event blocks. At the end of processing, the system carries on processing the current screen.