Module pool program in SAP is also called dialog program, which is an ABAP program of Type “M”, in this module pool program we can create a form like program using an environment called the layout editor which is similar with SDK (software development kit) you found in any other software such as visual studio .net.
The tcode we’re using to create a module pool program is SE51, you can also use the Object Navigator in SE80.
What we create in SE51 is actually the user interface or screen, so where’s the source code? The source code will still be created in ABAP editor SE38, but we can directly interact with the ABAP editor from the flow logic section in the Module pool program.
As you can see in below picture, you need to have a program first in Se38 because this will become the flow logic (source code) of your program. After that you can put a screen number to create the module program.

Even though the objects (label, textbox, pushbutton) are not as rich as the toolbox you find in Visual studio .net, the tools there are quite enough to create a working form to do the inserting, updating and deleting data in the database, but I usually create my own transparent tables to interact with a module pool program.
Just like other Object oriented programming languages, In Module pool program you also will find EVENTS. There are 2 different events, the first one is Process before output (PBO) which is a part of the Dynpro code, this event is triggered when the form is very initialize or before the user can do anything with the any object or data in the screen.
The second event is process after input (PAI), this event will be triggered after user interacted with the data, e.g input some data or any screen object.
To call a module program you also using TCODE just like report program you create in SE38, the difference is that when you create the TCODE you must choose “Program And Screen (Dialog Transaction)” option.

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