Why and how to use events in abap objects

Question

Dear all,
  Please explain me why and how to use events in abap objects with real time example
regards
pankaj giri

Answer 1

Hi Pankaj,
I will try to explain why to use events... How to use is a different topic.. which others have already answered...
This is same from your prev. post...
Events :
Technically speaking :
" Events are notifications an object receives from, or transmits to, other objects or applications. Events allow objects to perform actions whenever a specific occurrence takes place. Microsoft Windows is an event-driven operating system, events can come from other objects, applications, or user input such as mouse clicks or key presses. "
Lets say you have an ALV - An editable one ...
Lats say - Once you press some button  you want some kind of validation to be done.
How to do this ?
Raise an Event - Which is handled by a method and write the validation code.
Now you might argue, that I can do it in this way : Capture the function code - and call the validate method.
Yes, in this case it can be done.. But lets say .. you change a field in the ALV and you want the validation to be done as soon as he is done with typing.
Where is the function code here ? No function code... But there is an event here - The data changed event.
So you can raise a data changed event that can be handled and will do the validation.
It is not user friendly that you ask the user to press a button (to get the function code) for validation each time he enters a data.
The events can be raised by a system, or by a program also. So in this case the data changed event is raised by a system that you can handle.
Also, Lets say on a particular action you want some code to trigger. (You can take the same example of validation code). In this case the code to trigger is in a separate class. The object of which is not available here at this moment. (This case happens very frequently).
Advantage with events : Event handlers can be in a separate class also.
e.g : In the middle of some business logic .. you encounter a error. You want to send this information to the UI (to user - in form of a pop up) and then continue with some processing.
In many cases - A direct method call to trigger the pop up is not done. Because (in ideal cases) the engine must not interact with UI directly - Because the UI could be some other application - like a windows UI but the error comes from some SAP program.
So - A event is raised from the engine that is handled in the UI and a pop up is triggered.
Here -- I would have different classes (lets say for different Operating Systems). And all these classes must register to the event ERROR raised in application.
And these different classes for different Operation systems will have different code to raise a pop-up.
Now you can imagine : If you coded a pop-up for Windows (in your application logic) .. it will not work for Mac or Linux. But of you raise a event.. that is handled separately by a different UI classes for Win, Linux or Mac  they will catch this event and process accordingly.
May be I complicated this explanation .... but I couldn't think of a simpler and concrete example.
Cheers.
Varun.