ABAP_to_the_Future

7
Exception Classes and Design by Contract
PERFORM unlock_monster USING mo_chosen_monster.
ENDFORM."User Command
Listing 7.2 Using a NO_CHECK Exception During User Command Processing
In the preceding example, ZCX_BC_USER_CANCELLED inherits from CX_NO_CHECK.
Almost every one of the 15 possible user commands shows the user one or more
Are you sure? pop-up boxes, any of which the user can cancel out of, aborting
the whole process, and the code that ca lls the pop-ups can be many levels down
the call stack from the USER_COMMAND routine.
Traditionally, in order to deal with users deciding to cancel processing, you’d
have to declare a global variable called GF_CANCEL or some such, and after coming
back from each of the 15 different po p-up boxes you’d have a check, like CHECK
GF_CANCEL = ABAP_FALSE , and would repeat this statement at every level in the
call stack. Remember, for some user co mmands you might show the user three
different sorts of pop-up boxes, one after the other, any one of which the user can
cancel out of.
However, the code in Listing 7.2 takes a different approach: the instant the user
clicks a Cancel button, you raise the ZCX_BC_USER_CANCELLED exception, and the
program flow jumps right back to the top of the call stack, unlocks the monster,
and you’re done. This prevents the code from being littered with checks for the
same thing.
In conclusion, CX_NO_CHECK seems ideal when your routine has no idea what to do
with the problem encountered and you want to pass the error information
upwards until some part of the program does know how to respond.
7.1.4 Deciding which Type of Exception Class to Use
Many people are trying to work out which of the three types of exception class to
use in what situation, and most documentation on the subject is very vague.
There are no hard and fast rules, but Figure 7.1 is a little decision tree to help you
decide what type of exception class to use.
For example, imagine you have created a pu blic method in a global class that is
going to be reused all over the place. If this method gets into trouble when an
exceptional situation comes along and ca nnot handle the problem locally, then
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