Wednesday, October 2, 2013

the Class constructor and singleton in abap



Hi Folks,
i am hacking on an ALV oo program and needed or wanted to implement the singleton pattern.

A singleton is a class of which only one instance exists and some means is create to access the only instance without making it global of course...
Think of it as a global variable hidden behind a puzzling coding interface. At the very end it is just a single global instance of a class, but far more cool and  good looking :-)

Apart from jokes it makes sense. Obviously not in 10 lines programs. But in complex applications having a global in the scope of a whole project can be problematic... the singleton can be anice way to control this "global" and access it in a discplined and controlled manner

In my case i have program that runs on one person. So my "app" object is bound to the person and there is only one throuout the processing.

So i need a means to have a central object instance which can be accessed by all classes that need it.
In abap it can be done like in the following coding.

The class-method is a method that is run per "class" and not per "object" ( the instance of the class)
The class constructor is run only once per program before the class is called for the first time.

So the class construcor instantiates the object ( the only existing instance )
The class method gives access just to this one and only existing oblect of the given singleton class.

So in my program i can call it like this
...
DATA lr_my_app TYPE REF TO zcl_my_app.
  lr_my_app =  zcl_my_app=>get_singleton( ).
  CALL METHOD gr_my_app->init
....

Here the class example.


CLASS zcl_my_app DEFINITION.
  PUBLIC SECTION.
    class-METHODS: class_constructor.

    METHODS: init .

    CLASS-METHODS: get_ref RETURNING value(r_ref) TYPE REF TO zcl_my_app.

  PRIVATE SECTION.
    CLASS-DATA ar_singleton TYPE REF TO zcl_my_app.


CLASS zcl_my_app IMPLEMENTATION.

  METHOD: class_constructor.
    CREATE OBJECT ar_singleton.
  ENDMETHOD.                    "constructor

  METHOD: init.
*Do whatever
  ENDMETHOD.                    "init

  METHOD: get_ref.
    r_ref = ar_singleton.
  ENDMETHOD.                    "get_ref


One last word: i have seen implementations in the internet like this:

method GET_OBJECT.
" returning  value(R_OBJECT) type ref to ZCL_blabla .
*
  if zcl_blabla=>r_singleton is not bound.
    create object zcl_blabla=>r_singleton.
  endif.
*
  r_object = zcl_sblabla=>r_singleton .
endmethod.
 

Can i say that it is the same but at the same time it really sucks :-) LOL ?


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