ABAP设计模式之---“简单工厂模式(Simple Factory Pattern)”

1. 定义 

用一个单独的工厂类(factory class)，来控制实例化过程。

2. 解读

类比：

• 简单工厂模式，类似于现实世界的工厂，也即，工厂根据客户需要，生成相应的产品。
• 在程序中，简单工厂模式，根据client的需求，返回相应的实例

解析：

• 类的功能与类的实例化的过程分离
• 隐藏生产类（production class）的功能信息
• 工厂类提升实例化过程的灵活性（不同的场景，返回不同的实例；动态返回实例）
• 工厂类的引入，隔离了调用方与服务提供方，当服务调整时，调用方无需改动
• 工厂类是测试隔离（Test Isolation， Test Double）的实现途径

结合应用：

• 简单工厂模式经常与策略模式结合使用

3. 举例

3.1 Factory根据类型返回实例

对于已知的不同场景，可以利用工厂类，在不同场景下返回对应的实例。

REPORT zsimple_factory_pattern.
**********************************************************************
* A generic interface
**********************************************************************
INTERFACE lif_generic_interface.
  METHODS:
    method_1,
    method_2.
ENDINTERFACE.

CLASS lcl_factory DEFINITION DEFERRED.
**********************************************************************
* Specific class A to implement this interface
* 1) Production class shall be CREATE PRIVATE
* 2) Factory is the friend of production class
* 3) Factory is the only place to get the instance of production class
**********************************************************************
CLASS lcl_class_a DEFINITION FINAL CREATE PRIVATE FRIENDS lcl_factory.
  PUBLIC SECTION.
    INTERFACES: lif_generic_interface.
ENDCLASS.

CLASS lcl_class_a IMPLEMENTATION.
  METHOD lif_generic_interface~method_1.

  ENDMETHOD.

  METHOD lif_generic_interface~method_2.

  ENDMETHOD.
ENDCLASS.
**********************************************************************
* Specific class B to implement this interface
**********************************************************************
CLASS lcl_class_b DEFINITION FINAL CREATE PRIVATE FRIENDS lcl_factory.
  PUBLIC SECTION.
    INTERFACES: lif_generic_interface.
ENDCLASS.

CLASS lcl_class_b IMPLEMENTATION.
  METHOD lif_generic_interface~method_1.

  ENDMETHOD.

  METHOD lif_generic_interface~method_2.

  ENDMETHOD.
ENDCLASS.
**********************************************************************
* Factory class to control the instantiation
**********************************************************************
CLASS lcl_factory DEFINITION FINAL ABSTRACT.
  PUBLIC SECTION.
    " create per existing type
    CLASS-METHODS create
      IMPORTING
        iv_type            TYPE c
      RETURNING
        VALUE(ro_instance) TYPE REF TO lif_generic_interface.
ENDCLASS.

CLASS lcl_factory IMPLEMENTATION.
  METHOD create.
    CASE iv_type.
      WHEN 'A'.
        ro_instance = NEW lcl_class_a(  ).
      WHEN 'B'.
        ro_instance = NEW lcl_class_b(  ).
      WHEN OTHERS.
        ASSERT 1 = 2.
    ENDCASE.
  ENDMETHOD.
ENDCLASS.

START-OF-SELECTION.

3.2 Factory 动态返回实例

此种模式，更适用于架构性的设计。在架构中，允许不断增加实现相同接口的类，以及在不同场景下实例化它们。

**********************************************************************
* Factory class to control the instantiation
**********************************************************************
CLASS lcl_factory DEFINITION FINAL ABSTRACT.
  PUBLIC SECTION.
    " if there are multiple class to be implemented by scenario
    " a dedicated customizing table could be defined
    " the type of instantiation can be determined dynamically
    CLASS-METHODS create_from_db
      IMPORTING
        iv_type            TYPE c
      RETURNING
        VALUE(ro_instance) TYPE REF TO lif_generic_interface.
ENDCLASS.

CLASS lcl_factory IMPLEMENTATION.
  METHOD create_from_db.
    DATA:
      lv_classname         TYPE seoclname.

    " layout of table ZFACTORYSIMPLE (client independent)
    "    Field   Key   Data Element
    "    TYPE    X     CHAR01
    "    CLASS   _     SEOCLNAME
*    SELECT SINGLE class FROM zfactorysimple
*                        INTO lv_classname
*                       WHERE type = iv_type.
    IF sy-subrc = 0.
      CREATE OBJECT ro_instance TYPE (lv_classname).
    ELSE.
      " in case getting no concrete class is not a valid situation, make sure
      " implementation is complete (table entry is existing)
      ASSERT 1 = 2.
    ENDIF.
  ENDMETHOD.
ENDCLASS.

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