delphi - 适当的对象创建 - 寻找通用解决方案

Question

有 3 个类(可能更多)，它们具有相同的过程(过程填充)。它们几乎相同，仅在对象创建方面有所不同。我想要的只是在基类中编写一个通用过程，它将永远取代这种臭名昭著的重复代码。我不太确定，如果我能准确地表达我在做什么，但看看下面的代码看看。

  TGrandFather = class(TObject)

  end;

  TFather = class(TGrandFather)

  end;

  TSon = class(TFather)

  end;

  TGrandson.... and so on... 



  TGrandFathers = class (TList)
  public
    procedure Populate(Amount:Integer);
  end;

  TFathers = class (TGrandFathers)
  public
    procedure Populate(Amount:Integer);
  end;

  TSons = class (TFathers)
  public
    procedure Populate(Amount:Integer);
  end;

  TGrandsons.... 
...

procedure TGrandFathers.Populate(Amount:Integer);
var i:integer;
    xGrandFather:TGrandFather;
begin
   for i := 0 to Amount do
   begin
   xGrandFather:=TGrandFather.Create;
   Add(xGrandFather);
   end;
end;

procedure TFathers.Populate(Amount:Integer);
var i:integer;
    xFather:TFather;
begin
   for i := 0 to Amount do
   begin
   xFather:=TFather.Create;    //this is the point, which makes trouble
   Add(xFather);
   end;
end;

procedure TSons.Populate(Amount:Integer);
var i:integer;
    xSon:TSon;
begin
   for i := 0 to Amount do
   begin
   xSon:=TSon.Create;          //this is the point, which makes trouble
   Add(xSon);
   end;
end;

procedure Grandsons... 

谢谢...

Answer 1

要回答您的问题，如果您想走您要去的路线，您可以通过“class of”使用元类。这段代码演示了您将如何实现这一点。层次结构需要清理，但您应该通过此代码了解正在发生的事情的要点。

元类是一个类，其实例是类。这允许您构建一个更通用的框架，因为您可以使用您的元类来创建您需要的类。

type
  TGrandFather = class(TObject)

  end;

  TStrangeHeirarchyClass = class of TGrandFather;

  TFather = class(TGrandFather)

  end;

  TSon = class(TFather)

  end;

  TGrandFathers = class(TList)
  protected
    procedure PopulateInternal(aAmount:Integer; aContainedClass:
        TStrangeHeirarchyClass);
  public
    procedure Populate(Amount:Integer);
  end;

  TFathers = class (TGrandFathers)
  public
    procedure Populate(Amount:Integer);
  end;

  TSons = class (TFathers)
  public
    procedure Populate(Amount:Integer);
  end;

implementation

procedure TGrandFathers.Populate(Amount:Integer);
begin
  PopulateInternal(Amount, TGrandFather);
end;

procedure TGrandFathers.PopulateInternal(aAmount:Integer; aContainedClass:
    TStrangeHeirarchyClass);
var
  i:integer;
  xFamilyMember:TGrandFather;
begin
  for i := 0 to aAmount do
  begin
    xFamilyMember := aContainedClass.Create;
    Add(xFamilyMember);
  end;
end;

procedure TFathers.Populate(Amount:Integer);
begin
  PopulateInternal(Amount, TFather);
end;

procedure TSons.Populate(Amount:Integer);
begin
  PopulateInternal(Amount, TSon);
end;

它的工作方式是元类 TStrangeHeirarchyClass ，您可以像使用常规数据类型一样使用它，它存储您想要使用的基础类。您可以将类型作为参数传递(就像我在上面的代码示例中所做的那样)或将其作为属性或字段存储在类中，如下所示:

  TGrandFathers = class(TList)
  private
    FContainedClass: TStrangeHeirarchyClass;
  public
    procedure Populate(Amount:Integer);
    property ContainedClass: TStrangeHeirarchyClass read 
        FContainedClass write FContainedClass;
  end;

一旦你设置了这个属性，你就可以使用它来创建它被设置为的类类型的实例。因此，设置 ContainedClass作为 TFather将导致调用 ContainedClass.Create创建 TFather 的实例.

正如大卫在评论中指出的那样，如果您使用元类并覆盖默认构造函数，您将遇到问题。您在构造函数中的代码将永远不会运行。您要么需要使用虚拟构造函数，要么重写现有的 AfterConstruction方法是由构造函数调用的虚拟方法。如果您使用 AfterConstruction，这样的事情就是一个例子。 :

  TGrandFathers = class(TList)
  protected
    FContainedClass: TStrangeHeirarchyClass;
  public
    procedure AfterConstruction; override;
    procedure Populate(Amount:Integer);
  end;

  TFathers = class (TGrandFathers)
  public
    procedure AfterConstruction; override;
  end;

  TSons = class (TFathers)
  public
    procedure AfterConstruction; override;
  end;

implementation

procedure TGrandFathers.AfterConstruction;
begin
  inherited;
  FContainedClass := TGrandFather;
  // Other construction code
end;

procedure TGrandFathers.Populate(aAmount:Integer);
var
  i:integer;
  xFamilyMember:TGrandFather;
begin
  for i := 0 to aAmount do
  begin
    xFamilyMember := FContainedClass.Create;
    Add(xFamilyMember);
  end;
end;

procedure TFathers.AfterConstruction;
begin
  inherited;
  FContainedClass := TFather;
  // Other construction code
end;

procedure TSons.AfterConstruction;
begin
  inherited;
  FContainedClass := TSon;
  // Other construction code
end;

不过，您的层次结构看起来很奇怪。我认为这样的事情会更合适:

type
  TRelationType = (ptSon, ptFather, ptGrandfather);

  TPerson = class;

  TRelation = class(TObject)
  strict private
    FRelationship: TRelationType;
    FRelation: TPerson;
  public
    property Relation: TPerson read FRelation write FRelation;
    property Relationship: TRelationType read FRelationship write FRelationship;
  end;

  TRelationList = class(TList)
    //...
  end;

  TPerson = class(TObject)
  strict private
    FPersonName: string;
    FRelations: TRelationList;
  public
    procedure AfterConstruction; override;
    procedure BeforeDestruction; override;
    property PersonName: string read FPersonName write FPersonName;
    property Relations: TRelationList read FRelations;
  end;

implementation

procedure TPerson.AfterConstruction;
begin
  inherited;
  FRelations := TRelationList.Create;
end;

procedure TPerson.BeforeDestruction;
begin
  FRelations.Free;
  inherited;
end;