多接口继承Delphi答案

【问题标题】：Multiple Interface inherited Delphi多接口继承Delphi
【发布时间】：2020-12-27 21:23:46
【问题描述】：

我有下面的代码。 IAnimal 是我的应用程序中所有动物的基本接口。

为什么我不能用我想要的类型声明 var，得到一个实现基接口 IAnimal 的对象并调用方法？

type
  IAnimal = interface
  end;

  ICat = interface(IAnimal)
    procedure Hunt;
  end;

  IBird = interface(IAnimal)
    procedure Fly;
  end;

  TCat = class(TInterfacedObject, ICat)
    procedure Hunt;
  end;

  TBird = class(TInterfacedObject, IBird)
    procedure Fly;
  end;

  TAnimalType = (atCat, atBird);

  TAnimalFactory = class
    class function GetAnimal(aType: TAnimalType): IAnimal;
  end;

procedure TCat.Hunt;
begin
  Writeln('I hunt');
end;

procedure TBird.Fly;
begin
  Writeln('I fly');
end;
        
class function TAnimalFactory.GetAnimal(aType: TAnimalType): IAnimal;
begin
  case aType of
    atCat: Result := TCat.Create;
    atBird: Result := TBird.Create;
  end;
end;

var
  i: ICat;
begin
    i := TAnimalFactory.GetAnimal(atCat);
    // [dcc32 Error] Project1.dpr(63): E2010 Incompatible types: 'ICat' and 'IAnimal'

    i.Hunt;
end.

【问题讨论】：

解决方案是类型转换：i := TAnimalFactory.GetAnimal(atCat) as ICat;
感谢您的回复，我会尝试使用类型转换。结果的错误我在 Stackoverflow 中编码:-)
你还应该声明实现对象实现了 IAnimal

标签： delphi inheritance interface

【解决方案1】：

在现代 Delphi 版本中，您可以使用泛型，如下所示：

interface
uses
  System.Classes,
  System.Generics.Collections;

  ICat = interface(IAnimal)
    ['{15E79A9B-CF33-4672-8892-FCBC7A778C57}']  // Ctrl+Shift+G to generate GUID
    procedure Hunt;
  end;

  IBird = interface(IAnimal)
    ['{C9318161-2827-4D8C-AE0F-4D7B9A686F60}']
    procedure Fly;
  end;

  TAnimalFactory = class
    class function GetAnimal<Intf: IAnimal>: Intf;
  end;

implementation
uses
  System.SysUtils,
  System.TypInfo;
  { TAnimalFactory }

class function TAnimalFactory.GetAnimal<Intf>: Intf;
var
  G: TGUID;
  tmp: IInterface;
begin
  G := GetTypeData(TypeInfo(Intf))^.Guid;

  if G = ICat then
    tmp:=TCat.Create;
  if G = IBird then
    tmp:=TBird.Create;

  if not Supports(tmp, G, Result) then
    Result:=nil;
end;

var
  i: ICat;
begin
  i := TAnimalFactory.GetAnimal<ICat>;
  if Assigned(i) then  // if you not sure about interface support
    i.Hunt;
end;

【讨论】：

【解决方案2】：

我创建了我认为您会喜欢的代码。这就是你所做的，在 cmets 中使用了 Zed 和 Bosshoss 建议的修复程序。我做了一个 VCL 应用程序，还在基础接口中添加了一个方法。

unit Unit7;

interface

uses
  System.SysUtils, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

type
  IAnimal = interface
    procedure WhatILike;
  end;

  ICat = interface(IAnimal)
    procedure Hunt;
  end;

  IBird = interface(IAnimal)
    procedure Fly;
  end;

  TCat = class(TInterfacedObject, IAnimal, ICat)
    procedure WhatILike;
    procedure Hunt;
  end;

  TBird = class(TInterfacedObject, IAnimal, IBird)
    procedure WhatILike;
    procedure Fly;
  end;

  TAnimalType = (atCat, atBird);

  TAnimalFactory = class
    class function GetAnimal(aType: TAnimalType): IAnimal;
  end;

  TForm7 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  end;

var
  Form7: TForm7;

implementation

{$R *.dfm}

procedure TCat.Hunt;
begin
  ShowMessage('I hunt');
end;

procedure TBird.Fly;
begin
  ShowMessage('I fly');
end;

class function TAnimalFactory.GetAnimal(aType: TAnimalType): IAnimal;
begin
  case aType of
    atCat  : Result := IAnimal(TCat.Create);
    atBird : Result := IAnimal(TBird.Create);
    else     Result := nil;
  end;
end;

procedure TForm7.Button1Click(Sender: TObject);
var
    ABird : IBird;
    ACat  : ICat;
begin
    ACat  := ICat(TAnimalFactory.GetAnimal(atCat));
    ABird := IBird(TAnimalFactory.GetAnimal(atBird));

    ACat.Hunt;
    ABird.Fly;
    ACat.WhatILike;
    ABird.WhatILike;
end;


procedure TCat.WhatILike;
begin
    ShowMessage('I like to hunt');
end;

procedure TBird.WhatILike;
begin
    ShowMessage('I like to fly');
end;

end.

【讨论】：