Question

由于Delphi中缺少多重继承，我需要使用接口委派。这对我来说是一个非常新的主题，我在将重写与接口委派相结合时遇到了问题。

班级TMyNode必须从TBaseClass继承，并且需要实施IAddedStuff。我想在IAddedStuff中使用TAddedStuffDefaultImplementation的所有函数的默认实现，因此我不需要在任何地方都有重复的getter / setter代码。所以，我已经使用DefaultBehavior委派了这些内容。

问题是，TAddedStuffDefaultImplementation意味着拥有虚拟方法，所以我想直接在TMyNode中覆盖它们。如果我写FDefaultImplementation: TAddedStuffDefaultImplementation;而不是FDefaultImplementation: IAddedStuff;，这确实有用。

但是现在，由于某些原因，TAddedStuffDefaultImplementation会增加x: TBaseClass;的参考计数器，因此无法释放它。我该怎么办？

我的简化复制代码如下：

program Project2;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.SysUtils;

type
  IAddedStuff = interface(IInterface)
  ['{9D5B00D0-E317-41A7-8CC7-3934DF785A39}']
    function GetCaption: string; {virtual;}
  end;

  TAddedStuffDefaultImplementation = class(TInterfacedObject, IAddedStuff)
    function GetCaption: string; virtual;
  end;

  TBaseClass = class(TInterfacedObject);

  TMyNode = class(TBaseClass, IAddedStuff)
  private
    FDefaultImplementation: TAddedStuffDefaultImplementation;
  public
    property DefaultBehavior: TAddedStuffDefaultImplementation read FDefaultImplementation
      write FDefaultImplementation implements IAddedStuff;
    destructor Destroy; override;

    // -- IAddedStuff
    // Here are some functions which I want to "override" in TMyNode.
    // All functions not declared here, should be taken from FDefaultImplementation .
    function GetCaption: string; {override;}
  end;

{ TAddedStuffDefaultImplementation }

function TAddedStuffDefaultImplementation.GetCaption: string;
begin
  result := 'PROBLEM: CAPTION NOT OVERRIDDEN';
end;

{ TMyNode }

destructor TMyNode.Destroy;
begin
  if Assigned(FDefaultImplementation) then
  begin
    FDefaultImplementation.Free;
    FDefaultImplementation := nil;
  end;

  inherited;
end;

function TMyNode.GetCaption: string;
begin
  Result := 'OK: Caption overridden';
end;

var
  x: TBaseClass;
  gn: IAddedStuff;
  s: string;
begin
  x := TMyNode.Create;
  try
    TMyNode(x).DefaultBehavior := TAddedStuffDefaultImplementation.Create;
    Assert(Supports(x, IAddedStuff, gn));
    WriteLn(gn.GetCaption);
  finally
    WriteLn('RefCount = ', x.RefCount);
    // x.Free; // <-- FREE fails since FRefCount is 1
  end;
  ReadLn(s);
end.

Answer 1

如果您委派IAddedStuff，那么您还应该在另一个类上实现非默认行为，并通过构造函数注入传递它。

此外，如果要混合对象和接口引用，请确保引用计数不会发生冲突。使用接口委派时，容器对象的引用会发生变化。

program Project1;

{$APPTYPE CONSOLE}

uses
  Classes,
  SysUtils;

type
  IAddedStuff = interface(IInterface)
  ['{9D5B00D0-E317-41A7-8CC7-3934DF785A39}']
    function GetCaption: string; {virtual;}
  end;

  TAddedStuffDefaultImplementation = class(TInterfacedObject, IAddedStuff)
    function GetCaption: string; virtual;
  end;

  TAddedStuffOverriddenImplementation = class(TAddedStuffDefaultImplementation)
    function GetCaption: string; override;
  end;

  TBaseClass = class(TInterfacedPersistent);

  TMyNode = class(TBaseClass, IAddedStuff)
  private
    FAddedStuff: IAddedStuff;
    property AddedStuff: IAddedStuff read FAddedStuff implements IAddedStuff;
  public
    constructor Create(const addedStuff: IAddedStuff);
  end;

{ TAddedStuffDefaultImplementation }

function TAddedStuffDefaultImplementation.GetCaption: string;
begin
  result := 'PROBLEM: CAPTION NOT OVERRIDDEN';
end;

{ TAddedStuffOverriddenImplementation }

function TAddedStuffOverriddenImplementation.GetCaption: string;
begin
  Result := 'OK: Caption overridden';
end;

{ TMyNode }

constructor TMyNode.Create;
begin
  FAddedStuff := addedStuff;
end;

var
  x: TBaseClass;
  gn: IAddedStuff;
begin
  x := TMyNode.Create(TAddedStuffOverriddenImplementation.Create);
  try
    Assert(Supports(x, IAddedStuff, gn));
    WriteLn(gn.GetCaption);
  finally
    x.Free;
  end;
  Readln;
  ReportMemoryLeaksOnShutdown := True;
end.

修改

在评论中讨论后，我建议如下：

program Project1; {$APPTYPE CONSOLE} uses Classes, SysUtils; type IAddedStuff = interface(IInterface) ['{9D5B00D0-E317-41A7-8CC7-3934DF785A39}'] function GetCaption: string; end; TAddedStuffDefaultImplementation = class(TInterfacedObject, IAddedStuff) function GetCaption: string; virtual; end; TBaseClass = class(TInterfacedPersistent); TMyNode = class(TBaseClass, IAddedStuff) private FAddedStuff: IAddedStuff; property AddedStuff: IAddedStuff read FAddedStuff implements IAddedStuff; public constructor Create; end; TAddedStuffOverriddenImplementation = class(TAddedStuffDefaultImplementation) private FMyNode: TMyNode; public constructor Create(AMyNode: TMyNode); function GetCaption: string; override; end; { TAddedStuffDefaultImplementation } function TAddedStuffDefaultImplementation.GetCaption: string; begin result := 'PROBLEM: CAPTION NOT OVERRIDDEN'; end; { TMyNode } constructor TMyNode.Create; begin FAddedStuff := TAddedStuffOverriddenImplementation.Create(Self); end; { TAddedStuffOverriddenImplementation } constructor TAddedStuffOverriddenImplementation.Create(AMyNode: TMyNode); begin FMyNode := AMyNode; end; function TAddedStuffOverriddenImplementation.GetCaption: string; begin Result := 'OK: Caption overridden'; end; var x: TBaseClass; gn: IAddedStuff; begin x := TMyNode.Create; try Assert(Supports(x, IAddedStuff, gn)); WriteLn(gn.GetCaption); finally x.Free; end; ReadLn; ReportMemoryLeaksOnShutdown := True; end.

接口委派+覆盖

1 个答案: