接口委托+覆盖

Question

由于 Delphi 中缺少多重继承，我需要使用接口委托。这对我来说是一个非常新的话题，我在将覆盖与接口委托结合使用时遇到了问题。

TMyNode 类必须继承自TBaseClass，并且需要实现IAddedStuff。我想在TAddedStuffDefaultImplementation 中拥有IAddedStuff 的所有功能的默认实现，所以我不需要到处都有getter/setter 的重复代码。所以，我已经使用DefaultBehavior 委派了这些事情。

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

但是现在，由于某些原因TAddedStuffDefaultImplementation 会增加x: TBaseClass; 的 Ref-Counter，所以它不能被释放。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

如果您委托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，如果您正在混合对象和接口引用，请确保引用计数不冲突。使用接口委托时，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.

编辑：

在 cmets 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.