Implementing List Enumerator OfType in Delphi

Question

I am using Delphi XE to implement an enumerator that allows filtering the elements of the list by type. I have quickly assembled a test unit as follows:

uni         


        

Answer 1

A worked version using system.IEnumerable and system.IEnumerator

unit uTestList;

interface

uses Generics.Collections;

type
  TListItemBase = class(TObject)
  end; { TListItemBase }

  TListItemChild1 = class(TListItemBase)
  end;

  TListItemChild2 = class(TListItemBase)
  end;

  TTestList = class;

  TOfTypeEnumerator = class(TInterfacedObject, IEnumerator, IEnumerator)
  private
    FTestList : TList;
    FIndex : Integer;
  protected
    constructor Create(Owner : TList); overload;

    function GetCurrent: TObject;
    function GenericGetCurrent : TFilter;
    function MoveNext : Boolean;
    procedure Reset;

    function IEnumerator.GetCurrent = GenericGetCurrent;
  end;

  TOfTypeEnumeratorFactory = class(TInterfacedObject, IEnumerable, IEnumerable)
  private
    FTestList : TList;
  public
    constructor Create(Owner : TList); overload;
    function GetEnumerator : IEnumerator;
    function GenericGetEnumerator : IEnumerator;
    function IEnumerable.GetEnumerator = GenericGetEnumerator;
  end;

  TTestList = class(TList)
  public
    function OfType() : IEnumerable;
  end; { TTestList }


implementation

{ TOfTypeEnumerator }

constructor TOfTypeEnumerator.Create(Owner: TList);
begin
  inherited Create;
  FTestList := Owner;
  FIndex := -1;
end;

function TOfTypeEnumerator.GenericGetCurrent: TFilter;
begin
  Result := TFilter(TObject(FTestList[FIndex]));
end;

function TOfTypeEnumerator.GetCurrent: TObject;
begin
  Result := TObject( FTestList[FIndex] );
end;

function TOfTypeEnumerator.MoveNext: Boolean;
begin
  repeat
    Inc(FIndex);
  until (FIndex >= FTestList.Count) or FTestList[FIndex].InheritsFrom(TFilter);
  Result := FIndex < FTestList.Count;
end;

procedure TOfTypeEnumerator.Reset;
begin
  FIndex := -1;
end;

{ TOfTypeEnumeratorFactory }

constructor TOfTypeEnumeratorFactory.Create(Owner: TList);
begin
  inherited Create;
  FTestList := Owner;
end;

function TOfTypeEnumeratorFactory.GetEnumerator: IEnumerator;
begin
  Result := GenericGetEnumerator;
end;

function TOfTypeEnumeratorFactory.GenericGetEnumerator: IEnumerator;
begin
  Result := TOfTypeEnumerator.Create(FTestList);
end;

{ TTestList }

function TTestList.OfType: IEnumerable;
begin
  Result := TOfTypeEnumeratorFactory.Create(self);
end;

end.

A test procedure:

var
  MyElem: TListItemBase;
  MyElem1: TListItemChild1;
  MyElem2: TListItemChild2;
begin
  Memo1.Clear;
  for MyElem in FTestList.OfType() do
  begin
    Memo1.Lines.Add('----------');
  end;
  for MyElem1 in FTestList.OfType() do
  begin
    Memo1.Lines.Add('==========');
  end;
  for MyElem2 in FTestList.OfType() do
  begin
    Memo1.Lines.Add('++++++++++');
  end;