Event in descendant of TCollectionItem

Question

I wrote simple code (see below): a descendant of TCollectionItem with an event. But when I click OnDone event in object inspector I get the message:

"Cannot create a method for an unnamed component".

What is wrong with this code?

unit MainComponent2;

interface

uses Windows, SysUtils, Classes;

type
  TMyField = class(TCollectionItem)
  private
    FName: string;
    FOnDone: TNotifyEvent;
    FText: string;
  protected
    function GetDisplayName : String; override;
  public
    constructor Create(ACollection: TCollection);override;
    function GetNamePath: string;override;
  published
    property Name: string read FName write FName;
    property Text: string read FText write FText;
    property OnDone: TNotifyEvent read FOnDone write FOnDone;
  end;

  TMyFields = class(TOwnedCollection)
  private
    function GetItem(Index: Integer): TMyField;
    procedure SetItem(Index: Integer; const Value: TMyField);
  protected
    procedure Update(Item: TmyField);reintroduce;
  public
    constructor Create(AOwner: TComponent);
    function Add: TMyField;
    function Insert(Index: Integer): TMyField;
    property Items[Index: Integer]: TMyField read GetItem write SetItem; default;
  end;


  TMainComponent2 = class(TComponent)
  private
    FMyFields: TMyFields;
    function GetItem(index: integer): TMyField;
    procedure SetItem(index: integer; const Value: TMyField);
    procedure SetMyFields(const Value: TMyFields);
  public
    constructor Create(AOwner: TComponent);override;
    destructor Destroy;override;

    property Items[index: integer]: TMyField read GetItem write SetItem;
  published
    property MyFields: TMyFields read FMyFields write SetMyFields;
  end;

procedure Register;

implementation

{ TMainComponent2 }

constructor TMainComponent2.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FMyFields := TMyFields.Create(AOwner);
end;

destructor TMainComponent2.Destroy;
begin
  FMyFields.Free;
  inherited;
end;

function TMainComponent2.GetItem(index: integer): TMyField;
begin
  result := FMyFields.Items[index] as TMyField;
end;

procedure TMainComponent2.SetItem(index: integer; const Value: TMyField);
begin
  FMyFields.Items[index] := Value;
end;

procedure TMainComponent2.SetMyFields(const Value: TMyFields);
begin
  FMyFields.Assign(Value);
end;

{ TMyField }

constructor TMyField.Create(ACollection: TCollection);
begin
  inherited Create(ACollection);
  FName := ClassName + IntToStr(ID);
end;

function TMyField.GetDisplayName: String;
begin
  result := FName;
end;

function TMyField.GetNamePath: string;
begin
  Result := Format('%s%d',['MyField', Index])
end;

{ TMyFields }

function TMyFields.Add: TMyField;
begin
  result := (inherited Add) as TMyField;
end;

constructor TMyFields.Create(AOwner: TComponent);
begin
  inherited Create(AOwner, TMyField);
end;

function TMyFields.GetItem(Index: Integer): TMyField;
begin
  result := (inherited GetItem(Index)) as TMyField;
end;

function TMyFields.Insert(Index: Integer): TMyField;
begin
  result := (inherited Insert(Index)) as TMyField;
end;

procedure TMyFields.SetItem(Index: Integer; const Value: TMyField);
begin
  inherited SetItem(Index, Value);
end;

procedure TMyFields.Update(Item: TmyField);
begin
  inherited Update(Item);
end;

procedure Register;
begin
  RegisterComponents('MyComponents', [TMainComponent2]);
end;

end.

Answer 1

You forgot to include the inherited name path of TMyField:

function TMyField.GetNamePath: string;
begin
  Result := inherited GetNamePath + Format('MyField%d',[Index]);
end;

Also, you have given the wrong owner for the MyFields property. Use Self instead of AOwner. AOwner is the form on which you drop the component.

constructor TMainComponent2.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FMyFields := TMyFields.Create(Self);
end;

B.t.w. Why did you reintroduce TMyFields.Update? This breaks the inheritance chain. Use override when possible, or add another method.