Use DefineProperties to replace TPersistent properties e.g. TFont

Question

I'm updating some properties in a component. In order to avoid missing property errors I'm using DefineProperties to read the old properties from the stream. Most properties work fine e.g. Integer, but I can't get properties based on TPersistent to work. The ReadProperty(TPersistent) procedure in TReader is protected, not public and requires a hack to access it. Even then, the ReadFontProperty procedure is never called and the missing property exception occurs.

How do I read the TFont property?

Here's some sample code of how I'm trying to do it.

...

type
  TMyComponent = class(TComponent)
  strict private
    // Removed 
    //FIntegerProperty: Integer;
    //FFontProperty: TFont;

    // New
    FNewIntegerProperty: Integer;
    FNewFontProperty: TFont;

    procedure ReadIntegerProperty(Reader: TReader);
    procedure ReadFontProperty(Reader: TReader);
  protected
    procedure DefineProperties(Filer: TFiler); override;
  published
    // Removed properties
    //property IntegerProperty: Integer read FIntegerProperty write FIntegerProperty;
    //property FontProperty: TFont read FFontProperty write SetFontProperty;

    // New properties
    property NewIntegerProperty: Integer read FNewIntegerProperty write FNewIntegerProperty;
    property NewFontProperty: TFont read FNewFontProperty write SetNewFontProperty;
  end;

implementation

procedure TMyComponent.DefineProperties(Filer: TFiler);
begin
  inherited;

  // This works
  Filer.DefineProperty('IntegerProperty', ReadIntegerProperty, nil, FALSE);

  // This doesn't
  Filer.DefineProperty('FontProperty', ReadFontProperty, nil, FALSE);
end;

procedure TMyComponent.ReadIntegerProperty(Reader: TReader);
begin
  FNewIntegerProperty:= Reader.ReadInteger;
end;

type
  THackReader = class(TReader);

procedure TMyComponent.ReadFontProperty(Reader: TReader);
begin
  { TODO : This doesn't work. How do we read fonts? }
  THackReader(Reader).ReadProperty(FNewFontProperty);
end;

...

Update 1

Tried David's suggestion using the following code:

Filer.DefineProperty('Font.CharSet', ReadFontCharSet, nil, False);

...

procedure TMyComponent.ReadFontCharSet(Reader: TReader);
begin
  Reader.ReadInteger;
end;

I get an Invalid Property Value error. I guess it's something to do with Charset being of type TFontCharset (= System.UITypes.TFontCharset = 0..255). How do I read this type of property?

Answer 1

In order to do this you need to work with each individual published property of TFont and you will need to use fully qualified names.

Filer.DefineProperty('FontProperty.Name', ReadFontName, nil, False);
Filer.DefineProperty('FontProperty.Height', ReadFontHeight, nil, False);
Filer.DefineProperty('FontProperty.Size', ReadFontSize, nil, False);
// and so on for all the other published properties of TFont

ReadFontName, ReadFontHeight etc. should read the old property values into the newly named component.

procedure TMyComponent.ReadFontName(Reader: TReader);
begin
  FNewFontProperty.Name := Reader.ReadString;
end;

// etc. etc.

Update

You ask how to read the Charset property. This is complex because it can be written either as a textual identifier (see the FontCharsets constant in Graphics.pas), or as a plain integer value. Here is some rapidly hacked together code that will read your Charset.

procedure TMyComponent.ReadFontCharset(Reader: TReader);

  function ReadIdent: string;
  var
    L: Byte;
    LResult: AnsiString;
  begin
    Reader.Read(L, SizeOf(Byte));
    SetString(LResult, PAnsiChar(nil), L);
    Reader.Read(LResult[1], L);
    Result := UTF8ToString(LResult);
  end;

  function ReadInt8: Shortint;
  begin
    Reader.Read(Result, SizeOf(Result));
  end;

  function ReadInt16: Smallint;
  begin
    Reader.Read(Result, SizeOf(Result));
  end;

var
  Ident: string;
  CharsetOrdinal: Integer;

begin
  Beep;
  case Reader.ReadValue of
  vaIdent:
    begin
      Ident := ReadIdent;
      if not IdentToCharset(Ident, CharsetOrdinal) then begin
        raise EReadError.Create('Could not read MyFont.Charset');
      end;
      FNewFontProperty.Charset := CharsetOrdinal;
    end;
  vaInt8:
    FNewFontProperty.Charset := ReadInt8;
  vaInt16:
    FNewFontProperty.Charset := ReadInt16;
  else
    raise EReadError.Create('Could not read FontProperty.Charset');
  end;
end;