TObjectDispatch (Delphi)

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Description

This example demonstrates the use of message dispatching on TObject and any descendants. Using the same techniques, a form of message-based OOP can be acheived where an object implements a certain interface based on messages.

Code

{ Define the object interface messages. }
const
  INTF_MESSAGE_LOWER_STR = 1;
  INTF_MESSAGE_UPPER_STR = 2;

type
  { String container message that will be passed. }
  TStrCntrMessage = packed record
    MessageId : Word;
    MsgText   : String;
  end;

  { String handler object that will serve requests. }
  TStringHandler = class
  private
    procedure StrUpCase(var AMessage : TStrCntrMessage);
      message INTF_MESSAGE_UPPER_STR;

    procedure StrLowCase(var AMessage : TStrCntrMessage);
      message INTF_MESSAGE_LOWER_STR;

  public
    procedure DefaultHandler(var Message); override;

  end;

{ TStringHandler }

procedure TStringHandler.DefaultHandler(var Message);
var
  MessageId : Word absolute Message;
begin
  { Display the message, then pass the control to the default handler. }
  MessageDlg('Unhandled message ' + IntToStr(MessageId) +
    ' received!', mtInformation, [mbOK], 0);

  inherited;
end;

procedure TStringHandler.StrLowCase(
  var AMessage: TStrCntrMessage);
begin
  AMessage.MsgText := LowerCase(AMessage.MsgText);
end;

procedure TStringHandler.StrUpCase(
  var AMessage: TStrCntrMessage);
begin
  AMessage.MsgText := UpperCase(AMessage.MsgText);
end;

{ TForm2 }

procedure TForm2.FormCreate(Sender: TObject);
var
  Hndlr: TStringHandler;
  Msg: TStrCntrMessage;
begin
  { Create the object instance, dispatch a message, and free it. }
  Hndlr := TStringHandler.Create();

  { Make a string lowercase, using messaging. }
  Msg.MessageId := INTF_MESSAGE_LOWER_STR;
  Msg.MsgText := 'Hello World';

  Hndlr.Dispatch(Msg);

  MessageDlg('Lower-cased string: ' + Msg.MsgText,
    mtInformation, [mbOK], 0);

  { Make a string uppercase, using messaging. }
  Msg.MessageId := INTF_MESSAGE_UPPER_STR;
  Msg.MsgText := 'Hello World';

  Hndlr.Dispatch(Msg);

  MessageDlg('Upper-cased string: ' + Msg.MsgText,
    mtInformation, [mbOK], 0);

  Hndlr.Free;
end;

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