Delphi event handling, how to create own event

Here’s a short-but-complete console application that shows how to create your own event in Delphi. Includes everything from type declaration to calling the event. Read the comments in the code to understand what’s going on.

program Project23;

{$APPTYPE CONSOLE}

uses
  SysUtils;

type
  // Declare an event type. It looks allot like a normal method declaration except
  // it suffixed by "of object". That "of object" tells Delphi the variable of this
  // type needs to be assigned a method of an object, not just any global function
  // with the correct signature.
  TMyEventTakingAStringParameter = procedure(const aStrParam:string) of object;

  // A class that uses the actual event
  TMyDummyLoggingClass = class
  public
    OnLogMsg: TMyEventTakingAStringParameter; // This will hold the "closure", a pointer to
                                              // the method function itself + a pointer to the
                                              // object instance it's supposed to work on.
    procedure LogMsg(const msg:string);
  end;

  // A class that provides the required string method to be used as a parameter
  TMyClassImplementingTheStringMethod = class
  public
    procedure WriteLine(const Something:string); // Intentionally using different names for
                                                 // method and params; Names don't matter, only the
                                                 // signature matters.
  end;

  procedure TMyDummyLoggingClass.LogMsg(const msg: string);
  begin
    if Assigned(OnLogMsg) then // tests if the event is assigned
      OnLogMsg(msg); // calls the event.
  end;

  procedure TMyClassImplementingTheStringMethod.WriteLine(const Something: string);
  begin
    // Simple implementation, writing the string to console
    Writeln(Something);
  end;

var Logging: TMyDummyLoggingClass; // This has the OnLogMsg variable
    LoggingProvider: TMyClassImplementingTheStringMethod; // This provides the method we'll assign to OnLogMsg

begin
  try
    Logging := TMyDummyLoggingClass.Create;
    try

      // This does nothing, because there's no OnLogMsg assigned.
      Logging.LogMsg('Test 1');

      LoggingProvider := TMyClassImplementingTheStringMethod.Create;
      try
        Logging.OnLogMsg := LoggingProvider.WriteLine; // Assign the event
        try

          // This will indirectly call LoggingProvider.WriteLine, because that's what's
          // assigned to Logging.OnLogMsg
          Logging.LogMsg('Test 2');

        finally Logging.OnLogMsg := nil; // Since the assigned event includes a pointer to both
                                         // the method itself and to the instance of LoggingProvider,
                                         // need to make sure the event doesn't out-live the LoggingProvider                                             
        end;
      finally LoggingProvider.Free;
      end;
    finally Logging.Free;
    end;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.