Delphi Create Class with Event

Question

I'm trying to create a class in a Delphi 12 application to manage a certain type of data in my application. The app has two sets of this data, so that's why I'm making a class and abstracting the properties and methods of the data.

I followed the code in Delphi event handling, how to create own event to create the class shown below:

unit PathList;

//https://stackoverflow.com/questions/5786595/delphi-event-handling-how-to-create-own-event?rq=3
interface

Type
  TListChangedEvent = procedure(Sender: TObject) of object;

  TPathList = Class
  private
    FListChangedEvent: TListChangedEvent;
    FPathType: String;
    FSize: Integer;
  public
    property ListChangedEvent: TListChangedEvent read FListChangedEvent
      write FListChangedEvent;
    constructor Create(pathType: String);
    property Size: Integer read FSize;
    property pathType: String read FPathType;
  protected
    procedure DoListChangedEvent(Sender: TObject);
  private
  end;

implementation

{ TPathList }

procedure TPathList.DoListChangedEvent(Sender: TObject);
begin
  if Assigned(FListChangedEvent) then
    FListChangedEvent(self);
end;

constructor TPathList.Create(pathType: String);
// must call the constructor to set the path type
begin
  FPathType := pathType;
end;

end.

What I can't figure out, and what I can't find an example of, is how do I create the event listener in another unit in my app? Can someone please show me how?

Answer 1

Simply declare a class procedure of matching signature, and assign it to the event like any other variable, ie:

uses
  PathList;

type
  SomeOtherClass = class(...)
  private
    FList: TPathList;
    procedure ListChanged(Sender: TObject);
    ...
  public
    constructor Create(...);
    destructor Destroy; override;
  end;

constructor SomeOtherClass.Create(...);
begin
  ...
  FList := TPathList.Create;
  FList.ListChangedEvent := ListChanged;
  ...
end;

destructor SomeOtherClass.Destroy;
begin
  ...
  FList.Free;
  inherited;
end;

procedure SomeOtherClass.ListChanged(Sender: TObject);
begin
  // do something...
end;