// Based on: https://sourcemaking.com/design_patterns/observer/delphi
unit ObserverPattern;

interface

uses
  System.Classes;

type
  TSubjectObserver = class
  private
    FEnabled: Boolean;
    FOnChange: TNotifyEvent;
  protected
    procedure Change(Subject: TObject);
  public
    constructor Create();
    property Enabled: Boolean read FEnabled write FEnabled;
    property OnChange: TNotifyEvent read FOnChange write FOnChange;
  end;

  TSubject = class
  private
    FObservers: TList;
  protected
    procedure Change; // Call this method to dispatch change
  public
    constructor Create();
    procedure RegisterObserver(Observer: TSubjectObserver);
    procedure UnregisterObserver(Observer: TSubjectObserver);
  end;

implementation

constructor TSubjectObserver.Create();
begin
  inherited Create();
  self.FEnabled := True;
end;

procedure TSubjectObserver.Change(Subject: TObject);
begin
  if Assigned(FOnChange) then
    FOnChange(Subject);
end;

constructor TSubject.Create();
begin
  inherited Create();
  FObservers := TList.Create();
end;

procedure TSubject.Change;
var
  Obs: TSubjectObserver;
  I: Integer;
begin
  for I := 0 to FObservers.Count - 1 do
  begin
    Obs := FObservers[I];
    if Obs.Enabled then
      Obs.Change(self);
  end;
end;

procedure TSubject.RegisterObserver(Observer: TSubjectObserver);
begin
  if FObservers.IndexOf(Observer) = -1 then
  begin
    FObservers.Add(Observer);
    // Update nieuwe observer
    if Observer.Enabled then
      Observer.Change(self);
  end;
end;

procedure TSubject.UnregisterObserver(Observer: TSubjectObserver);
begin
  FObservers.Remove(Observer);
end;

end.