Design patterns

Design Patterns GoF in Delphi — Skill

Use this skill when the user requests implementation of design patterns in Delphi. Always apply together with naming conventions (T/I/E/F/A/L) and memory management (try..finally).

When to Use

• Create Factory, Abstract Factory or Builder for creating complex objects
• Implement Strategy to vary algorithms (calculation of freight, taxes, export)
• Use Observer for decoupled notifications (Domain Events)
• Apply Command for undo/redo, job queues or auditing
• Use Decorator to add responsibilities without inheritance
• Implement Adapter for integration with legacy systems
• Use Facade to simplify complex subsystems (e.g. NFe emission)
• Apply Template Method to algorithms with variations (reports, exports)
• Use State for behavior that changes depending on the state of the object
• Use Chain of Responsibility for validation or processing pipelines

---

🏗️ Creational Patterns

Singleton — Global Configuration

unit MeuApp.Infra.AppConfig;

interface

type
  TAppConfig = class
  private
    class var FInstance: TAppConfig;
    FDatabaseUrl: string;
    FApiKey: string;
    constructor Create;
  public
    class function GetInstance: TAppConfig;
    class procedure ReleaseInstance;
    property DatabaseUrl: string read FDatabaseUrl write FDatabaseUrl;
    property ApiKey: string read FApiKey write FApiKey;
  end;

implementation

constructor TAppConfig.Create;
begin
  inherited Create;
  FDatabaseUrl := 'localhost:5432/myapp';
end;

class function TAppConfig.GetInstance: TAppConfig;
begin
  if not Assigned(FInstance) then
    FInstance := TAppConfig.Create;
  Result := FInstance;
end;

class procedure TAppConfig.ReleaseInstance;
begin
  FreeAndNil(FInstance);
end;

initialization
finalization
  TAppConfig.ReleaseInstance;
end.

Factory Method — Creation with Polymorphism

unit MeuApp.Domain.Report.Factory;

interface

uses
  MeuApp.Domain.Report.Intf;

type
  IReportExporter = interface
    ['{A1B2-...}']
    procedure Export(const AData: TReportData; const AFilePath: string);
  end;

  //Factory method — each subclass decides which exporter to create
  TReportExporterFactory = class abstract
  public
    function CreateExporter: IReportExporter; virtual; abstract;
    //Template Method usando Factory Method
    procedure ExportReport(const AData: TReportData; const AFilePath: string);
  end;

  TPdfReportFactory = class(TReportExporterFactory)
    function CreateExporter: IReportExporter; override;
  end;

  TExcelReportFactory = class(TReportExporterFactory)
    function CreateExporter: IReportExporter; override;
  end;

implementation

procedure TReportExporterFactory.ExportReport(const AData: TReportData; const AFilePath: string);
var
  LExporter: IReportExporter;
begin
  LExporter := CreateExporter;
  LExporter.Export(AData, AFilePath);
end;

Abstract Factory — Family of Related Objects

unit MeuApp.Infra.UI.Factory;

interface

type
  IButton = interface ['{...}'] procedure Render; end;
  IInputField = interface ['{...}'] procedure Render; end;
  IDialog = interface ['{...}'] procedure Show(const AMsg: string); end;

  //Abstract Factory
  IUIComponentFactory = interface
    ['{B2C3-...}']
    function CreateButton(const ACaption: string): IButton;
    function CreateInputField(const APlaceholder: string): IInputField;
    function CreateDialog: IDialog;
  end;

  TVCLComponentFactory = class(TInterfacedObject, IUIComponentFactory)
    function CreateButton(const ACaption: string): IButton;
    function CreateInputField(const APlaceholder: string): IInputField;
    function CreateDialog: IDialog;
  end;

  TFMXComponentFactory = class(TInterfacedObject, IUIComponentFactory)
    function CreateButton(const ACaption: string): IButton;
    function CreateInputField(const APlaceholder: string): IInputField;
    function CreateDialog: IDialog;
  end;

Builder — Step by Step Construction

unit MeuApp.Infra.Query.Builder;

interface

uses
  System.SysUtils,
  System.Classes,
  System.Generics.Collections;

type
  ///<summary>
  ///Builder for fluent construction of parameterized SQL queries.
  ///</summary>
  TQueryBuilder = class
  private
    FSelect: string;
    FFrom: string;
    FWheres: TStringList;
    FOrderBy: string;
    FLimit: Integer;
  public
    constructor Create;
    destructor Destroy; override;

    function Select(const AFields: string): TQueryBuilder;
    function From(const ATable: string): TQueryBuilder;
    function Where(const ACondition: string): TQueryBuilder;
    function OrderBy(const AField: string; ADesc: Boolean = False): TQueryBuilder;
    function Limit(ACount: Integer): TQueryBuilder;
    function Build: string;
  end;

implementation

constructor TQueryBuilder.Create;
begin
  inherited Create;
  FWheres := TStringList.Create;
  FLimit := 0;
end;

destructor TQueryBuilder.Destroy;
begin
  FWheres.Free;
  inherited Destroy;
end;

function TQueryBuilder.Select(const AFields: string): TQueryBuilder;
begin
  FSelect := AFields;
  Result := Self;
end;

function TQueryBuilder.From(const ATable: string): TQueryBuilder;
begin
  FFrom := ATable;
  Result := Self;
end;

function TQueryBuilder.Where(const ACondition: string): TQueryBuilder;
begin
  FWheres.Add(ACondition);
  Result := Self;
end;

function TQueryBuilder.OrderBy(const AField: string; ADesc: Boolean): TQueryBuilder;
begin
  FOrderBy := AField;
  if ADesc then FOrderBy := FOrderBy + ' DESC';
  Result := Self;
end;

function TQueryBuilder.Limit(ACount: Integer): TQueryBuilder;
begin
  FLimit := ACount;
  Result := Self;
end;

function TQueryBuilder.Build: string;
var
  LSql: TStringBuilder;
begin
  LSql := TStringBuilder.Create;
  try
    LSql.Append('SELECT ').Append(FSelect);
    LSql.Append(' FROM ').Append(FFrom);
    if FWheres.Count > 0 then
      LSql.Append(' WHERE ').Append(String.Join(' AND ', FWheres.ToStringArray));
    if not FOrderBy.IsEmpty then
      LSql.Append(' ORDER BY ').Append(FOrderBy);
    if FLimit > 0 then
      LSql.Append(' LIMIT ').Append(FLimit.ToString);
    Result := LSql.ToString;
  finally
    LSql.Free;
  end;
end;

---

🔧 Structural Patterns

Adapter — Legacy Integration

unit MeuApp.Infra.Payment.Adapter;

interface

type
  //Legacy system — cannot be changed
  TLegacyPaymentGateway = class
    procedure ProcessarPagamento(AValor: Double; ACodigoCartao: string);
  end;

  //Interface expected by the modern domain
  IPaymentGateway = interface
    ['{C3D4-...}']
    procedure ProcessPayment(AAmount: Currency; const ACardToken: string);
  end;

  //Adapter: translates the new call to the legacy one
  TLegacyPaymentAdapter = class(TInterfacedObject, IPaymentGateway)
  private
    FLegacy: TLegacyPaymentGateway;
  public
    constructor Create(ALegacy: TLegacyPaymentGateway);
    destructor Destroy; override;
    procedure ProcessPayment(AAmount: Currency; const ACardToken: string);
  end;

implementation

constructor TLegacyPaymentAdapter.Create(ALegacy: TLegacyPaymentGateway);
begin
  inherited Create;
  if not Assigned(ALegacy) then
    raise EArgumentNilException.Create('ALegacy gateway cannot be nil');
  FLegacy := ALegacy;
end;

destructor TLegacyPaymentAdapter.Destroy;
begin
  FLegacy.Free;  //The adapter has the legacy
  inherited Destroy;
end;

procedure TLegacyPaymentAdapter.ProcessPayment(AAmount: Currency; const ACardToken: string);
begin
  FLegacy.ProcessarPagamento(AAmount, ACardToken);
end;

Decorator — Extension without Inheritance

unit MeuApp.Infra.Logger.Decorators;

interface

type
  ILogger = interface
    ['{D4E5-...}']
    procedure Log(const ALevel, AMessage: string);
  end;

  TConsoleLogger = class(TInterfacedObject, ILogger)
    procedure Log(const ALevel, AMessage: string);
  end;

  //Decorator: adds timestamp
  TTimestampDecorator = class(TInterfacedObject, ILogger)
  private
    FInner: ILogger;
  public
    constructor Create(AInner: ILogger);
    procedure Log(const ALevel, AMessage: string);
  end;

  //Decorator: filters by minimum level
  TLevelFilterDecorator = class(TInterfacedObject, ILogger)
  private
    FInner: ILogger;
    FMinLevel: string;
  public
    constructor Create(AInner: ILogger; const AMinLevel: string);
    procedure Log(const ALevel, AMessage: string);
  end;

implementation

procedure TConsoleLogger.Log(const ALevel, AMessage: string);
begin
  Writeln(Format('[%s] %s', [ALevel, AMessage]));
end;

procedure TTimestampDecorator.Log(const ALevel, AMessage: string);
begin
  FInner.Log(ALevel, Format('%s | %s', [FormatDateTime('yyyy-mm-dd hh:nn:ss', Now), AMessage]));
end;

procedure TLevelFilterDecorator.Log(const ALevel, AMessage: string);
begin
  if ALevel >= FMinLevel then //DEBUG filter in production
    FInner.Log(ALevel, AMessage);
end;

Facade — Simplifying Subsystems

unit MeuApp.Application.NFe.Facade;

interface

uses
  MeuApp.Domain.NFe.Entity,
  MeuApp.Domain.NFe.Repository.Intf;

type
  ///<summary>
  ///Facade that simplifies the complete process of issuing NF-e,
  ///hiding XML, digital signature and communication with SEFAZ.
  ///</summary>
  TNFeFacade = class
  private
    FXmlBuilder: TNFeXmlBuilder;
    FSigner: TDigitalSigner;
    FTransmitter: TSefazTransmitter;
    FRepository: INFeRepository;
    procedure GenerateXml(ANFe: TNFe);
    procedure SignXml(ANFe: TNFe);
    procedure TransmitToSefaz(ANFe: TNFe);
    procedure PersistResult(ANFe: TNFe);
  public
    constructor Create(ARepository: INFeRepository);
    destructor Destroy; override;
    ///<summary>Issue NF-e: generates XML → signs → transmits → persists.</summary>
    procedure EmitirNFe(ANFe: TNFe);
    ///<summary>Cancels NF-e already issued.</summary>
    procedure CancelarNFe(const AChaveAcesso: string; const AMotivo: string);
  end;

Proxy — Access Control

unit MeuApp.Infra.Customer.Repository.Proxy;

interface

uses
  MeuApp.Domain.Customer.Entity,
  MeuApp.Domain.Customer.Repository.Intf,
  MeuApp.Domain.User.Entity;

type
  ///<summary>
  ///Security proxy that checks permissions before delegating to the real repository.
  ///</summary>
  TSecureCustomerRepositoryProxy = class(TInterfacedObject, ICustomerRepository)
  private
    FReal: ICustomerRepository;
    FCurrentUser: TUser;
    procedure CheckPermission(const AAction: string);
  public
    constructor Create(AReal: ICustomerRepository; ACurrentUser: TUser);
    function FindById(AId: Integer): TCustomer;
    function FindAll: TObjectList<TCustomer>;
    procedure Insert(ACustomer: TCustomer);
    procedure Update(ACustomer: TCustomer);
    procedure Delete(AId: Integer);
  end;

implementation

procedure TSecureCustomerRepositoryProxy.CheckPermission(const AAction: string);
begin
  if not FCurrentUser.HasPermission(AAction) then
    raise EAuthorizationException.CreateFmt(
      'User "%s" does not have permission: %s', [FCurrentUser.Login, AAction]);
end;

procedure TSecureCustomerRepositoryProxy.Delete(AId: Integer);
begin
  CheckPermission('CUSTOMER_DELETE');
  FReal.Delete(AId);
end;

---

🎭 Behavioral Patterns

Strategy — Variation of Algorithms

unit MeuApp.Domain.Tax.Strategies;

interface

type
  ITaxStrategy = interface
    ['{E5F6-...}']
    function Calculate(ABaseValue: Currency): Currency;
    function GetDescription: string;
  end;

  TSimplesTaxStrategy = class(TInterfacedObject, ITaxStrategy)
  public
    function Calculate(ABaseValue: Currency): Currency;
    function GetDescription: string;
  end;

  TLucroPresumidoStrategy = class(TInterfacedObject, ITaxStrategy)
  public
    function Calculate(ABaseValue: Currency): Currency;
    function GetDescription: string;
  end;

  TIsencaoStrategy = class(TInterfacedObject, ITaxStrategy)
  public
    function Calculate(ABaseValue: Currency): Currency;
    function GetDescription: string;
  end;

implementation

const
  SIMPLES_RATE = 0.06;
  LUCRO_PRESUMIDO_RATE = 0.15;

function TSimplesTaxStrategy.Calculate(ABaseValue: Currency): Currency;
begin
  Result := ABaseValue * SIMPLES_RATE;
end;

function TSimplesTaxStrategy.GetDescription: string;
begin
  Result := 'Simples Nacional (6%)';
end;

function TIsencaoStrategy.Calculate(ABaseValue: Currency): Currency;
begin
  Result := 0;
end;

function TIsencaoStrategy.GetDescription: string;
begin
  Result := 'Isento de impostos';
end;

Observer — Domain Events

unit MeuApp.Domain.Order.Events;

interface

uses
  System.Generics.Collections,
  MeuApp.Domain.Order.Entity;

type
  IOrderEventObserver = interface
    ['{F6A7-...}']
    procedure OnOrderPlaced(AOrder: TOrder);
    procedure OnOrderCancelled(AOrder: TOrder);
  end;

  TOrderEventPublisher = class
  private
    FObservers: TList<IOrderEventObserver>;
  public
    constructor Create;
    destructor Destroy; override;
    procedure Subscribe(AObserver: IOrderEventObserver);
    procedure Unsubscribe(AObserver: IOrderEventObserver);
    procedure NotifyOrderPlaced(AOrder: TOrder);
    procedure NotifyOrderCancelled(AOrder: TOrder);
  end;

  //Observers concretos
  TEmailOrderNotifier = class(TInterfacedObject, IOrderEventObserver)
    procedure OnOrderPlaced(AOrder: TOrder);    //send confirmation by email
    procedure OnOrderCancelled(AOrder: TOrder); //sends cancellation notice
  end;

  TStockReservationObserver = class(TInterfacedObject, IOrderEventObserver)
    procedure OnOrderPlaced(AOrder: TOrder);    //reserve stock
    procedure OnOrderCancelled(AOrder: TOrder); //releases stock
  end;

implementation

constructor TOrderEventPublisher.Create;
begin
  inherited Create;
  FObservers := TList<IOrderEventObserver>.Create;
end;

destructor TOrderEventPublisher.Destroy;
begin
  FObservers.Free;
  inherited Destroy;
end;

procedure TOrderEventPublisher.NotifyOrderPlaced(AOrder: TOrder);
var
  LObserver: IOrderEventObserver;
begin
  for LObserver in FObservers do
    LObserver.OnOrderPlaced(AOrder);
end;

Command — Wrapping Actions

unit MeuApp.Application.Commands;

interface

uses
  MeuApp.Domain.Customer.Entity,
  MeuApp.Domain.Customer.Repository.Intf;

type
  ICommand = interface
    ['{A7B8-...}']
    procedure Execute;
    procedure Undo;
    function GetDescription: string;
  end;

  TCreateCustomerCommand = class(TInterfacedObject, ICommand)
  private
    FRepository: ICustomerRepository;
    FCustomerData: TCustomerDTO;
    FCreatedId: Integer;
  public
    constructor Create(ARepository: ICustomerRepository; AData: TCustomerDTO);
    procedure Execute;
    procedure Undo;
    function GetDescription: string;
  end;

  TDeleteCustomerCommand = class(TInterfacedObject, ICommand)
  private
    FRepository: ICustomerRepository;
    FCustomerId: Integer;
    FBackup: TCustomer;
  public
    constructor Create(ARepository: ICustomerRepository; ACustomerId: Integer);
    destructor Destroy; override;
    procedure Execute;
    procedure Undo;
    function GetDescription: string;
  end;

  //Command History (for Undo/Redo)
  TCommandHistory = class
  private
    FHistory: TStack<ICommand>;
    FRedoStack: TStack<ICommand>;
  public
    constructor Create;
    destructor Destroy; override;
    procedure Execute(ACommand: ICommand);
    procedure Undo;
    procedure Redo;
    function CanUndo: Boolean;
    function CanRedo: Boolean;
  end;

Template Method — Algorithm Skeleton

unit MeuApp.Application.Report.Generator;

interface

type
  TReportData = record
    Title: string;
    StartDate: TDate;
    EndDate: TDate;
  end;

  ///<summary>
  ///Abstract base that defines the skeleton of the report generation algorithm.
  ///Subclasses implement variable steps.
  ///</summary>
  TReportGenerator = class abstract
  protected
    FData: TReportData;
    procedure LoadData; virtual; abstract;
    procedure ValidateData; virtual;           //hook with default implementation
    procedure ProcessData; virtual; abstract;
    procedure FormatOutput; virtual; abstract;
    procedure SaveOutput(const APath: string); virtual; abstract;
    procedure SendNotification; virtual;       //optional hook — default: noop
  public
    //Template Method — cannot be overwritten (final)
    procedure Generate(AData: TReportData; const ASavePath: string);
  end;

  TSalesReportGenerator = class(TReportGenerator)
  protected
    procedure LoadData; override;
    procedure ProcessData; override;
    procedure FormatOutput; override;
    procedure SaveOutput(const APath: string); override;
    procedure SendNotification; override;
  end;

implementation

procedure TReportGenerator.Generate(AData: TReportData; const ASavePath: string);
begin
  FData := AData;
  LoadData;
  ValidateData;
  ProcessData;
  FormatOutput;
  SaveOutput(ASavePath);
  SendNotification;
end;

procedure TReportGenerator.ValidateData;
begin
  if FData.Title.Trim.IsEmpty then
    raise EValidationException.Create('Report title cannot be empty');
  if FData.StartDate > FData.EndDate then
    raise EValidationException.Create('StartDate must be before EndDate');
end;

procedure TReportGenerator.SendNotification;
begin
  //Default hook: empty. Subclasses can override to send email etc.
end;

Chain of Responsibility — Validation Pipeline

unit MeuApp.Application.Validation.Chain;

interface

uses
  MeuApp.Domain.Customer.Entity;

type
  TValidationResult = record
    IsValid: Boolean;
    ErrorMessage: string;
    class function Ok: TValidationResult; static;
    class function Fail(const AMessage: string): TValidationResult; static;
  end;

  IValidationHandler = interface
    ['{B8C9-...}']
    procedure SetNext(AHandler: IValidationHandler);
    function Validate(ACustomer: TCustomer): TValidationResult;
  end;

  TBaseValidationHandler = class(TInterfacedObject, IValidationHandler)
  private
    FNext: IValidationHandler;
  public
    procedure SetNext(AHandler: IValidationHandler);
    function Validate(ACustomer: TCustomer): TValidationResult; virtual;
  end;

  TNameValidationHandler = class(TBaseValidationHandler)
    function Validate(ACustomer: TCustomer): TValidationResult; override;
  end;

  TCpfValidationHandler = class(TBaseValidationHandler)
    function Validate(ACustomer: TCustomer): TValidationResult; override;
  end;

  TEmailValidationHandler = class(TBaseValidationHandler)
    function Validate(ACustomer: TCustomer): TValidationResult; override;
  end;

implementation

function TBaseValidationHandler.Validate(ACustomer: TCustomer): TValidationResult;
begin
  //Delegates to the next handler if there is one
  if Assigned(FNext) then
    Result := FNext.Validate(ACustomer)
  else
    Result := TValidationResult.Ok;
end;

function TNameValidationHandler.Validate(ACustomer: TCustomer): TValidationResult;
begin
  if ACustomer.Name.Trim.IsEmpty then
    Exit(TValidationResult.Fail('Nome é obrigatório'));
  if ACustomer.Name.Length < 3 then
    Exit(TValidationResult.Fail('Nome deve ter ao menos 3 caracteres'));
  Result := inherited Validate(ACustomer);  //continue the chain
end;

State — Behavior by State

unit MeuApp.Domain.Order.States;

interface

uses
  MeuApp.Domain.Order.Entity;

type
  IOrderState = interface
    ['{C9D0-...}']
    procedure Confirm(AOrder: TOrder);
    procedure Ship(AOrder: TOrder);
    procedure Deliver(AOrder: TOrder);
    procedure Cancel(AOrder: TOrder);
    function GetStatusDescription: string;
  end;

  //Condition: New (to be confirmed)
  TNewOrderState = class(TInterfacedObject, IOrderState)
  public
    procedure Confirm(AOrder: TOrder);
    procedure Ship(AOrder: TOrder);   //throws EInvalidOperationException
    procedure Deliver(AOrder: TOrder);
    procedure Cancel(AOrder: TOrder);
    function GetStatusDescription: string;
  end;

  //Status: Confirmed (awaiting shipment)
  TConfirmedOrderState = class(TInterfacedObject, IOrderState)
  public
    procedure Confirm(AOrder: TOrder);  //throws EInvalidOperationException
    procedure Ship(AOrder: TOrder);
    procedure Deliver(AOrder: TOrder);
    procedure Cancel(AOrder: TOrder);
    function GetStatusDescription: string;
  end;

  //Status: In transit
  TShippedOrderState = class(TInterfacedObject, IOrderState)
  public
    procedure Confirm(AOrder: TOrder);
    procedure Ship(AOrder: TOrder);
    procedure Deliver(AOrder: TOrder);
    procedure Cancel(AOrder: TOrder);  //requires calling the carrier
    function GetStatusDescription: string;
  end;

implementation

procedure TNewOrderState.Confirm(AOrder: TOrder);
begin
  AOrder.ConfirmedAt := Now;
  AOrder.SetState(TConfirmedOrderState.Create);  //transition
end;

procedure TNewOrderState.Ship(AOrder: TOrder);
begin
  raise EInvalidOperationException.Create('Pedido ainda não confirmado. Confirme antes de enviar.');
end;

function TNewOrderState.GetStatusDescription: string;
begin
  Result := 'Novo — aguardando confirmação';
end;

---

📌 Guide to Choosing the Right Pattern

Need	Standard
Create objects with type variation	Factory Method / Abstract Factory
Create complex objects step by step	Builder
Ensure Global Single Instance	Singleton
Copy objects	Prototype
Adapt incompatible interface	Adapter
Add responsibilities dynamically	Decorator
Simplify complex system	Facade
Control access to an object	Proxy
Compose objects in tree structure	Composite
Vary algorithm without changing context	Strategy
Notify multiple objects about changes	Observer
Encapsulate actions with undo/redo	Command
Algorithm with variations	Template Method
Pass request through handler chain	Chain of Responsibility
Behavior that changes with the state	State

✅ Final Checklist

• All dependencies injected via constructor (DIP)
• Every interface implementation uses TInterfacedObject (ARC)
• No .Create without try..finally (except on ARC interfaces)
• Each class has a single responsibility (SRP)
• DUnitX tests cover each pattern with Fakes/Stubs
• XMLDoc in Portuguese in public methods
• Prefixes: T class, I interface, E exception, F field, A param, L location