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 — cada subclasse decide qual exportador criar
  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 para construção fluente de queries SQL parametrizadas.
  /// </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
  // Sistema legado — not pode ser alterado
  TLegacyPaymentGateway = class
    procedure ProcessarPagamento(AValor: Double; ACodigoCartao: string);
  end;

  // Interface esperada pelo domínio moderno
  IPaymentGateway = interface
    ['{C3D4-...}']
    procedure ProcessPayment(AAmount: Currency; const ACardToken: string);
  end;

  // Adapter: traduz a chamada nova para a legada
  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;  // O adapter possui o legado
  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: adiciona timestamp
  TTimestampDecorator = class(TInterfacedObject, ILogger)
  private
    FInner: ILogger;
  public
    constructor Create(AInner: ILogger);
    procedure Log(const ALevel, AMessage: string);
  end;

  // Decorator: filtra por nível mínimo
  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 // Filtra DEBUG em produção
    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>
  ///   Fachada que simplifica o processo completo de emissão de NF-e,
  ///   ocultando XML, assinatura digital e comunicaction com 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>Emite NF-e: gera XML → assina → transmite → persiste.</summary>
    procedure EmitirNFe(ANFe: TNFe);
    /// <summary>Cancela NF-e já emitida.</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>
  ///   Proxy de segurança que verifica permissões before de delegar ao repositório real.
  /// </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);    // envia confirmaction por e-mail
    procedure OnOrderCancelled(AOrder: TOrder); // envia aviso de cancelamento
  end;

  TStockReservationObserver = class(TInterfacedObject, IOrderEventObserver)
    procedure OnOrderPlaced(AOrder: TOrder);    // reserva estoque
    procedure OnOrderCancelled(AOrder: TOrder); // libera estoque
  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;

  // Histórico de Commands (para 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>
  ///   Base abstrata que define o esqueleto do algoritmo de geraction de relatório.
  ///   Subclasses implementam os passos variáveis.
  /// </summary>
  TReportGenerator = class abstract
  protected
    FData: TReportData;
    procedure LoadData; virtual; abstract;
    procedure ValidateData; virtual;           // hook com implementaction default
    procedure ProcessData; virtual; abstract;
    procedure FormatOutput; virtual; abstract;
    procedure SaveOutput(const APath: string); virtual; abstract;
    procedure SendNotification; virtual;       // hook opcional — default: noop
  public
    // Template Method — not pode ser sobrescrito (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
  // Hook default: vazio. Subclasses podem sobrescrever para enviar e-mail 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
  // Delega para o próximo handler se houver
  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);  // continua a cadeia
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;

  // Estado: Novo (a confirmar)
  TNewOrderState = class(TInterfacedObject, IOrderState)
  public
    procedure Confirm(AOrder: TOrder);
    procedure Ship(AOrder: TOrder);   // lança EInvalidOperationException
    procedure Deliver(AOrder: TOrder);
    procedure Cancel(AOrder: TOrder);
    function GetStatusDescription: string;
  end;

  // Estado: Confirmado (aguardando envio)
  TConfirmedOrderState = class(TInterfacedObject, IOrderState)
  public
    procedure Confirm(AOrder: TOrder);  // lança EInvalidOperationException
    procedure Ship(AOrder: TOrder);
    procedure Deliver(AOrder: TOrder);
    procedure Cancel(AOrder: TOrder);
    function GetStatusDescription: string;
  end;

  // Estado: Em trânsito
  TShippedOrderState = class(TInterfacedObject, IOrderState)
  public
    procedure Confirm(AOrder: TOrder);
    procedure Ship(AOrder: TOrder);
    procedure Deliver(AOrder: TOrder);
    procedure Cancel(AOrder: TOrder);  // requer chamada a transportadora
    function GetStatusDescription: string;
  end;

implementation

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