design-patterns

Design Patterns GoF — Rules

Use these rules when implementing design patterns in Object Pascal/Delphi.

Interface GUIDs: every ['{...}'] below is a real, unique GUID generated for this example. When writing new interfaces, always generate a fresh GUID (IDE: Ctrl+Shift+G, or TGUID.NewGuid.ToString) — never emit the literal placeholder text {GUID}, and never reuse a GUID across two different interfaces.

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🏗️ Creational Patterns

Factory Method

//Product Interface
IButton = interface
  ['{EDF8C141-96C2-417D-98E8-2D3AB67DB22A}']
  procedure Render;
end;

//Abstract Creator
TButtonFactory = class abstract
public
  function CreateButton: IButton; virtual; abstract;
end;

//Concrete implementations
TVCLButton = class(TInterfacedObject, IButton)
  procedure Render;
end;

TFMXButton = class(TInterfacedObject, IButton)
  procedure Render;
end;

Abstract Factory

//Abstract factory interface
IUIFactory = interface
  ['{F104148C-C1E6-42AD-85C5-AE16DD3B98FB}']
  function CreateButton: IButton;
  function CreateDialog: IDialog;
end;

//Concrete factories by platform
TVCLFactory = class(TInterfacedObject, IUIFactory)
  function CreateButton: IButton;
  function CreateDialog: IDialog;
end;

TFMXFactory = class(TInterfacedObject, IUIFactory)
  function CreateButton: IButton;
  function CreateDialog: IDialog;
end;

Singleton

//Singleton thread-safe em Delphi
TAppConfig = class
private
  class var FInstance: TAppConfig;
  constructor Create;
public
  class function GetInstance: TAppConfig;
  class procedure ReleaseInstance;
  property DatabaseUrl: string read FDatabaseUrl write FDatabaseUrl;
end;

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

⚠️ For multi-threaded environments use TCriticalSection when creating the instance.

Builder

//Builder for building complex queries
TQueryBuilder = class
private
  FSql: TStringBuilder;
  FParams: TDictionary<string, Variant>;
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): TQueryBuilder;
  function Build: string;
end;

//Fluent use (Fluent Interface)
var LSql: string;
begin
  LSql := TQueryBuilder.Create
    .Select('id, name, email')
    .From('customers')
    .Where('active = 1')
    .OrderBy('name')
    .Build;
end;