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Child Injectors

InjectContainer supports hierarchical composition: a parent TInject can hold child TInject instances. When Get<T> or GetInterface<I> cannot find a service in the parent, it cascades into each registered child injector.

Registering a child injector​

procedure AddInject(const ATag: string; const AInstance: TInject);

The child injector is stored as a TServiceData with mode imSingleton under the given tag, so it participates in the cascade automatically.

Example: layered architecture​

uses
  Inject;

var
  LInfra  : TInject;
  LDomain : TInject;

begin
  // Infrastructure injector — data access services
  LInfra := TInject.Create;
  LInfra.Singleton<TDatabaseConnection>;
  LInfra.Singleton<TFileRepository>;

  // Domain injector — business logic services  
  LDomain := TInject.Create;
  LDomain.Singleton<TOrderService>;

  // Compose: parent holds both children
  GetInjector.AddInject('infra', LInfra);
  GetInjector.AddInject('domain', LDomain);

  // Resolving from the root finds services in any child
  var LConn  := GetInjector.Get<TDatabaseConnection>; // from LInfra
  var LOrder := GetInjector.Get<TOrderService>;        // from LDomain
end;

Cascade order​

Resolution is attempted in this order:

  1. Current injector's own FRepositoryReference / FRepositoryInterface.
  2. Each child TInject stored in FInstances (in insertion order).
  3. If none found: Get<T> returns nil; GetInterface<I> raises EServiceNotFound.

AddInstance — registering existing objects​

procedure AddInstance<T: class>(const AInstance: TObject);

Registers an already-constructed object as a Singleton without using RTTI construction. Useful for bridging external dependencies (e.g., a third-party object created before the DI container started).

var LExternalLogger := TThirdPartyLogger.Create('config.ini');
GetInjector.AddInstance<TThirdPartyLogger>(LExternalLogger);

// Later
var LLogger := GetInjector.Get<TThirdPartyLogger>;
caution

The container takes ownership of instances registered via AddInstance and will free them when the injector is destroyed or Remove<T> is called.