Registering Singletons

A Singleton registration means the container creates the instance on first resolution and returns the same object for every subsequent call within the same injector.

Method signature

procedure Singleton<T: class, constructor>(
  const AOnCreate: TProc<T> = nil;
  const AOnDestroy: TProc<T> = nil;
  const AOnConstructorParams: TConstructorCallback = nil); overload;

Declared in unit Inject, class TInject.

Basic usage

uses
  Inject;

// Register — the instance is created immediately and cached
GetInjector.Singleton<TMyService>;

// Resolve — always returns the same instance
var LSvc := GetInjector.Get<TMyService>;

With an OnCreate callback

Use AOnCreate to configure the instance right after construction:

GetInjector.Singleton<TConnectionPool>(
  procedure(const APool: TConnectionPool)
  begin
    APool.MaxConnections := 20;
    APool.ConnectionString := 'localhost:3050';
  end
);

With an OnDestroy callback

GetInjector.Singleton<TConnectionPool>(
  nil, // AOnCreate
  procedure(const APool: TConnectionPool)
  begin
    APool.CloseAll;
  end
);

With custom constructor parameters

When the class constructor requires arguments that cannot be auto-resolved from the container, supply AOnConstructorParams:

GetInjector.Singleton<TMyService>(
  nil, nil,
  function: TConstructorParams
  begin
    Result := [TValue.From('ConfigValue'), TValue.From(42)];
  end
);

Lifecycle behavior

Event	Behavior
First Get<T> call	Instance is constructed and stored
Subsequent Get<T> calls	Cached instance is returned — no new allocation
Remove<T> call	OnDestroy fires, instance freed, entry removed from container
Container destruction	All owned instances are freed

caution

Singleton creates the instance at registration time (not lazily). If you want construction to be deferred until first resolution, use SingletonLazy instead.