iocc v2.2.2

IOCC

Simple class dependency injection library for Typescript

Table of Contents

• Features
• Installation
• Quick Start
• Documentation
  • Container Initialization
  • Dependency Registration
  • Dependency Resolution
  • Resetting Scoped Dependencies
  • Protection against captive dependencies
• FAQ

Features

• Multiple container support
• Injection using type hinting & decorators
• Support singleton and transient dependencies
• Run on both browser and server
• Zero dependencies except reflect-metadata
• Protection against captive dependencies

Installation

This library uses reflect-metadata for dependency resolution, so you have to install it also.

npm install reflect-metadata
npm install iocc

Quick Start

Import the reflect-metadata library on the project entry point.

// index.ts

import "reflect-metadata";

Declare and register some dependencies

import { Singleton, Scoped, Transient } from "iocc";

@Singleton()
class BookService {
	//
}

@Scoped()
class LibrarianService {
	public constructor(
		private readonly bookService: BookService,
	) {
		//
	}
}

@Transient()
class ShelfService {
	public constructor(
		private readonly bookService: BookService,
		private readonly librarianService: LibrarianService,
	) {
		//
	}
}

@Transient()
class LibraryService {
	public constructor(
		private readonly shelfService: ShelfService,
		private readonly librarianService: LibrarianService,
	) {
		//
	}
}

Resolve them anywhere

import { ContainerFactory, IContainer } from "iocc";

const container: IContainer = ContainerFactory.getContainer();

const libraryService: LibraryService = container.resolve(LibraryService);

Documentation

Container Initialization

Container is initialized in two ways:

Explicitly initializing via container factory:

Container can be initialized using the factory function ContainerFactory.getContainer():

import { ContainerFactory, IContainer } from "iocc";

const defaultContainer: IContainer = ContainerFactory.getContainer(); // Default container instance

const TestContainerIdentifier: symbol = Symbol("TEST_CONTAINER");
const container: IContainer = ContainerFactory.getContainer(TestContainerIdentifier); // Test container instance

These two instances of IContainer are separated and contain their own dependencies. No dependency from one container can interact or use dependency from the other container.

Using Singleton or Transient decorators:

Container can be also be initialized using the injection decorators:

import { Container, IContainer, Singleton, Transient } from "iocc";

// New default container is initialized and "Library" is registered
@Singleton()
class Library {
	//
}

// Existing default container is used and "Hall" is also registered
@Singleton()
class Hall {
	//
}

// New TEST_CONTAINER container is initialized and "Book" is registered
const TestContainerIdentifier: symbol = Symbol("TEST_CONTAINER");

@Transient(TestContainerIdentifier)
class Book {
	//
}

// Existing TEST_CONTAINER container is used and "City" is also registered
@Transient(TestContainerIdentifier)
class City {
	//
}

There are two separate containers being initialized, one is the default container and the other one using the identifier TestContainerIdentifier. Note that at the same time, along with container initialization, the classes are also registered with the respective containers.

Dependency Registration

Dependencies are registered in three ways:

Using decorators

This is the simplest form of dependency registration

import { Singleton, Scoped, Transient } from "iocc";

// Register a singleton class in default container
@Singleton()
class Library {
	//
}

// Register a scoped class in default container
@Scoped()
class Shelf {
	//
}

// Register a transient class in default container
@Transient()
class Book {
	//
}

const TEST_CONTAINER = Symbol("TEST_CONTAINER");

// Register a singleton class in TEST_CONTAINER container
@Singleton(TEST_CONTAINER)
class City {
	//
}

// Register a singleton class in TEST_CONTAINER container
@Scoped(TEST_CONTAINER)
class Citizen {
	//
}

// Register a transient class in TEST_CONTAINER container
@Transient(TEST_CONTAINER)
class Hall {
	//
}

Using explicit container instance

Dependencies can be registered using container instance by passing dependency to the container

import { Container, IContainer } from "iocc";

class Book {
	//
}

class Shelf {
	//
}

class Library {
	//
}

const container: IContainer = Container.of();

// As transient
container.registerTransient(Book);

// As scoped
container.registerScoped(Shelf);

// As singleton
container.registerSingleton(Library);

Using explicit container instance and injection token

Dependencies can be registered using container instance by passing injection token and dependency to the container. This is useful, when you want to depend upon abstraction instead of implementation details

import { Token, Container, IContainer } from "iocc";

interface IBook {
	//
}

class Book implements IBook {
	//
}

interface IShelf {
	//
}

class Shelf implements IShelf {
	//
}

interface ILibrary {
	//
}

class Library implements ILibrary {
	//
}

const container: IContainer = Container.of();

// As transient
const bookToken = new Token<IBook>("BOOK");
container.registerTransient(bookToken, Book);

const shelfToken = new Token<IShelf>("SHELF");
container.registerScoped(shelfToken, Shelf);

// As singleton
const libraryToken = new Token<ILibrary>("LIBRARY");
container.registerSingleton(libraryToken, Library);

Dependency registration options

During dependency registration, you can pass an options object to customize the registration behavior. The options are

Dependency Resolution

Dependencies are resolved in three ways:

Using container instance:

Dependencies that are registered in the container without the injection token can be resolved from the container simply by passing the required dependency to the container

import { Container, IContainer } from "iocc";

class Book {
	//
}

const container: IContainer = Container.of();
container.registerSingleton(Book);

const book: Book = container.resolve(Book);

Using container instance and injection token:

Dependencies that are registered in the container with the injection token can be resolved from the container simply by passing the injection token to the container

import { Token, Container, IContainer } from "iocc";

interface IBook {
	//
}

class Book implements IBook {
	//
}

const BookToken: Token<IBook> = new Token<IBook>("BOOK");

const container: IContainer = Container.of();
container.registerSingleton(BookToken, Book);

const book: IBook = container.resolve(BookToken);

Auto resolution in other dependencies:

Dependencies that are utilized in other dependencies can be resolved by:

• If they are registered using the decorator or explicitly in the container instance without using token, then they will be inferred automatically

import { Container, IContainer, Singleton } from "iocc";

class Book {
	//
}

// Registered using decorator
@Singleton()
class Library {
	public constructor(
		// It will be inferred and injected at runtime
		private readonly book: Book,
	) {
		//
	}
}

const container: IContainer = Container.of();

// Registered using container instance without injection token
container.registerSingleton(Book);

const library: Library = container.resolve(Library);

• If they are registered explicitly in the container instance with injection token, then they should be injected using the Inject decorator

import { Token, Inject, Container, IContainer, Singleton } from "iocc";

interface IBook {
	//
}

class Book implements IBook {
	//
}

const BookToken: Token<IBook> = new Token<IBook>("BOOK");

// Registered using decorator
@Singleton()
class Library {
	public constructor(
		// It must be injected using the decorator as it is registerd using the injection token
		@Inject(BookToken) private readonly book: IBook,
	) {
		//
	}
}

const container: IContainer = Container.of();

// Registered using container instance with injection token
container.registerSingleton(BookToken, Book);

const library: Library = container.resolve(Library);

Resetting Scoped Dependencies

Scoped dependencies depends upon the definition of "SCOPE" of the application that is using the IOCC library. A classic example of that would be request scoped dependencies, i.e. dependencies that will be instantiated once per request life cycle and gets destroyed at the end of request lifecycle.

To denote the end of scope, call the resetScopedDependencies method on the container instance.

import { IContainer, ContainerFactory } from "iocc";

class ScopedDependency {
	public counter: number = 0;
}

const container: IContainer = ContainerFactory.getContainer();

container.registerScoped(ScopedDependency);

const scopedDependencyOne: ScopedDependency = container.resolve(ScopedDependency);
console.log(scopedDependencyOne.counter); // 0
scopedDependencyOne.counter++;
console.log(scopedDependencyOne.counter); // 1

// Scope ended
container.resetScopedDependencies();

const scopedDependencyTwo: ScopedDependency = container.resolve(ScopedDependency);
console.log(scopedDependencyTwo.counter); // 0
scopedDependencyTwo.counter++;
console.log(scopedDependencyTwo.counter); // 1

Protection against captive dependencies

According to the documentation at Microsoft

The term Captive Dependency was coined by Mark Seemann, and refers to the misconfiguration of service lifetimes, where a longer-lived service holds a shorter-lived service captive.

Consider this example:

import { v4 as uuid } from "uuid";
import { Singleton, Transient, Container, IContainer } from "iocc";

@Transient()
class TaxCodeCalculator {
	private readonly _taxCode: string;

	public get taxCode(): string {
		return this._taxCode;
	}

	public constructor() {
		this._taxCode = uuid();
	}
}

@Singleton()
class OrderApiGateway {
	public constructor(
		// A transient dependency is injected inside a singleton dependency
		private readonly taxCalculator: TaxCodeCalculator
	) {
		//
	}

	public getOrderNumber(): string {
		return `SDNU-${taxCalculator.taxCode}`;
	}
}

const conainer: IContainer = Container.of();

// Somewhere in the application
const orderApiGateway = container.resolve(OrderApiGateway);
console.log(orderApiGateway.getOrderNumber()); // prints abe91712-2e55-442c-95f7-d3704c11a254

In the above example, when you call the orderApiGateway.getOrderNumber() in the same application lifecycle, you will expect to have a different order number each time, as TaxCodeCalculator is a transient dependency, but since it is injected in a singleton OrderApiGateway whose instance is created once per application lifecycle, it will keep the TaxCodeCalculator also alive throughout the application lifecycle.

IOCC protects you from these captive dependencies, so when you try to resolve the OrderApiGateway, it will produce the following error:

CaptiveDependencyException [Error]: Captive dependency detected: Singleton[OrderApiGateway] -> Transient[TaxCodeCalculator]

The dependency lifetime order in descending order is

SINGLETON > SCOPED > TRANSIENT

FAQ

• Can it resolve type hinted dependency when used as typed imports?

No. When you try to resolve the dependencies like this:

// file1.ts

import { Singleton } from "iocc";

@Singleton()
export class Book {
	//
}

// file2.ts

import { Singleton } from "iocc";
// Book is a type import instead of a value import!!
import type { Book } from "./file1";

@Singleton()
export class Library {
	public constructor(
		// This will fail
		private readonly book: Book,
	) {
		//
	}
}

• Can I add a proxy instance on a dependency?

Yes, but there is a catch. Consider an example where the constructor initialization is trapped using proxies:

const Decorator = <T>(target: Constructable<T>): Constructable<T> => {
	return new Proxy(target, {
		construct(concrete: Constructable<T>, args: Array<any>) {
			//
		}
	})
}

@Singleton()
class DemoService {
	//
}

@Singleton()
@Decorator
class UserService {
	public constructor(
		private readonly demoService: DemoService,
	) {
		//
	}
}

Here the UserService instance will be created, but demoService will be undefined inside of userService. That is because when we returned the proxy constructor from the Decorator function, the metadata properties from UserService are lost.

To work around this issue, use the helper function copyMetadata like this inside the Decorator function:

import { copyMetadata } from "iocc";

const Decorator = <T>(target: Constructable<T>): Constructable<T> => {
	const proxifiedTarget = new Proxy(target, {
		construct(concrete: Constructable<T>, args: Array<any>) {
			//
		}
	});

	// This will copy the metadata properties from the original class constructor to the proxified one.
	copyMetadata(target, proxifiedTarget);

	return proxifiedTarget;
}

It will preserve the metadata on the proxy constructor