sack

Sack

An Inversion-of-Control container for all your dependency injection needs.

Installation

npm install sack

Sack works on both on the server with NodeJS and the client with Browserify.

Introduction

This is a simple Inversion of Control Container. It provides the mechanism you need to have a centralized store of lazily-created, dynamically resolved object instances that other types can passively request get injected.

This means that you can have objects use, access, or compose other objects without having to explicitly know how to create/setup/initialize them.

It lets you have a highly-decoupled and framework-independent domain objects.

Philosophy

As with any IoC container, you want to only have your code be aware of the container the topmost/single entry point that bootstraps the rest of your application.

It is very much so an antipattern to have traces of the IoC container all over your codebase. The idea is to allow your business logic and domain classes to be totally unaware that they are getting injected by a container, keeping them free of Sack-specific code.

This is done by embracing and reifying the pattern of dependency injection via constructor parameters. Instead of having all of your objects be aware of how to find, access, setup, initialize, and manage other objects, they simply implicitly state their need for a dependency as a constructor parameter.

It allows your business logic classes to go from this:

function UserController()
{
  var connection = new DbConnection(global.settings.conConfig);
  this.users = connection.selectUsers();
}

To this:

function UserController(users)
{
  this.users = users;
}

This removes the knowledge and logic from UserController on how to connect to a database.

In large applications, things like model stores, application configs, service handles, etc. are all used across several different parts of an application. Relying on every consumer object to manage its dependencies violates DRY and SRP, making your codebase difficult to maintain as it scales.

The goal is to reduce how tightly coupled your various objects are by removing the knowledge of how to create other dependency objects, and simply rely on expressing what kind of objects we need.

This facilitates testing, as complex dependencies that are injected can be substituted with mocks, and the consuming classes are not tied to a specific implementation.

Usage

Dependencies should be managed from a Container instance:

var Container = require('sack').Container;

var container = new Container();

Registering Objects

Register a class constructor that will get executed every time the dependency is resolved, creating a new instance:

container.register('service', MyService);

Register a constructor function that will get executed one time when the first time the dependency is resolved, and then re-used after that (singleton pattern):

container.register('service', MyService).asSingleton();

Register an existing object instance as a dependency:

container.register('service', someService).asInstance();

Register a (lazily evaluated) callback to provide the dependency on every request:

container.register('service', function() {
  return new MyService();
});

Registered a callback to provide the dependency the first time it is requested, and then re-use it all subsequent times (singleton via callback):

container.register('service', function() {
  return new MyService();
}).asSingleton();

Resolving Objects

You can create / request objects via the make() function by passing in the string tag used during registration:

var service = container.make('service');

Not that you should typically not be using Sack this way, but rather expressing dependencies as explained below:

Expressing Dependencies

Whenever Sack creates an object, it satisfies that object's dependencies by resolving them out of the container as well.

An object expresses its dependency as a constructor parameter, whose name must match a registered object.

function UserEditController(users)
{
  this.users = users;
}

UserEditController.prototype.refreshUsers()
{
  this.users.refresh();
}

Assuming we have registered some implementation for users:

container.register('users', LocalStorageUsers).asSingleton();

Then making UserEditController via the container will resolve the dependency automatically.

var controller = container.make(UserEditController);

Strong vs Weak Dependencies

By default, all registered dependencies are considered "strong", that is, they cannot be overridden. Attempting to register a dependency with the same name as another one will result in an error:

container.register('server', express());
container.register('server', http.createServer());

> Error: Cannot override: server

Registering a dependency as weak allows it to be overriden later:

container.register('config', {}).asWeak();
container.register('config', new ConfigStore());

// No error

Architecture Patterns

Sack is not a framework, but it is something that should mostly live at the top of your application lifecycle.

A typical pattern is to have some bootstrap file (main.js) that creates a Container, that then passes it to several services that each create and wire up their own set of dependencies. The services and ONLY the services are aware of the IoC container.

Tern Support

The source files are all decorated with JSDoc3-style annotations that work great with the Tern code inference system. Combined with the Node plugin (see this project's .tern-project file), you can have intelligent autocomplete for methods in this library.

Testing

Testing is done with Tape and can be run with the command npm test.

Automated CI cross-browser testing is provided by Testling.

License

Copyright 2014 Brandon Valosek

Sack is released under the MIT license.