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service-container

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A Symfony2-style dependency injection library for Node.js

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# Service Container

A Symfony2-style dependency injection library for Node.js

# Purpose

Inspired by the [Symfony2 Service Container](http://fabien.potencier.org/article/13/introduction-to-the-symfony-service-container)

A service container increases the usability of classes by allowing a configuration
file to specify which classes should be used to construct a class.  The container
will construct an instance of the class for you using the parameters that you
specify, allowing you to specify different classes to use in different situations
or different environments.

For example, if you have a class that makes HTTP requests.  In your production
environment you would want this to be a real http client.  In your unit test
environment, there is significant complication and overhead in using a real HTTP
client, so you would probably want to use a mock class instead that delivers canned
and predictable responses.

With a service container, you can reconfigure your class through a configuration
file without actually touching your code.

The Symfony 2 Guide Book provides a great [explanation](http://symfony.com/doc/current/book/service_container.html)

# Installation

npm install service-container

Or add to your package.json dependencies.

# Usage

There are two pieces to using the service container, the container itself and the
`services.json` configuration files.

The `services.json` file specifies [3 types of dependency injection](http://symfony.com/doc/current/components/dependency_injection/types.html):
* Constructor Injection
* Setter Injection
* Property Injection

Constructor injection allows you to specify JSON literal parameters as arguments
for your class instances or it will allow you to specify another service as an
argument (be careful about circular dependencies!).

Setter injection will attempt to call the method that you specify with the arguments
(either parameters or other services) that you define.  This is useful for adding
dependency injection for existing libraries that do not conform to Constructor
Injection.

Property injection will directly attempt to set the property on the object with
the argument that you specify, either another service or a JSON literal parameter.

## Initializing and using the container

Create a container like:

```javascript
var ServiceContainer = require('service-container');

// Create an instance of the container with the environment option set
// This will include both services.json and services_[ENV].json files to override
// any environment-specific parameters
var options = {
  env: 'test',
  ignoreNodeModulesDirectory: true
};
var container = ServiceContainer.buildContainer(__dirname, options);
```

There are two available options:
* `env` - Which will cause the builder to search for `services_[ENV].json` files
* `ignoreNodeModulesDirectory` - Which will prevent a recursive search through your dependent modules

Get an instance of a service like:

```javascript
// Get an example mailer service
var mailer = container.get('mailer');
mailer.sendMail('Hello');

// Get a parameter
var maxcount = container.getParameter('mailer.maxcount');
```

The section below goes over how to configure and construct services.

## services.json

When initializing the container, you'll pass it the root directory for it to search
for the services.json files.  The Builder will recursively search through directories
below the root directory to search for services.json files.  If the `env` option
is specified then files named services_[ENV].json will also be parsed.  Allowing
you to override any parameters setup earlier.

The hierarchy of services and parameters are as follows:
* services.json files at lower folder levels are overridden by those at higher levels
* services.json files are overriden by services_[ENV].json files
* parameters are loaded and overriden by any parameters in the parameters.json file (see below)

This allows you to override the specifics of a module from the application level
and allows you to override any parameter based on whether this is your dev, test,
qa or production environment.

```javascript
{
  "parameters": {
    "dependency_service.file":"./CoolClass",   // File paths are relative to the services.json file
    "dependency2_service.file":"./OtherClass",
    "my_service.file":"MyClass",
    "my_service.example_param":"yes",          // Parameter names are arbitrary
    "my_service.obj_param":{isExample: true}   // Can use literals as parameters
  },
  "services": {
    "dependency_service": {
      "class":"%depency_service.file%",
      "arguments":[]  // No Arguments
    },
    "dependency2_service": {
      "class":"%depency2_service.file%",
      "arguments":[]
    },
    "my_service": {
      "class":"%my_service.file%" // Parameters use % symbols, services use @
      "arguments": ["@dependency_service", "%my_service.example_param%", "@?optional_service"] // Optional services have @? at the beginning
      "calls": [
        ["setSecondDependency", ["@dependency2_service"]] // Method calls have the method name and an array of arguments
      ],
      "properties": {
        "myServiceProperty":"%my_service.obj_param%"
      }
    }
  }
}
```

### Other Service Options

* `"constructorMethod"` - A specific constructor function if including a library that
  serves as a namespace for many constructors.  For example: `mylibrary.SomeClass`
  the constructorMethod would be "SomeClass".
* `"isObject"` - When including a service that is an object and not a constructor
  like the node module `fs`
* `"isSingleton"` - Create only one of these objects as a service which will be
  passed around everytime the service is called.  The default behavior is that
  new objects will be created everytime the user requests this service from the
  container.

### parameters.json

The service container will also load its configurations through files named
`paramters.json`.  Only the parameters block is loaded from these files and they
override any previously seen parameters.  This file is specifically meant to be
out of version control and maintain deployment-specific details such as a DB user
and password, or the log level of a specific environment, etc.  These are details
that should be configured on the server level manually or through your configuration
manager.

### Importing Other JSON Service Configuration Files

To support more complex service.json file organizations, the `"imports"` key allows
any JSON file to be parsed like a services.json file.  In terms of service and parameter
hierarchy, the imports are at the same level as the file that they are found in.
The imports are parsed top to bottom, and are all overridden by whatever content
is in the services.json file.

```javascript
{
  "imports": [
    "../my_other_services.json",
    "./lib/stuff.json"
  ],
  "parameters": {
    // ...
  },
  "services": {
    // ...
  }
}
```

### Namespaces

In addition to imports, another key feature for a complex application is the
use of namespaces.  Within any of your `services.json` files, you can apply a
namespace which will be prefixed to all of the paramters and services defined
in that file.  In addition, the namespace will be prepended to any files that
are imported via a name-spaced file and if they have their own namespace, it
will be applied after the namespace from the importing file.

All of the references within a `services.json` file using a namespace will prefer
parameters and services using that namespace and if no matching parameter or
service can be found, it will graduate to the non-namespaced version of the
service or parameter name.

So for example:

```javascript
{
  "namespace": "api_models.v2",
  "parameters": {
    "user.class": "./User.js"
  },
  "services": {
    "user": {"class": "%user.class%"}
  }
}
```

In the example above, you would actually retrieve the user service from the
container by including the namespace:

var User = container.get('api_models.v2.user');

And although the user class is specified by `user.class`, it will actually
look up the parameter named `api_models.v2.user.class` first, and if that is
not found, it will use the parameter `user.class`.

## Examples

Check out the `example` directory to see some of the more common use cases for a
service container.

Included:
* Basic usage
* Overriding/Using mocks with the environment option
* Using an argument as a service

##TODO
* tags
* scoped services

## Run the Tests

make unittest

## Create Unit Test Coverage

make coverage

Open the file `coverage.html` that gets generated in the root directory

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