q-di

Asynchronous dependency injection using Q promises

npm install q-di
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q-di

Dependency injection capable of handling asynchronous dependencies using the promise library Q. Some inspiration drawn from the Java DI frameworks Guice and Dagger.

How to use

A set of dependencies (henceforth referred to as a "module") is specified via a Javascript object. Each dependency is a named function. The arguments of the function specify the prerequisite dependencies that must be satisfied before the dependency itself can be satisfied. A module for a hypothetical server might look like (source code in CoffeeScript for succinctness):

MODULE = {
  server  : (cache, database) -> new MyServer(cache, database)
  cache   : -> new MyCache()
  database: -> new MyDatabase()
}

Here we see that constructing a server object depends on first constructing a cache object and a database object. To actually construct a server object, we need to create an Injector and call the correspondingly named function on it:

di = require('q-di')
injector = new di.Injector(MODULE)
injector.server() # construct a server (returns a Q promise)

Promises/asynchrony

Dependencies can return promises instead of raw values. For example, the following module introduces a 100 ms delay when constructing a foo object:

MODULE = {
  foo : -> Q.delay(100).then(-> 'foo')
}

# Prints "foobar" (after 100 ms):
new di.Injector(MODULE).foo().done((result) -> console.log(result))

The above example is contrived, but more realistically, some of your components may need to perform IO asynchronously before they are fully initialized. For example, your server object might depend on a database object which needs to first establish a connection to the database before it's initialized.

Explicit argument names

Names of dependency arguments can also be specified explicitly, as shown below. This method for specifying dependencies might be useful if you want to use periods in your dependency names (e.g., to namespace things, such as 'server.backend.cache'):

MODULE = {
  server: {
    args   : ['services.cache', 'services.database']
    create : (c, d) -> new MyServer(c, d)
  }
  # ...
}

Hierarchical dependencies

Dependency names containing periods ('.') can be treated hierarchically if the hierarchical flag is set to true, as below:

MODULE = {
  'services.component1' : -> new MyComponent1()
  'services.component2' : -> new MyComponent2()
  'services.component3' :
    args   : ['services.component1', 'services.component2']
    create : (c1, c2) -> new MyComponent3(c1, c2)
}

injector = new di.Injector(MODULE, { hierarchical: true })

# Returns promise for services "container object" containing the 3 components.
injector.services()

# Returns promise for component 1.
injector['services.component1']()

This introduces implicit "container object" dependencies into the created injector which will automatically create all dependencies within the container when invoked. Containers can be nested arbitrarily deep:

injector = new di.Injector({
  'foo.bar.baz' : -> ...
}, { hierarchical: true })

# Returns promise for bar container.
injector.foo().then -> foo.bar

# Also returns promise for bar container.
injector['foo.bar']()

This is another feature which can be useful when organizing larger codebases.

Notes

Scoping

All constructed objects are singleton-scoped to the injector. This means that calling a method on an injector multiple times will return the same instance of the object (actually, promise), e.g. the following returns true:

injector.foo() == injector.foo() # always equal

Objects are not shared across injectors, however, e.g. if injector1 and injector2 are separate objects:

injector1.foo() != injector2.foo() # never equal

Cycles

Cyclic dependencies will result in an error being thrown, e.g. the following fails:

new di.Injector({ foo: (foo) -> }).foo() # throws error

Promise coercion

The Q library is compatible with many other promise implementations (as long as they follow the Promises/A+ specification). This means module dependencies may return non-Q promises. Note, however, that the injector will always coerce such promises into Q promises.

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