@mft/di
v1.0.4
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Dependency injection
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DI
Simple, predictable dependency injection
Example
function myDependencyFactory() {
return {
doTheThing() {
return "done"
}
}
}
function myServiceFactory({myDependency}) {
return {
doAThing() {
return myDependency.doTheThing()
},
}
}
const container = require("@mft/di").createContainer("application")
container.registerFactory("myService", myServiceFactory)
container.registerFactory("myDependency", myDependencyFactory)
const myService = container.resolve("myService")
myService.doAThing() // "done"
Terminology
- component - a reusable piece of your application
- factory - a function which creates a component
- registration - the process of mapping an ID to a component factory
- resolution - the process of mapping an ID to component instance
- resolver - object which performs resolution
- container - object which contains component registrations and instances, performs resolution
- lifetime - how long a component instance is expected to live
- scope - which components, and instances, are available to inject into a component instance
Concepts
The purpose of this library is to enable easy creation of your application's components, without having to worry about those components' dependencies.
Components
Components are the individual pieces of your application. They can be functions, objects, classes, strings... whatever you want.
An example of a component with two dependencies. Here, we have a factory function, which takes an object with named dependencies, and returns an instance of the component (in this case, an object).
Notice that there is no direct dependency on the DI library here. The only requirement is that factory functions take their dependencies as named arguments (an object). This makes the code portable, easy to test, and possible to wire together manually.
// my-component.js
const factory = ({dep1, dep2}) => {
const instance = {
get() {
return dep1.getSomething() + dep2.getSomethingElse()
},
}
return instance
}
Registration and resolution
When we have enough components in our application, and the dependency tree starts to get deeper (e.g. A
depends on B
which depends on C
etc.), wiring these components together can become tedious.
What we can do instead is register all our component factories with a 'container', and let it be responsible for creating our components (and their dependencies) for us.
Here is how we would register the component factories for the above example, and manually resolve (create) an instance of the component, with its dependencies injected.
const di = require("@mft/di")
const container = di.createContainer("application")
// register all component factories
container.register("myComponent", require("./my-component"))
container.register("dep1", require("./dep1"))
container.register("dep1", require("./dep2"))
// resolve an instance of the component
const myInstance = container.resolve("myComponent")
myInstance.get()
Here we've called container.resolve()
directly to manually resolve an instance of myComponent
. But how do dep1
and dep2
get resolved? Let's have a look at myComponent
again, but slightly re-write it to illustrate what's happening.
// my-component.js
const factory = (resolver) => {
const dep1 = resolver.dep1 // resolved when this property is accessed
const instance = {
get() {
const dep2 = resolver.dep2 // lazily resolve dep2
return dep1.getSomething() + dep2.getSomethingElse()
},
}
return instance
}
Here we can see that what actually gets injected is a 'resolver' object. Accessing properties on the resolver is what triggers resolution of dependencies.
Lifetimes
A lifetime is a statement about how long a container caches a reference to a component instance.
There are two lifetimes currently supported:
TRANSIENT
- no reference cached
- one instance per resolution
REGISTRATION
- reference cached in container in which the factory was registered
- one instance per registration container
TRANSIENT
is the default lifetime.
To explicitly set a lifetime, use the registerFactory
function like so:
container.registerFactory("id", factory, di.lifetimes.REGISTRATION)
Child containers
Sometimes you don't want a component to live for the entire life of your application, but also don't want to create a new instance every time it's resolved. For example, in an HTTP server, you might want to create some components which hold some data associated with a request.
For this, we can use child containers. Create one like so:
const di = require("@mft/di")
const express = require('express')
const app = express()
const container = di.createContainer("application")
app.get("/", (req, res) => {
req.container = container.child("request")
// register some components
// resolve and use a component
})
Now, each request can register its own components and have its own instances.
It is expected that a child container will have a shorter lifetime that its parent. Here, for example, request
container will live only as long as the request, but the application
container will live for the lifetime of the application.
With that in mind, there are two simple rules which dictate how instances get resolved when using child containers:
- instances get resolved in the container they were registered in
- a container can only resolve dependencies from itself or a parent container
This means nothing can depend on something with a shorter lifetime. For example, no components in the application
container could depend on a component in the request
container. However, components in the request
container can depend on instances from the application
container.
API
const di = require("@mft/di")
DI
di.createContainer(scopeName: string) -> container
di.lifetimes.TRANSIENT
di.lifetimes.REGISTRATION
Container
Services and values can be registered with a container. It is responsible for resolving an ID to an instance.
container.registerFactory(id: string, factory: function [, lifetime]) -> container
container.registerValue(id: string, value: any) -> container
container.registerValues(values: {string: any}) -> container
container.resolve(id: string) -> componentInstance
container.child(scopeName: string) -> container
Factory functions
Factory functions are used to create the component instances.
factory(resolver) -> componentInstance
Resolver
The resolver gets injected into each factory function. It is a Proxy which resolves component instances on property lookup.
resolver[id] -> componentInstance
Further reading
Inversion of Control Containers and the Dependency Injection pattern - Martin Fowler