Async-Injection


A robust lightweight dependency injection library for TypeScript.

About

Async-Injection is a small IoC container with support for both synchronous and asynchronous dependency injection, as well as isolated and/or hierarchical scopes.

Installation

You can get the latest release using npm:

``
$ npm install async-injection --save
`

Basic Usage (synchronous)

Here we 'get' a new transaction handling object, that itself, relies on a shared service:

`typescript
@Injectable()
class SharedService {
constructor(@Inject('LogLevel') @Optional('warn') private logLevel: string) { }
}

@Injectable()
class TransactionHandler {
constructor(svc: SharedService) { }
}

// Create a simple container (we will bind providers into it).
const container = new Container();

// A single instance will be created and shared by everyone.
container.bindClass(SharedService).asSingleton();

// A new instance will be created each time one is requested.
container.bindClass(TransactionHandler);

// If we omit this line, the logLevel of SharedService will be initialized to 'warn'
container.bindConstant('LogLevel', 'info');

// In our request processing code (which would be an anti-pattern)...
// Instantiate a new transaction handler (it will be injected with the shared service).
const tx = container.get(TransactionHandler);
`
NOTE:
The examples in this ReadMe are contrived to quickly communicate concepts and usage.
Your real world project should of course follow best practices like separation of concerns, having a composition root, and should avoid anti-patterns like service locator.

Scopes

Scopes can be created using multiple Containers, and/or a hierarchy of Containers.

IoC Modules

Why reinvent the wheel? TypeScript is great!
Implement the "module" you want and just import it:

my-http-ioc-module.ts
`typescript
import {myContainer} from './app';
import {Logger, HttpClient} from './services';
import {HttpClientGotWrapper} from './impl';

myContainer.bind(Logger).asSingleton();
myContainer.bind(HttpClient, HttpClientGotWrapper);
`

Asynchronous Support


For simplicity, it is recommended that you use traditional synchronous injection for any class where that is possible.
But when that's just to much work, you can "blend" synchronous and asynchronous injection.
To support "blending", we introduce three new methods on the Container which will be explained below.

Asynchronous Usage

Perhaps in the example above, our SharedService is useless until it has established a database connection.
Of course such a simple scenario could easily be handled in user-land code, but as application complexity grows, this becomes more tedious and difficult to maintain.
Let's modify the example as follows:
`typescript
@Injectable()
class SharedService {
constructor() { }
connect(): Promise { ... }
}

const container = new Container();

// Bind a factory function that awaits until it can fully create a SharedService.
container.bindAsyncFactory(SharedService, async () => {
let svc = new SharedService();
return await svc.connect();
}).asSingleton();

// A new transient instance will be created each time one is requested.
container.bindClass(TransactionHandler);

// Wait for all bound asynchronous factory functions to complete.
// This step is optional. You could omit and use Container.resolve instead (see alternative below).
await container.resolveSingletons(true);
// We are now connected to the database

// In our request processing code...
const tx = container.get(TransactionHandler);
`
As an alternative, we could remove the call to Container.resolveSingletons, and in our request processing code, simply call Container.resolve.
`typescript
const tx = await container.resolve(TransactionHandler);
`

Important - Container.resolve vs Container.get


Blending synchronous and asynchronous injection adds complexity to your application.
The key to successful blending is to think of the object you are requesting, not as an object, but as a tree of objects with your object at the top.
Keep in mind that you may have transient objects which need to be created each time, as well as existing singleton objects in your dependency tree.
If you know ahead of time that every object which you depend on is immediately (synchronously) available, or if they are asynchronous singletons which have already been resolved (via Container.resolveSingletons, or a previous call to Container.resolve), then no need to wait, you can just Container.get the tree.
Otherwise you need to await the full resolution of the tree with await Container.resolve.

@PostConstruct Support

It is not always possible to fully initialize your object in the class constructor.
This (albeit contrived) demo shows that the Employee class is not yet initialized when the Person subclass tries to call the overridden state method.
`typescript
class Person {
public constructor() { this.describe(); }
protected state() { return "relaxing"; }
public describe() { console.log("Hi I'm '" + this.state() + "'"); }
}
class Employee extends Person {
constructor(private manager: boolean) { super(); }
protected state() { return this.manager ? "busy" : "producing"; }
}
// This will print:
// "Hi I'm 'producing", even though the author probably expected
// "Hi I'm busy", because they passed true for the 'manager' parameter.
new Employee(true);
`
Can we refactor code to work around this? Sure. You may have to submit a couple of PR's, re-write legacy code that has no unit tests, trash encapsulation, skip a few nights sleep, etc. But why?
A PostConstruct annotation ensure's your initialization method is working on a fully constructed version of your object.
Even better, since constructors cannot be asynchronous, PostConstruct gives you an easy way to asynchronously prepare an object before it's put into service.

@PostConstruct Usage

Post construction methods can be either synchronous or asynchronous.

`typescript
class A {
public constructor() { }

// Called before the object is placed into the container (or is returned from get/resolve)
@PostConstruct()
public init(): void {
// ... initialization code here
}
}
class D {
public constructor() { }

// Will not be placed into the container (or returned) until the Promise has been resolved.
@PostConstruct()
public init(): Promise {
// ... initialization code here
}
}
`

$3


- Ensure your post construction method signature properly declares it's return type.
WARNING! An unspecified return type signature where the type is implied by return new Promise(...) is not sufficient! You must explicitly declare the return type.
- Container.get will throw an exception if you try to retrieve a class with @PostConstruct on a method that returns a Promise, but which does not declare it's return type to be Promise.
- The library will not invoke @PostConstruct on an object returned from a factory. It is the factory's responsibility to construct and initialize before returning.
- You will likely want a Container.resolveSingletons(true) call between your last Container.bindXXX() call and any Container.get` call.

API Overview

Async-Injection tries to follow the common API patterns found in most other DI implementations. Please refer to the examples above or the linked elements below for specific syntax.
- The
Container class implements a
Binder interface, which allows you to bind a
Constant,
Factory,
AsyncFactory, or
Class value to an
InjectableId (aka key) within a
Container.
- The
Container also implements an
Injector interface which allows you to synchronously
get or asynchronously
resolve anything that has been bound.
- When binding a
Factory,
AsyncFactory or
Class to an
InjectableId, you can chain the result of the call to specify the binding as a
Singleton, and/or configure an
Error Handler.
- Containers may be nested by passing a parent Container to the
constructor of a child Container.
- To bind a
Class into the
Container, you must add the
@Injectable decorator (aka annotation) to your class (see examples above).
- You may optionally add a
@PostConstruct decorator to a method of your class to perform synchronous or asynchronous initialization of a new instance.
- By default, Async-Inject will examine the parameters of a class constructor and do it's best to match those to bound
InjectableIds.
- You may use the
@Inject decorator to explicitly declare which
InjectableId should be used (generally required for a
Constant binding as in the logLevel example above).
- The
@Optional decorator allows you to specify a default value for a class constructor parameter in the event that no matching
InjectableId can be found.
- The Container's
resolveSingletons method may be used to wait for any bound asynchronous Singletons to finish initialization before continuing execution of your application.

Acknowledgements

Thanks to all the contributors at InversifyJS. It is a powerful, clean, flexible, inspiring design.

Thanks to everyone at NestJS for giving us Asynchronous providers.

Thanks to Darcy Rayner for describing a DI implementation so simply and clearly.

Thanks to Carlos Delgado for the idea of a "QuerablePromise" which allowed us to blend asynchronous DI with the simplicity of synchronous DI.

MIT License

Copyright (c) 2020 Frank Stock

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