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Introduction

Dependency Injection is a technique where the construction of dependencies of a class or function is separated from its behavior, in order to keep the code loosely coupled.

For example, the Sequence Action authenticate supports different authentication strategies (e.g. HTTP Basic Auth, OAuth2, etc.). Instead of hard-coding some sort of a lookup table to find the right strategy instance, authenticate uses dependency injection to let the caller specify which strategy to use.

The example below shows a simplified implementation of authenticate action, please refer to the source code of @loopback/authenticate for the full working version.

class AuthenticateActionProvider {
  constructor(@inject(AuthenticationBindings.STRATEGY) strategy) {
    this.strategy = strategy;
  }

  value(): AuthenticateFn {
    return request => this.action(request);
  }

  // this is the function invoked by "authenticate()" sequence action
  action(request: Request) {
    const adapter = new StrategyAdapter(this.strategy);
    const user = await adapter.authenticate(request);
    return user;
  }
}

Dependency Injection makes the code easier to extend and customize, because the dependencies can be easily rewired by the application developer. It makes the code easier to test in isolation (in a pure unit test), because the test can inject a custom version of the dependency (a mock or a stub). This is especially important when testing code interacting with external services like a database or an OAuth2 provider. Instead of making expensive network requests, the test can provide a lightweight implementation returning pre-defined responses.

Configuring what to inject

Now that we write a class that gets the dependencies injected, you are probably wondering where are these values going to be injected from and how to configure what should be injected. This part is typically handled by an IoC Container, where IoC means Inversion of Control.

In LoopBack, we use Context to keep track of all injectable dependencies.

There are several different ways for configuring the values to inject, the simplest options is to call app.bind(key).to(value). Building on top of the example above, one can configure the app to use a Basic HTTP authentication strategy as follows:

// TypeScript example

import {BasicStrategy} from 'passport-http';
import {RestApplication, RestServer} from '@loopback/rest';
// basic scaffolding stuff happens in between...

// The REST server has its own context!
const server = await app.getServer(RestServer);
server.bind(AuthenticationBindings.STRATEGY).to(new BasicStrategy(loginUser));

function loginUser(username, password, cb) {
  // check that username + password are valid
}

However, when you want to create a binding that will instantiate a class and automatically inject required dependencies, then you need to use .toClass() method:

server
  .bind(AuthenticationBindings.AUTH_ACTION)
  .toClass(AuthenticateActionProvider);

const provider = await server.get(AuthenticationBindings.AUTH_ACTION);
// provider is an AuthenticateActionProvider instance
// provider.strategy was set to the value returned
// by server.get('authentication.strategy')

When a binding is created via .toClass(), Context will create a new instance of the class when resolving the value of this binding, injecting constructor arguments and property values as configured via @inject decorator.

Note that the dependencies to be injected could be classes themselves, in which case Context will recursively instantiate these classes first, resolving their dependencies as needed.

In this particular example, the class is a Provider. Providers allow you to customize the way how a value is created by the Context, possibly depending on other Context values. A provider is typically bound using .toProvider() API:

app
  .bind(AuthenticationBindings.AUTH_ACTION)
  .toProvider(AuthenticateActionProvider);

const authenticate = await app.get(AuthenticationBindings.AUTH_ACTION);
// authenticate is the function returned by provider's value() method

You can learn more about Providers in Creating Components.

Flavors of Dependency Injection

LoopBack supports three kinds of dependency injection:

  1. constructor injection: the dependencies are provided as arguments of the class constructor.
  2. property injection: the dependencies are stored in instance properties after the class was constructed.
  3. method injection: the dependencies are provided as arguments of a method invocation. Please note that constructor injection is a special form of method injection to instantiate a class by calling its constructor.

Constructor injection

This is the most common flavor that should be your default choice.

class ProductController {
  constructor(@inject('repositories.Product') repo) {
    this.repo = repo;
  }

  async list() {
    return await this.repo.find({where: {available: true}});
  }
}

Property injection

Property injection is usually used for optional dependencies which are not required for the class to function or for dependencies that have a reasonable default.

class InfoController {
  @inject('logger', {optional: true})
  private logger = ConsoleLogger();

  status() {
    this.logger.info('Status endpoint accessed.');
    return {pid: process.pid};
  }
}

Method injection

Method injection allows injection of dependencies at method invocation level. The parameters are decorated with @inject or other variants to declare dependencies as method arguments.

class InfoController {
  greet(@inject(AuthenticationBindings.CURRENT_USER) user: UserProfile) {
    return `Hello, ${userProfile.name}`;
  }
}

Optional dependencies

Sometimes the dependencies are optional. For example, the logging level for a Logger provider can have a default value if it is not set (bound to the context).

To resolve an optional dependency, set optional flag to true:

const ctx = new Context();
await ctx.get('optional-key', {optional: true});
// returns `undefined` instead of throwing an error

Here is another example showing optional dependency injection using properties with default values:

// Optional property injection
export class LoggerProvider implements Provider<Logger> {
  // Log writer is an optional dependency and it falls back to `logToConsole`
  @inject('log.writer', {optional: true})
  private logWriter: LogWriterFn = logToConsole;

  // Log level is an optional dependency with a default value `WARN`
  @inject('log.level', {optional: true})
  private logLevel: string = 'WARN';
}

Optional dependencies can also be used with constructor and method injections.

An example showing optional constructor injection in action:

export class LoggerProvider implements Provider<Logger> {
  constructor(
    // Log writer is an optional dependency and it falls back to `logToConsole`
    @inject('log.writer', {optional: true})
    private logWriter: LogWriterFn = logToConsole,
    // Log level is an optional dependency with a default value `WARN`
    @inject('log.level', {optional: true}) private logLevel: string = 'WARN',
  ) {}
}

An example of optional method injection, where the prefix argument is optional:

export class MyController {
  greet(@inject('hello.prefix', {optional: true}) prefix: string = 'Hello') {
    return `${prefix}, world!`;
  }
}

Circular dependencies

LoopBack can detect circular dependencies and report the path which leads to the problem.

Consider the following example:

import {Context, inject} from '@loopback/context';

interface Developer {
  // Each developer belongs to a team
  team: Team;
}

interface Team {
  // Each team works on a project
  project: Project;
}

interface Project {
  // Each project has a lead developer
  lead: Developer;
}

class DeveloperImpl implements Developer {
  constructor(@inject('team') public team: Team) {}
}

class TeamImpl implements Team {
  constructor(@inject('project') public project: Project) {}
}

class ProjectImpl implements Project {
  constructor(@inject('lead') public lead: Developer) {}
}

const context = new Context();

context.bind('lead').toClass(DeveloperImpl);
context.bind('team').toClass(TeamImpl);
context.bind('project').toClass(ProjectImpl);

try {
  // The following call will fail
  context.getSync('lead');
} catch (e) {
  console.error(e.toString());
}

When the user attempts to resolve “lead” binding, LoopBack detects a circular dependency and prints the following error:

Error: Circular dependency detected:
  lead --> @DeveloperImpl.constructor[0] -->
  team --> @TeamImpl.constructor[0] -->
  project --> @ProjectImpl.constructor[0] -->
  lead

Additional resources