The Dependency Injection pattern

Dependency injection is an important application design pattern. It's used so widely that almost everyone just calls it DI.

Angular has its own dependency injection framework, and you really can't build an Angular application without it.

This page covers what DI is and why it's useful.

When you've learned the general pattern, you're ready to turn to the Angular Dependency Injection guide to see how it works in an Angular app.

Why dependency injection?

To understand why dependency injection is so important, consider an example without it. Imagine writing the following code:

export class Car {

  public engine: Engine;
  public tires: Tires;
  public description = 'No DI';

  constructor() {
    this.engine = new Engine();
    this.tires = new Tires();
  }

  // Method using the engine and tires
  drive() {
    return `${this.description} car with ` +
      `${this.engine.cylinders} cylinders and ${this.tires.make} tires.`;
  }
}

The Car class creates everything it needs inside its constructor. What's the problem? The problem is that the Car class is brittle, inflexible, and hard to test.

This Car needs an engine and tires. Instead of asking for them, the Car constructor instantiates its own copies from the very specific classes Engine and Tires.

What if the Engine class evolves and its constructor requires a parameter? That would break the Car class and it would stay broken until you rewrote it along the lines of this.engine = new Engine(theNewParameter). The Engine constructor parameters weren't even a consideration when you first wrote Car. You may not anticipate them even now. But you'll have to start caring because when the definition of Engine changes, the Car class must change. That makes Car brittle.

What if you want to put a different brand of tires on your Car? Too bad. You're locked into whatever brand the Tires class creates. That makes the Car class inflexible.

Right now each new car gets its own engine. It can't share an engine with other cars. While that makes sense for an automobile engine, surely you can think of other dependencies that should be shared, such as the onboard wireless connection to the manufacturer's service center. This Car lacks the flexibility to share services that have been created previously for other consumers.

When you write tests for Car you're at the mercy of its hidden dependencies. Is it even possible to create a new Engine in a test environment? What does Engine depend upon? What does that dependency depend on? Will a new instance of Engine make an asynchronous call to the server? You certainly don't want that going on during tests.

What if the Car should flash a warning signal when tire pressure is low? How do you confirm that it actually does flash a warning if you can't swap in low-pressure tires during the test?

You have no control over the car's hidden dependencies. When you can't control the dependencies, a class becomes difficult to test.

How can you make Car more robust, flexible, and testable?

That's super easy. Change the Car constructor to a version with DI:

public description = 'DI';

constructor(public engine: Engine, public tires: Tires) { }

public engine: Engine;
public tires: Tires;
public description = 'No DI';

constructor() {
  this.engine = new Engine();
  this.tires = new Tires();
}

See what happened? The definition of the dependencies are now in the constructor. The Car class no longer creates an engine or tires. It just consumes them.

This example leverages TypeScript's constructor syntax for declaring parameters and properties simultaneously.

Now you can create a car by passing the engine and tires to the constructor.

// Simple car with 4 cylinders and Flintstone tires.
let car = new Car(new Engine(), new Tires());

How cool is that? The definition of the engine and tire dependencies are decoupled from the Car class. You can pass in any kind of engine or tires you like, as long as they conform to the general API requirements of an engine or tires.

Now, if someone extends the Engine class, that is not Car's problem.

The consumer of Car has the problem. The consumer must update the car creation code to something like this:

class Engine2 {
  constructor(public cylinders: number) { }
}
// Super car with 12 cylinders and Flintstone tires.
let bigCylinders = 12;
let car = new Car(new Engine2(bigCylinders), new Tires());

The critical point is this: the Car class did not have to change. You'll take care of the consumer's problem shortly.

The Car class is much easier to test now because you are in complete control of its dependencies. You can pass mocks to the constructor that do exactly what you want them to do during each test:

class MockEngine extends Engine { cylinders = 8; }
class MockTires  extends Tires  { make = 'YokoGoodStone'; }

// Test car with 8 cylinders and YokoGoodStone tires.
let car = new Car(new MockEngine(), new MockTires());

You just learned what dependency injection is.

It's a coding pattern in which a class receives its dependencies from external sources rather than creating them itself.

Cool! But what about that poor consumer? Anyone who wants a Car must now create all three parts: the Car, Engine, and Tires. The Car class shed its problems at the consumer's expense. You need something that takes care of assembling these parts.

You could write a giant class to do that:

import { Engine, Tires, Car } from './car';

// BAD pattern!
export class CarFactory {
  createCar() {
    let car = new Car(this.createEngine(), this.createTires());
    car.description = 'Factory';
    return car;
  }

  createEngine() {
    return new Engine();
  }

  createTires() {
    return new Tires();
  }
}

It's not so bad now with only three creation methods. But maintaining it will be hairy as the application grows. This factory is going to become a huge spiderweb of interdependent factory methods!

Wouldn't it be nice if you could simply list the things you want to build without having to define which dependency gets injected into what?

This is where the dependency injection framework comes into play. Imagine the framework had something called an injector. You register some classes with this injector, and it figures out how to create them.

When you need a Car, you simply ask the injector to get it for you and you're good to go.

let car = injector.get(Car);

Everyone wins. The Car knows nothing about creating an Engine or Tires. The consumer knows nothing about creating a Car. You don't have a gigantic factory class to maintain. Both Car and consumer simply ask for what they need and the injector delivers.

This is what a dependency injection framework is all about.

Now that you know what dependency injection is and appreciate its benefits, turn to the Angular Dependency Injection guide to see how it is implemented in Angular.