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Angular 2's DI

The last example introduced a hypothetical Injector object. Angular 2 simplifies DI even further. With Angular 2, programmers almost never have to get bogged down with injection details.

Angular 2's DI system is very subtle. It's not obvious, but calling Angular 2's bootstrap function initializes Angular 2's injection framework.

For example:

import {bootstrap} from '@angular/platform-browser-dynamic';
import {App} from './path/to/your/root/component';

bootstrap(App)

Believe it or not, the above example creates the root injector. This example is too primitive though; the injector is not told about anything.

import {bootstrap} from '@angular/platform-browser-dynamic';
import {Injectable} from '@angular/core';
import {App} from './path/to/your/root/component';

@Injectable()
class Hamburger {
  constructor(private bun: Bun, private patty: Patty,
    private toppings: Toppings) {}
}

bootstrap(App, [Hamburger]);

In the above example the root injector is initialized, and told about the Hamburger class. Another way of saying this is that Angular 2 has been provided a Hamburger.

That seems pretty straightforward, but astute readers will be wondering how Angular 2 knows how to build Hamburger. What if Hamburger was a string, or a plain function?

Angular 2 assumes that it's being given a class.

What about Bun, Patty and Toppings? How is Hamburger getting those?

It's not, at least not yet. Angular 2 does not know about them yet. That can be changed easily enough:

import {bootstrap} from '@angular/platform-browser-dynamic';
import {Injectable} from '@angular/core';
import {App} from './path/to/your/root/component';

@Injectable()
class Hamburger {
  constructor(private bun: Bun, private patty: Patty,
    private toppings: Toppings) {}
}

@Injectable()
class Patty {}

@Injectable()
class Bun {}

@Injectable()
class Toppings {}

// provide injectables to Angular 2
bootstrap(App, [Hamburger, Patty, Bun, Toppings]);

Okay, this is starting to look a little bit more complete. The key takeaway here is bootstrap(App, [Hamburger, Patty, Bun, Toppings]). The second parameter is an array of providers.

Although it's still unclear how Hamburger is being told about its dependencies. Perhaps that is related to those odd @Injectable statements.