dependency injection

dependency-injection.ts

import 'reflect-metadata';

abstract class Singleton {
  static getInstance: () => Singleton;
}

const INJECTION_KEY = 'inject:';

const inject =
  (kls: typeof Singleton): PropertyDecorator =>
  (target, propertyKey: string) => {
    Reflect.defineMetadata(`${INJECTION_KEY}${propertyKey}`, kls, target);
  };

function injectable(target) {
  return new Proxy(target, {
    construct(cls: Function, args, receiver) {
      const obj = Reflect.construct(cls, args, receiver);
      Reflect.getMetadataKeys(obj)
        .filter((v) => v.startsWith(INJECTION_KEY))
        .forEach((key) => {
          const propertyKey = key.substring(INJECTION_KEY.length);
          const kls = Reflect.getMetadata(key, obj);
          Reflect.defineProperty(obj, propertyKey, {
            get: () => kls.getInstance(),
          });
        });
      return obj;
    },
  });
}

class Example {
  static instance: Example;
  static getInstance() {
    if (!this.instance) {
      this.instance = new this();
    }
    return this.instance;
  }
  prop = 42;
}

@injectable
class Application {
  @inject(Example)
  private example: Example;

  public callMe() {
    console.log(this.example.prop);
  }
}

const app = new Application();
app.callMe();