JavaScript Example

/**
 * The Flyweight stores a common portion of the state (also called intrinsic
 * state) that belongs to multiple real business entities. The Flyweight accepts
 * the rest of the state (extrinsic state, unique for each entity) via its
 * method parameters.
 */
class Flyweight {
  constructor(sharedState) {
    this.sharedState = sharedState;
  }

  operation(uniqueState) {
    const s = JSON.stringify(this.sharedState);
    const u = JSON.stringify(uniqueState);
    console.log(`Flyweight: Displaying shared (${s}) and unique (${u}) state.`);
  }
}

module.exports = Flyweight;

const Flyweight = require("./Flyweight");

/**
 * The Flyweight Factory creates and manages the Flyweight objects. It ensures
 * that flyweights are shared correctly. When the client requests a flyweight,
 * the factory either returns an existing instance or creates a new one, if it
 * doesn't exist yet.
 */
class FlyweightFactory {
  constructor(initialFlyweights) {
    this.flyweights = {};
    for (const state of initialFlyweights) {
      this.flyweights[this.getKey(state)] = new Flyweight(state);
    }
  }

  // Returns a Flyweight's string hash for a given state.
  getKey(state) {
    return state.join("_");
  }

  // Returns an existing Flyweight with a given state or creates a new one.
  getFlyweight(sharedState) {
    const key = this.getKey(sharedState);

    if (!(key in this.flyweights)) {
      console.log(
        "FlyweightFactory: Can't find a flyweight, creating new one."
      );
      this.flyweights[key] = new Flyweight(sharedState);
    } else {
      console.log("FlyweightFactory: Reusing existing flyweight.");
    }

    return this.flyweights[key];
  }

  listFlyweights() {
    const count = Object.keys(this.flyweights).length;
    console.log(`\nFlyweightFactory: I have ${count} flyweights:`);
    for (const key in this.flyweights) {
      console.log(key);
    }
  }
}

module.exports = FlyweightFactory;

const FlyweightFactory = require("./FlyweightFactory");

/**
 * The client code usually creates a bunch of pre-populated flyweights in the
 * initialization stage of the application.
 */
const factory = new FlyweightFactory([
  ["Chevrolet", "Camaro2018", "pink"],
  ["Mercedes Benz", "C300", "black"],
  ["Mercedes Benz", "C500", "red"],
  ["BMW", "M5", "red"],
  ["BMW", "X6", "white"],
  // ...
]);
factory.listFlyweights();

// ...

function addCarToPoliceDatabase(ff, plates, owner, brand, model, color) {
  console.log("\nClient: Adding a car to database.");
  const flyweight = ff.getFlyweight([brand, model, color]);

  // The client code either stores or calculates extrinsic state and passes it
  // to the flyweight's methods.
  flyweight.operation([plates, owner]);
}

addCarToPoliceDatabase(factory, "CL234IR", "James Doe", "BMW", "M5", "red");

addCarToPoliceDatabase(factory, "CL234IR", "James Doe", "BMW", "X1", "red");

factory.listFlyweights();