Flyweight Pattern

The Flyweight Pattern falls under the structural design patterns from the Gang Of Four.

The flyweight is an object that minimizes memory usage by sharing as much data as possible with similar objects - a way to use objects in large numbers when a simple representation would use an unacceptable amount of memory.

Usage

Use when:

Example

Here we will create a 1000 combatants that

interface Action {
  move(location: [number, number]): void;
  hit?(target: string, location: [number, number]): void;
}

class Combatant implements Action {
  private equipmentSet: string;
  number: number;

  constructor(set: string, number: number) {
    // initialization consumes time
    this.equipmentSet = set;
    this.number = number;
    console.log(`new Combatant ${number}`);
  }

  move(location: [number, number]): void {
    console.log(`move to ${location}`);
  }

  hit?(target: string, location: [number, number]): void {
    console.log(`damage ${target} at ${location}`);
  }
}

class CombatantAcademy {
  private static groups: { [set: string]: Combatant } = {};

  public static getCombatant(set: string, num: number) {
    let combatant = CombatantAcademy.groups[set];

    if (!combatant) {
      combatant = new Combatant(set, num);
      CombatantAcademy.groups[set] = combatant;
    } else {
      combatant.number = num;
      console.log(`shared Combatant ${combatant.number}`);
    }

    return combatant;
  }
}

(function main() {
  let start, end;

  // the larger you change the threshold, the bigger the difference
  const threshold = 50000;

  // inefficient creating without flyweight
  start = Math.floor(Date.now());
  for (let i = 0; i < threshold; i++) {
    new Combatant('normal-set', i); // creating fifty thousand real Combatants
  }
  end = Math.floor(Date.now());
  const withoutFlyweight = end - start;

  // efficient create with flyweight
  start = Math.floor(Date.now());
  for (let i = 0; i < threshold; i++) {
    CombatantAcademy.getCombatant('normal-set', i); // create 1 Combatant
  }
  end = Math.floor(Date.now());
  const withFlyweight = end - start;

  console.log('Without Flyweight', withoutFlyweight);
  console.log('With Flyweight', withFlyweight);
})();
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