flyweight.md

Definition

Save and use memory efficiently, increase performance by avoiding data duplication. Objects that have identical data can share that data instead of copying it everywhere it's needed.

Example code

class EnemyType {
  final String name;
  final ByteData imageData; // This attribute is of substantial size, so need to save memory space by not repeating it unnecessarily

  const EnemyType(this.name, this.imageData);
}

// Remove the extrinsic state from the Enemy class and add a cache for the flyweight objects
class Enemy {
  static final Map<String, EnemyType> types = {};

  final EnemyType type;
  int x;
  int y;

  Enemy(String typeName) : type = getType(typeName);

  void moveTo(int x, int y) {
    this.x = x;
    this.y = y;
  }

  void draw() {
    print("Drawing ${type.name}...");
  }

  static EnemyType getType(String typeName) {
    return types.putIfAbsent(typeName, () => EnemyType(
      typeName, 
      loadImageData(typeName),
    ));
  }
}

// Then, when creating Enemy class, only a single instance of an Enemy with the EnemyType - `Red Avenger` will be constructed regardless how many items of that type in the list
final List<Enemy> enemies = [
  Enemy("Red Avenger"),
  Enemy("Red Avenger"),
  Enemy("Blue Stinger"),
];

• Within the context of the pattern, the memory-hogging constant data is referred to as intrinsic state, data that every object of a given type needs access to in an unaltered form.
• Each object's mutable data is known as the extrinsic state, because it's often changed by events occurring outside the instance and may be unique to a given instance.