Flyweight pattern

In computer programming, flyweight is a software design pattern. A flyweight is an object that minimizes memory use by sharing as much data as possible with other similar objects; it is a way to use objects in large numbers when a simple repeated representation would use an unacceptable amount of memory. Often some parts of the object state can be shared, and it is common practice to hold them in external data structures and pass them to the flyweight objects temporarily when they are used.

A classic example usage of the flyweight pattern is the data structures for graphical representation of characters in a word processor. It might be desirable to have, for each character in a document, a glyph object containing its font outline, font metrics, and other formatting data, but this would amount to hundreds or thousands of bytes for each character. Instead, for every character there might be a reference to a flyweight glyph object shared by every instance of the same character in the document; only the position of each character (in the document and/or the page) would need to be stored internally.

Another example is string interning.

In other contexts the idea of sharing identical data structures is called hash consing.

History

According to the textbook Design Patterns: Elements of Reusable Object-Oriented Software,^[1] the flyweight pattern was first coined and extensively explored by Paul Calder and Mark Linton in 1990 to efficiently handle glyph information in a WYSIWYG document editor,^[2] although similar techniques were already used in other systems, e.g., an application framework by Weinand et al. (1988).^[3]

Immutability and equality

To enable safe sharing, between clients and threads, Flyweight objects must be immutable. Flyweight objects are by definition value objects. The identity of the object instance is of no consequence therefore two Flyweight instances of the same value are considered equal.

Example in C# (note Equals and GetHashCode overrides as well as == and != operator overloads):

public class CoffeeFlavour {
    private readonly string _flavour;
 
    public CoffeeFlavour(string flavour) {
        _flavour = flavour;
    }
 
    public string Flavour {
        get { return _flavour; }
    }
 
    public override bool Equals(object obj) {
        if (ReferenceEquals(null, obj)) return false;
        return obj is CoffeeFlavour && Equals((CoffeeFlavour)obj);
    }
 
    public bool Equals(CoffeeFlavour other) {
        return string.Equals(_flavour, other._flavour);
    }
 
    public override int GetHashCode() {
        return (_flavour != null ? _flavour.GetHashCode() : 0);
    }
 
    public static bool operator ==(CoffeeFlavour a, CoffeeFlavour b) {
        return Equals(a, b);
    }
 
    public static bool operator !=(CoffeeFlavour a, CoffeeFlavour b) {
        return !Equals(a, b);
    }
}

Concurrency

Special consideration must be made in scenarios where Flyweight objects are created on multiple threads. If the list of values is finite and known in advance the Flyweights can be instantiated ahead of time and retrieved from a container on multiple threads with no contention. If Flyweights are instantiated on multiple threads there are two options:

1. Make Flyweight instantiation single threaded thus introducing contention and ensuring one instance per value.
2. Allow concurrent threads to create multiple Flyweight instances thus eliminating contention and allowing multiple instances per value. This option is only viable if the equality criterion is met.

Example in C#

using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading;
 
public interface ICoffeeFlavourFactory {
    CoffeeFlavour GetFlavour(string flavour);
}
 
public class ReducedMemoryFootprint : ICoffeeFlavourFactory {
    private readonly object _cacheLock = new object();
    private readonly IDictionary<string, CoffeeFlavour> _cache = new Dictionary<string, CoffeeFlavour>();
 
    public CoffeeFlavour GetFlavour(string flavour) {
        if (_cache.ContainsKey(flavour)) return _cache[flavour];
        var coffeeFlavour = new CoffeeFlavour(flavour);
        ThreadPool.QueueUserWorkItem(AddFlavourToCache, coffeeFlavour);
        return coffeeFlavour;
    }
 
    private void AddFlavourToCache(object state) {
        var coffeeFlavour = (CoffeeFlavour)state;
        if (!_cache.ContainsKey(coffeeFlavour.Flavour)) {
            lock (_cacheLock) {
                if (!_cache.ContainsKey(coffeeFlavour.Flavour)) _cache.Add(coffeeFlavour.Flavour, coffeeFlavour);
            }
        }
    }
}
 
public class MinimumMemoryFootprint : ICoffeeFlavourFactory {
    private readonly ConcurrentDictionary<string, CoffeeFlavour> _cache = new ConcurrentDictionary<string, CoffeeFlavour>();
 
    public CoffeeFlavour GetFlavour(string flavour) {
        return _cache.GetOrAdd(flavour, flv => new CoffeeFlavour(flv));
    }
}

Simple implementation

Flyweight allows you to share bulky data which are common to each object. In other words, if you think that same data is repeating for every object, you can use this pattern to point to the single object and hence can easily save space.Here the FlyweightPointer creates a static member Company, which is used for every object of MyObject.

//IVSR: simple flyweight example in C#
    // Defines Flyweight object which repeats itself.
    public class FlyWeight
    {
        public string Company { get; set; }
        public string CompanyLocation { get; set; }
        public string CompanyWebSite { get; set; }
        //Bulky Data
        public byte[] CompanyLogo { get; set; } 
    }
    public static class FlyWeightPointer
    {
        public static FlyWeight Company = new FlyWeight
        {
            Company = "Abc",
            CompanyLocation = "XYZ",
            CompanyWebSite = "www.abc.com"
        };
    }
    public class MyObject
    {
        public string Name { get; set; }
        public FlyWeight Company
        {
            get
            {
                return FlyWeightPointer.Company;
            }
        }
    }

Example in Java

import java.util.concurrent.CopyOnWriteArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
 
// Instances of CoffeeFlavour will be the Flyweights
class CoffeeFlavour {
  private final String name;
 
  CoffeeFlavour(String newFlavor) {
    this.name = newFlavor;
  }
 
  @Override
  public String toString() {
    return name;
  }
}
 
// Menu acts as a factory and cache for CoffeeFlavour flyweight objects
class Menu {
  private Map<String, CoffeeFlavour> flavours = new HashMap<String, CoffeeFlavour>();
 
  CoffeeFlavour lookup(String flavorName) {
    if (!flavours.containsKey(flavorName))
      flavours.put(flavorName, new CoffeeFlavour(flavorName));
    return flavours.get(flavorName);
  }
 
  int totalCoffeeFlavoursMade() {
    return flavours.size();
  }
}
 
class Order {
  private final int tableNumber;
  private final CoffeeFlavour flavour;
 
  Order(int tableNumber, CoffeeFlavour flavor) {
    this.tableNumber = tableNumber;
    this.flavour = flavor;
  }
 
  void serve() {
    System.out.println("Serving " + flavour + " to table " + tableNumber);
  }
}
 
public class CoffeeShop {
  private final List<Order> orders = new CopyOnWriteArrayList<Order>();
  private final Menu menu = new Menu();
 
  void takeOrder(String flavourName, int table) {
    CoffeeFlavour flavour = menu.lookup(flavourName);
    Order order = new Order(table, flavour);
    orders.add(order);
  }
 
  void service() {
    for (Order order : orders) {
      order.serve();
      orders.remove(order);    
    }
  }
 
  String report() {
    return "\ntotal CoffeeFlavour objects made: "
        + menu.totalCoffeeFlavoursMade();
  }
 
  public static void main(String[] args) {
    CoffeeShop shop = new CoffeeShop();
 
    shop.takeOrder("Cappuccino", 2);
    shop.takeOrder("Frappe", 1);
    shop.takeOrder("Espresso", 1);
    shop.takeOrder("Frappe", 897);
    shop.takeOrder("Cappuccino", 97);
    shop.takeOrder("Frappe", 3);
    shop.takeOrder("Espresso", 3);
    shop.takeOrder("Cappuccino", 3);
    shop.takeOrder("Espresso", 96);
    shop.takeOrder("Frappe", 552);
    shop.takeOrder("Cappuccino", 121);
    shop.takeOrder("Espresso", 121);
 
    shop.service();
    System.out.println(shop.report());
  }
}

Example in Scala

/*
https://gist.github.com/pkinsky/111aee2f129c03ff1d0d
run as a script using `scala flyweight.scala`
expected output:
  Serving CoffeeFlavour(Espresso) to table 121
  Serving CoffeeFlavour(Cappuccino) to table 121
  Serving CoffeeFlavour(Frappe) to table 552
  Serving CoffeeFlavour(Espresso) to table 96
  Serving CoffeeFlavour(Cappuccino) to table 3
  Serving CoffeeFlavour(Espresso) to table 3
  Serving CoffeeFlavour(Frappe) to table 3
  Serving CoffeeFlavour(Cappuccino) to table 97
  Serving CoffeeFlavour(Frappe) to table 897
  Serving CoffeeFlavour(Espresso) to table 1
  Serving CoffeeFlavour(Frappe) to table 1
  Serving CoffeeFlavour(Cappuccino) to table 2
  total CoffeeFlavour objects made: 3
*/
 
class CoffeeFlavour(val name: String){
    override def toString = s"CoffeeFlavour($name)"
}
 
object CoffeeFlavour {
    import scala.collection.mutable.Map
    private val cache = Map.empty[String, CoffeeFlavour]
 
    def apply(name: String): CoffeeFlavour =
        cache.getOrElseUpdate(name, new CoffeeFlavour(name))
 
    def totalCoffeeFlavoursMade = cache.size
}
 
 
case class Order(tableNumber: Int, flavour: CoffeeFlavour){
 
  def serve: Unit =
    println(s"Serving $flavour to table $tableNumber")
}
 
object CoffeeShop {
  var orders = List.empty[Order]
 
  def takeOrder(flavourName: String, table: Int) {
    val flavour = CoffeeFlavour(flavourName)
    val order = Order(table, flavour)
    orders = order :: orders
  }
 
  def service: Unit = orders.foreach(_.serve)
 
  def report =
    s"total CoffeeFlavour objects made: ${CoffeeFlavour.totalCoffeeFlavoursMade}"
}
 
 
CoffeeShop.takeOrder("Cappuccino", 2)
CoffeeShop.takeOrder("Frappe", 1)
CoffeeShop.takeOrder("Espresso", 1)
CoffeeShop.takeOrder("Frappe", 897)
CoffeeShop.takeOrder("Cappuccino", 97)
CoffeeShop.takeOrder("Frappe", 3)
CoffeeShop.takeOrder("Espresso", 3)
CoffeeShop.takeOrder("Cappuccino", 3)
CoffeeShop.takeOrder("Espresso", 96)
CoffeeShop.takeOrder("Frappe", 552)
CoffeeShop.takeOrder("Cappuccino", 121)
CoffeeShop.takeOrder("Espresso", 121)
 
CoffeeShop.service
println(CoffeeShop.report)

Example in Ruby

# Flyweight Object
class Lamp
  attr_reader :color
  #attr_reader makes color attribute available outside 
  #of the class by calling .color on a Lamp instance
 
  def initialize(color)
    @color = color
  end
end
 
class TreeBranch
  def initialize(branch_number)
    @branch_number = branch_number
  end
 
  def hang(lamp)
    puts "Hang #{lamp.color} lamp on branch #{@branch_number}"
  end
end
 
# Flyweight Factory
class LampFactory
  def initialize
    @lamps = {}
  end
 
  def find_lamp(color)
    if @lamps.has_key?(color)
      # if the lamp already exists, reference it instead of creating a new one
      lamp = @lamps[color]
    else
      lamp = Lamp.new(color)
      @lamps[color] = lamp
    end
    lamp
  end
 
  def total_number_of_lamps_made
    @lamps.size
  end
end
 
class ChristmasTree
  def initialize
    @lamp_factory = LampFactory.new
    @lamps_hung = 0
 
    dress_up_the_tree
  end
 
  def hang_lamp(color, branch_number)
    TreeBranch.new(branch_number).hang(@lamp_factory.find_lamp(color))
    @lamps_hung += 1
  end
 
  def dress_up_the_tree
    hang_lamp('red', 1)
    hang_lamp('blue', 1)
    hang_lamp('yellow', 1)
    hang_lamp('red', 2)
    hang_lamp('blue', 2)
    hang_lamp('yellow', 2)
    hang_lamp('red', 3)
    hang_lamp('blue', 3)
    hang_lamp('yellow', 3)
    hang_lamp('red', 4)
    hang_lamp('blue', 4)
    hang_lamp('yellow', 4)
    hang_lamp('red', 5)
    hang_lamp('blue', 5)
    hang_lamp('yellow', 5)
    hang_lamp('red', 6)
    hang_lamp('blue', 6)
    hang_lamp('yellow', 6)
    hang_lamp('red', 7)
    hang_lamp('blue', 7)
    hang_lamp('yellow', 7)
    puts "Made #{@lamp_factory.total_number_of_lamps_made} total lamps"
  end
end

Example in Python

By default, instances of Python's new-style classes have a dictionary to store instance data. When there are many instances, the total space consumed by the many dictionaries can be large. Accordingly, new-style classes support a __slots__ class variable to save space. It works by suppressing the instance dictionary and instead making more compact objects with fixed pointers to predefined attributes. This forgoes the ability to easily add new attributes, but saves the space overhead of a sparse hash table for each instance. ^[4]

class Part(object):
 
    __slots__ = ['part_number', 'part_name', 'unit_cost']
 
    def __init__(self, part_number, part_name, unit_cost):
        self.part_number = part_number
        self.part_name = part_name
        self.unit_cost = unit_cost
 
    def total_cost(self, quantity):
        return self.unit_cost * quantity
 
assembly = [(30, Part('XL123', 'xwidget', 1.25)),
            (20, Part('CQ456', 'jcog', 3.75)),
            (12, Part('MT789', 'cgear', 2.50))]
 
print(sum(part.total_cost(quantity) for quantity, part in assembly))

See also

• Copy-on-write
• Memoization
• Multiton

References