The
Flyweight pattern has a single purpose: minimizing memory intake. If your program doesn’t struggle with a shortage of RAM, then you might just ignore this pattern for a while.
Let's create a
Car.cs class.
public class Car
{
public string Owner { get; set; }
public string Number { get; set; }
public string Company { get; set; }
public string Model { get; set; }
public string Color { get; set; }
}
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.
getKey(Car key) returns a Flyweight's string hash for a given state.
GetFlyweight(Car sharedState) Returns an existing
Flyweight with a given state or creates a new one.
public class FlyweightFactory
{
private List<Tuple<Flyweight, string>> flyweights = new List<Tuple<Flyweight, string>>();
public FlyweightFactory(params Car[] args)
{
foreach (var elem in args)
{
flyweights.Add(new Tuple<Flyweight, string>(new Flyweight(elem), this.getKey(elem)));
}
}
public string getKey(Car key)
{
List<string> elements = new List<string>();
elements.Add(key.Model);
elements.Add(key.Color);
elements.Add(key.Company);
if (key.Owner != null && key.Number != null)
{
elements.Add(key.Number);
elements.Add(key.Owner);
}
elements.Sort();
return string.Join("_", elements);
}
public Flyweight GetFlyweight(Car sharedState)
{
string key = this.getKey(sharedState);
if (flyweights.Where(t => t.Item2 == key).Count() == 0)
{
Console.WriteLine("FlyweightFactory: Can't find a flyweight, creating new one.");
this.flyweights.Add(new Tuple<Flyweight, string>(new Flyweight(sharedState), key));
}
else
{
Console.WriteLine("FlyweightFactory: Reusing existing flyweight.");
}
return this.flyweights.Where(t => t.Item2 == key).FirstOrDefault().Item1;
}
public void listFlyweights()
{
var count = flyweights.Count;
Console.WriteLine($"\nFlyweightFactory: I have {count} flyweights:");
foreach (var flyweight in flyweights)
{
Console.WriteLine(flyweight.Item2);
}
}
}
The Flyweight class stores a common portion of the state (also called intrinsic state) that belongs to multiple real business entities.
The Flyweight accepts the rest of the state (extrinsic state, unique for each entity) via its method parameters.
using System;
using Newtonsoft.Json;
public class Flyweight
{
private Car _sharedState;
public Flyweight(Car car)
{
this._sharedState = car;
}
public void Operation(Car uniqueState)
{
string s = JsonConvert.SerializeObject(this._sharedState);
string u = JsonConvert.SerializeObject(uniqueState);
Console.WriteLine($"Flyweight: Displaying shared {s} and unique {u} state.");
}
}
Now let's put this altogether in the Program class.
First we add a client function **addCarToPoliceDatabase**
The client code either stores or calculates extrinsic state and passes it to the flyweight's methods.
public static void addCarToPoliceDatabase(FlyweightFactory factory, Car car)
{
Console.WriteLine("\nClient: Adding a car to database.");
var flyweight = factory.GetFlyweight(new Car
{
Color = car.Color,
Model = car.Model,
Company = car.Company
});
flyweight.Operation(car);
}
Now let's create the Main method
The client code usually creates a bunch of pre-populated flyweights in the initialization stage of the application.
static void Main(string[] args)
{
var factory = new FlyweightFactory(
new Car { Company = "Chevrolet", Model = "Camaro2018", Color = "pink" },
new Car { Company = "Mercedes Benz", Model = "C300", Color = "black" },
new Car { Company = "Mercedes Benz", Model = "C500", Color = "red" },
new Car { Company = "BMW", Model = "M5", Color = "red" },
new Car { Company = "BMW", Model = "X6", Color = "white" }
);
factory.listFlyweights();
addCarToPoliceDatabase(factory, new Car {
Number = "CL234IR",
Owner = "James Doe",
Company = "BMW",
Model = "M5",
Color = "red"
});
addCarToPoliceDatabase(factory, new Car {
Number = "CL234IR",
Owner = "James Doe",
Company = "BMW",
Model = "X1",
Color = "red"
});
factory.listFlyweights();
}
Let's compile and run. We should get:
FlyweightFactory: I have 5 flyweights:
Camaro2018_Chevrolet_pink
black_C300_Mercedes Benz
C500_Mercedes Benz_red
BMW_M5_red
BMW_white_X6
Client: Adding a car to database.
FlyweightFactory: Reusing existing flyweight.
Flyweight: Displaying shared {"Owner":null,"Number":null,"Company":"BMW","Model":"M5","Color":"red"} and unique {"Owner":"James Doe","Number":"CL234IR","Company":"BMW","Model":"M5","Color":"red"} state.
Client: Adding a car to database.
FlyweightFactory: Can't find a flyweight, creating new one.
Flyweight: Displaying shared {"Owner":null,"Number":null,"Company":"BMW","Model":"X1","Color":"red"} and unique {"Owner":"James Doe","Number":"CL234IR","Company":"BMW","Model":"X1","Color":"red"} state.
FlyweightFactory: I have 6 flyweights:
Camaro2018_Chevrolet_pink
black_C300_Mercedes Benz
C500_Mercedes Benz_red
BMW_M5_red
BMW_white_X6
BMW_red_X1
The Ray Code is AWESOME!!!
[Wikipedia](https://en.wikipedia.org/wiki/Flyweight_pattern)
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