The flyweight pattern is used whenever you have large amount of small objects
that share common information.
The use of the pattern reduces the storage of those objects.
The flyweight distinguishes between inner state (sometime called intrinsic state)
and outer state (sometime called extrinsic state) of the object.
The inner state can be shared by the objects and therefore minimize the amount
of storage needed by the objects.
The outer state can be computed or stored outside the objects and are given to
the objects whenever it's needed.
For a UML diagram of the pattern go to
dofactory site.
Flyweight C# Example
Lets look at an example for the use of flyweight pattern:
#region Helper
public enum eNumbers
{
One,
Two,
Three,
// .. and so on with every number
}
#endregion
#region The Flyweight Factory
public class NumbersFactory
{
#region Members
private Dictionary<eNumbers, Number> _numbers =
new Dictionary<eNumbers, Number>();
#endregion
#region Methods
public Number GetNumber(eNumbers number)
{
if (!_numbers.ContainsKey(number))
{
switch (number)
{
case (eNumbers.One):
{
_numbers.Add(number, new One());
break;
}
case (eNumbers.Two):
{
_numbers.Add(number, new Two());
break;
}
case (eNumbers.Three):
{
_numbers.Add(number, new Three());
break;
}
// ... and so on with every number
default:
{
break;
}
}
}
return _numbers[number];
}
#endregion
}
#endregion
#region The Flyweight
public abstract class Number
{
#region Members
protected int _number;
protected string _numberName;
protected int _numberSize;
#endregion
#region Methods
public abstract void WriteNumber(int numberSize);
#endregion
}
#endregion
#region The Flyweight Concrete
public class One : Number
{
#region Ctor
public One()
{
this._number = 1;
this._numberName = "One";
}
#endregion
#region Methods
public override void WriteNumber(int numberSize)
{
this._numberSize = numberSize;
Console.WriteLine(string.Format("{0} is size {1}",
_numberName, _numberSize));
}
#endregion
}
public class Two : Number
{
#region Ctor
public Two()
{
this._number = 2;
this._numberName = "Two";
}
#endregion
#region Methods
public override void WriteNumber(int numberSize)
{
this._numberSize = numberSize;
Console.WriteLine(string.Format("{0} is size {1}",
_numberName, _numberSize));
}
#endregion
}
public class Three : Number
{
#region Ctor
public Three()
{
this._number = 3;
this._numberName = "Three";
}
#endregion
#region Methods
public override void WriteNumber(int numberSize)
{
this._numberSize = numberSize;
Console.WriteLine(string.Format("{0} is size {1}",
_numberName, _numberSize));
}
#endregion
}
// ... Four, Five and so on
#endregion
The example include 3 parts - the flyweight factory (NumberFactory), the flyweight
itself (Number) and flyweight concretes (One, Two, Three). You can see that lazy loading
is used in the factory. Whenever a number is needed and isn't in the factory I build it and
then return it in the GetNumber method.
Summary
To sum up, use the flyweight when you have a large amount of objects that
consume large amount of memory. Also use the pattern when you have groups of
objects that share common state.
From my experience, the pattern is rarely used. Even though, you should know the
pattern in order to use it when it is needed
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