• It uses a shared mechanism to hold and return a large number of objects efficiently.
  • Shared flyweight objects are immutable.
• Examples include, characters and line-styles in a word processor or digit receivers in a public switched telephone network application.
  • You will find flyweights mostly in utility-type applications (word processors, graphics programs, network apps).
  • They are rarely used in data-driven business type applications.
  • Flyweights are used internally in the .NET Framework as a string management technique to minimize memory usage for immutable strings.

Participants

• Flyweight (IApiEndPoint)
  • Declares an interface through which flyweights can receive and act on extrinsic state.
• ConcreteFlyweight (EndPoint1, EndPoint2)
  • Implements the Flyweight interface and adds storage for intrinsic state, if any.
  • This object must be shareable.
  • Any state it stores must be independent of the ConcreteFlyweight object's context.
• UnsharedConcreteFlyweight (Not Used)
  • Not all Flyweight sub-classes need to be shared.
  • The Flyweight interface enables sharing, but it doesn't enforce it.
• FlyweightFactory (ApiFactory)
  • Creates and manages flyweight objects.
  • Ensures that flyweight are shared properly.
    • When a client requests a flyweight, it supplies an existing instance or creates one, if none exists.

var factory = new ApiFactory();

var endPoint1 = factory.GetEndPoint("EndPoint1");
var client1 = endPoint1.GetClient("Api/Test1");
var endPoint2 = factory.GetEndPoint("EndPoint2");
var client2 = endPoint1.GetClient("Api/Test2");

public class ApiFactory
{
	private Dictionary<string, IApiEndPoint> _EndPoints = new Dictionary<string, IApiEndPoint>();

	public IApiEndPoint GetEndPoint(string name)
	{
		IApiEndPoint endPoint = null;
		if (_EndPoints.ContainsKey(name))
			endPoint = _EndPoints[name];
		else
		{
			switch (name)
			{
				case "EndPoint1": endPoint = new EndPoint1(); break;
				case "EndPoint2": endPoint = new EndPoint2(); break;
			}
			_EndPoints.Add(name, endPoint);
		}
		return endPoint;
	}
}

public interface IApiEndPoint
{
	string Name { get; set; }
	string Url { get; set; }
	HttpClient GetClient(string route);
}

public class EndPoint1 : IApiEndPoint
{
	public string Name { get; set; } = "Name1";
	public string Url { get; set; } "www.Test1.com";

	public HttpClient GetClient(string route) => throw new NotImplementedException();
}

public class EndPoint2 : IApiEndPoint
{
	public string Name { get; set; } = "Name2";
	public string Url { get; set; } = "www.Test2.com";

	public HttpClient GetClient(string route) => throw new NotImplementedException();
}

Rules of Thumb

• Whereas Flyweight shows how to make lots of little objects, Facade shows how to make a single object represent an entire subsystem.
• Flyweight is often combined with Composite to implement shared leaf nodes.
• Terminal symbols within Interpreter's abstract syntax tree can be shared with Flyweight.
• Flyweight explains when and how State objects can be shared.