Design Pattern -- 05. Creational. Prototype. Pattern (CSharp Sample)

Intent

• Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
• Co-opt one instance of a class for use as a breeder of all future instances.
• ThenewOperator considered harmful.

Problem

Application "hard wires" the class of object to create in each "new" expression.

Discussion

• Declare an abstract base class that specifies a pure virtual "clone" method, and, maintains a dictionary of all "cloneable" concrete derived classes. any class that needs a "polymorphic constructor" capability: derives itself from the abstract base class, registers its prototypical instance, and implementsclone()Operation.
• The client then, instead of writing code that invokes the "new" operator on a hard-wired class name, calla "clone" operation on the abstract base class, supplying a string or enumerated data type that designates the participating concrete derived class desired.

Structure

• The Factory knows how to find the correct Prototype, and each Product knows how to spawn new instances of itself.

Example

• The Prototype pattern specifies the kind of objects to create using a prototypical instance. prototypes of new products are often built prior to full production, but in this example, the prototype is passive and does not participant in copying itself. the mitotic division of a cell-resulting in two identical cells-is an example of a prototype that plays an active role in copying itself and thus, demonstrates the Prototype pattern. when a cell splits, two cells of identical genotvpe result. in other words, the cell clones itself.

Check list

1. Addclone()Method to the existing "product" hierarchy.
2. Design a "registry" that maintains a cache of prototypical objects. The registry cocould be encapsulated in a newFactoryClass, or in the base class of the "product" hierarchy.
3. Design a factory method that: may (or may not) accept arguments, finds the correct prototype object, CILSclone()On that object, and returns the result.
4. The client replaces all references tonewOperator with callto the factory method.

Rules of thumb

• Sometimes creational patterns are competitors: there are cases when either Prototype or Abstract Factory cocould be used properly. at other times they are complementory: Abstract Factory might store a set of Prototypes from which to clone and return product objects. abstract Factory, Builder, and Prototype can use Singleton in their implementations.
• Abstract Factory classes are often implemented with Factory Methods, but they can be implemented using Prototype.
• Factory Method: creation through inheritance. Protoype: creation through delegation.
• Often, designs start out using Factory Method (less complicated, more customizable, subclasses proliferate) and evolve toward Abstract Factory, Protoype, or Builder (more flexible, more complex) as the designer discovers where more flexibility is needed.
• Prototype doesn't require subclassing, but it does require an "initialize" operation. Factory Method requires subclassing, but doesn't require Initialize.
• Designs that make heavy use of the Composite and Decorator patterns often can benefit from Prototype as well.
• Prototype co-opts one instance of a class for use as a breeder of all future instances.
• Prototypes are useful when object initialization is expensive, and you anticipate few variations on the initialization parameters. in this context, Prototype can avoid expensive "creation from scratch", and support cheap cloning of a pre-initialized prototype.
• Prototype is unique among the other creational patterns in that it doesn't require a class-only an object. object-oriented versions like Self and Omega that do away with classes completely rely on prototypes for creating new objects.

CSharp Sample

using System;namespace DoFactory.GangOfFour.Prototype.Structural {   // MainApp test application  class MainApp  {     static void Main()    {      // Create two instances and clone each       ConcretePrototype1 p1 = new ConcretePrototype1("I");      ConcretePrototype1 c1 = (ConcretePrototype1)p1.Clone();      Console.WriteLine ("Cloned: {0}", c1.Id);      ConcretePrototype2 p2 = new ConcretePrototype2("II");      ConcretePrototype2 c2 = (ConcretePrototype2)p2.Clone();      Console.WriteLine ("Cloned: {0}", c2.Id);      // Wait for user       Console.Read();    }  }  // "Prototype"   abstract class Prototype  {    private string id;    // Constructor     public Prototype(string id)    {      this.id = id;    }    // Property     public string Id    {      get{ return id; }    }    public abstract Prototype Clone();  }  // "ConcretePrototype1"   class ConcretePrototype1 : Prototype  {    // Constructor     public ConcretePrototype1(string id) : base(id)     {    }    public override Prototype Clone()    {      // Shallow copy       return (Prototype)this.MemberwiseClone();    }  }  // "ConcretePrototype2"   class ConcretePrototype2 : Prototype  {    // Constructor     public ConcretePrototype2(string id) : base(id)     {    }    public override Prototype Clone()    {      // Shallow copy       return (Prototype)this.MemberwiseClone();    }  }}

Cloned: I Cloned: II