设计模式之组件模式

Composite Design Pattern

definition Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. Frequency of use:   medium high

UML class diagram

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participants

    The classes and/or objects participating in this pattern are:

• Component   (DrawingElement)
  • declares the interface for objects in the composition.
  • implements default behavior for the interface common to all classes, as appropriate.
  • declares an interface for accessing and managing its child components.
  • (optional) defines an interface for accessing a component's parent in the recursive structure, and implements it if that's appropriate.
• Leaf   (PrimitiveElement)
  • represents leaf objects in the composition. A leaf has no children.
  • defines behavior for primitive objects in the composition.
• Composite   (CompositeElement)
  • defines behavior for components having children.
  • stores child components.
  • implements child-related operations in the Component interface.
• Client  (CompositeApp)
  • manipulates objects in the composition through the Component interface.

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sample code in C#

This structural code demonstrates the Composite pattern which allows the creation of a tree structure in which individual nodes are accessed uniformly whether they are leaf nodes or branch (composite) nodes.

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// Composite pattern -- Structural example

using System; using System.Collections; namespace DoFactory.GangOfFour.Composite.Structural {    // MainApp test application    class MainApp   {      static void Main()     {        // Create a tree structure       Composite root = new Composite("root");       root.Add( new Leaf("Leaf A"));       root.Add( new Leaf("Leaf B"));       Composite comp = new Composite("Composite X");       comp.Add( new Leaf("Leaf XA"));       comp.Add( new Leaf("Leaf XB"));       root.Add(comp);       root.Add( new Leaf("Leaf C"));        // Add and remove a leaf       Leaf leaf = new Leaf("Leaf D");       root.Add(leaf);       root.Remove(leaf);        // Recursively display tree       root.Display(1);        // Wait for user       Console.Read();     }   }    // "Component"    abstract class Component   {      protected string name;      // Constructor      public Component( string name)     {        this.name = name;     }      public abstract void Add(Component c);      public abstract void Remove(Component c);      public abstract void Display( int depth);   }    // "Composite"    class Composite : Component   {      private ArrayList children = new ArrayList();      // Constructor      public Composite( string name) : base(name)     {       }      public override void Add(Component component)     {       children.Add(component);     }      public override void Remove(Component component)     {       children.Remove(component);     }      public override void Display( int depth)     {       Console.WriteLine( new String('-', depth) + name);        // Recursively display child nodes        foreach (Component component in children)       {         component.Display(depth + 2);       }     }   }    // "Leaf"    class Leaf : Component   {      // Constructor      public Leaf( string name) : base(name)     {       }      public override void Add(Component c)     {       Console.WriteLine("Cannot add to a leaf");     }      public override void Remove(Component c)     {       Console.WriteLine("Cannot remove from a leaf");     }      public override void Display( int depth)     {       Console.WriteLine( new String('-', depth) + name);     }   } }

Output -root ---Leaf A ---Leaf B ---Composite X -----Leaf XA -----Leaf XB ---Leaf C

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This real-world code demonstrates the Composite pattern used in building a graphical tree structure made up of primitive nodes (lines, circles, etc) and composite nodes (groups of drawing elements that make up more complex elements).

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// Composite pattern -- Real World example

using System; using System.Collections; namespace DoFactory.GangOfFour.Composite.RealWorld {       // Mainapp test application    class MainApp   {      static void Main()     {        // Create a tree structure       CompositeElement root =          new CompositeElement("Picture");       root.Add( new PrimitiveElement("Red Line"));       root.Add( new PrimitiveElement("Blue Circle"));       root.Add( new PrimitiveElement("Green Box"));       CompositeElement comp =          new CompositeElement("Two Circles");       comp.Add( new PrimitiveElement("Black Circle"));       comp.Add( new PrimitiveElement("White Circle"));       root.Add(comp);        // Add and remove a PrimitiveElement       PrimitiveElement pe =          new PrimitiveElement("Yellow Line");       root.Add(pe);       root.Remove(pe);        // Recursively display nodes       root.Display(1);        // Wait for user       Console.Read();     }   }    // "Component" Treenode    abstract class DrawingElement   {      protected string name;      // Constructor      public DrawingElement( string name)     {        this.name = name;     }      public abstract void Add(DrawingElement d);      public abstract void Remove(DrawingElement d);      public abstract void Display( int indent);   }    // "Leaf"    class PrimitiveElement : DrawingElement   {      // Constructor      public PrimitiveElement( string name) : base(name)     {       }      public override void Add(DrawingElement c)     {       Console.WriteLine(         "Cannot add to a PrimitiveElement");     }      public override void Remove(DrawingElement c)     {       Console.WriteLine(         "Cannot remove from a PrimitiveElement");     }      public override void Display( int indent)     {       Console.WriteLine(          new String('-', indent) + " " + name);     }   }    // "Composite"    class CompositeElement : DrawingElement   {      private ArrayList elements = new ArrayList();         // Constructor      public CompositeElement( string name) : base(name)     {       }      public override void Add(DrawingElement d)     {       elements.Add(d);     }      public override void Remove(DrawingElement d)     {       elements.Remove(d);     }      public override void Display( int indent)     {       Console.WriteLine( new String('-', indent) +         "+ " + name);        // Display each child element on this node        foreach (DrawingElement c in elements)       {         c.Display(indent + 2);       }     }   } }

Output -+ Picture --- Red Line --- Blue Circle --- Green Box ---+ Two Circles ----- Black Circle ----- White Circle