结构型模式 外观模式

   Definition

Provide a unified interface to a set of interfaces in a subsystem. Façade defines a higher-level interface that makes the subsystem easier to use.

UML class diagram

   Participants

The classes and/or objects participating in the Façade pattern are: Facade   (MortgageApplication) knows which subsystem classes are responsible for a request. delegates client requests to appropriate subsystem objects. Subsystem classes   (Bank, Credit, Loan) implement subsystem functionality. handle work assigned by the Facade object. have no knowledge of the facade and keep no reference to it.

Sample code in C# 
   

This structural code demonstrates the Facade pattern which provides a simplified and uniform interface to a large subsystem of classes.

// Facade pattern -- Structural example

using System;

// "SubSystem"

class SubSystemOne
{
  public void MethodOne()
  {
    Console.WriteLine("SubSystemOne Method");
  }
}

class SubSystemTwo
{
  public void MethodTwo()
  {
    Console.WriteLine("SubSystemTwo Method");
  }
}

class SubSystemThree
{
  public void MethodThree()
  {
    Console.WriteLine("SubSystemThree Method");
  }
}

class SubSystemFour
{
  public void MethodFour()
  {
    Console.WriteLine("SubSystemFour Method");
  }
}

// "Facade"

class Facade
{
  // Fields
  SubSystemOne one;
  SubSystemTwo two;
  SubSystemThree three;
  SubSystemFour four;

  // Constructors
  public Facade()
  {
    one = new SubSystemOne();
    two = new SubSystemTwo();
    three = new SubSystemThree();
    four = new SubSystemFour();
  }

  // Methods
  public void MethodA()
  {
    Console.WriteLine( "\nMethodA() ---- " );
    one.MethodOne();
    two.MethodTwo();
    four.MethodFour();
  }

  public void MethodB()
  {
    Console.WriteLine( "\nMethodB() ---- " );
    two.MethodTwo();
    three.MethodThree();
  }
}

/// <summary>
/// Client test
/// </summary>
public class Client
{
  public static void Main(string[] args)
  {
    // Create and call Facade
    Facade f = new Facade();
    f.MethodA();
    f.MethodB();

  }
}

Output

MethodA() ----
SubSystemOne Method
SubSystemTwo Method
SubSystemFour Method

MethodB() ----
SubSystemTwo Method
SubSystemThree Method


---

This real-world code demonstrates the Facade pattern as a MortgageApplication object which provides a simplified interface to a large subsystem of classes measuring the creditworthyness of an applicant.

// Facade pattern -- Real World example

using System;

// "SubSystem ClassA"

class Bank
{
  // Methods
  public bool SufficientSavings( Customer c )
  {
    Console.WriteLine("Check bank for {0}", c.Name );
    return true;
  }
}

// "SubSystem ClassB"

class Credit
{
  // Methods
  public bool GoodCredit( int amount, Customer c )
  {
    Console.WriteLine( "Check credit for {0}", c.Name );
    return true;
  }
}

// "SubSystem ClassC"

class Loan
{
  // Methods
  public bool GoodLoan( Customer c )
  {
    Console.WriteLine( "Check loan for {0}", c.Name );
    return true;
  }
}

class Customer
{
  // Fields
  private string name;

  // Constructors
  public Customer( string name )
  {
    this.name = name;
  }

  // Properties
  public string Name
  {
    get{ return name; }
  }
}

// "Facade"

class MortgageApplication
{
  // Fields
  int amount;
  private Bank bank = new Bank();
  private Loan loan = new Loan();
  private Credit credit = new Credit();

  // Constructors
  public MortgageApplication( int amount )
  {
    this.amount = amount;
  }

  // Methods
  public bool IsEligible( Customer c )
  {
    // Check creditworthyness of applicant
    if( !bank.SufficientSavings( c ) ) 
      return false;
    if( !loan.GoodLoan( c ) ) 
      return false;
    if( !credit.GoodCredit( amount, c )) 
      return false;

    return true;
  }
}
/// <summary>
/// Facade Client
/// </summary>
public class FacadeApp
{
  public static void Main(string[] args)
  {
    // Create Facade
    MortgageApplication mortgage =
                new MortgageApplication( 125000 );
    // Call subsystem through Facade
    mortgage.IsEligible(
             new Customer( "Gabrielle McKinsey" ) );

  }
}

Output

Check bank for Gabrielle McKinsey
Check loan for Gabrielle McKinsey
Check credit for Gabrielle McKinsey