Facade pattern

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The Facade pattern is a software engineering design pattern commonly used with Object-oriented programming.

A facade is an object that provides a simplified interface to a larger body of code, such as a class library. A facade can:

  • make a software library easier to use and understand, since the facade has convenient methods for common tasks;
  • make code that uses the library more readable, for the same reason;
  • reduce dependencies of outside code on the inner workings of a library, since most code uses the facade, thus allowing more flexibility in developing the system;
  • wrap a poorly-designed collection of APIs with a single well-designed API (as per task needs).

An Adapter is used when the wrapper must respect a particular interface and must support a polymorphic behavior. On the other hand, a facade is used when one wants an easier or simpler interface to work with.

Contents

[edit] Structure

Image:FacadeDesignPattern.png

Facade
The facade class interacts Packages 1, 2, and 3 with the rest of the application.
Clients
The objects using the Facade Pattern to access resources from the Packages.
Packages
Software library / API collection accessed through the Facade Class.

[edit] Examples

[edit] Java

This is an abstract example of how a client ("you") interacts with a facade (the "computer") to a complex system (internal computer parts, like CPU and HardDrive).

/* Complex parts */
 
class CPU {
	public void freeze() { ... }
	public void jump(long position) { ... }
	public void execute() { ... }
}
 
class Memory {
	public void load(long position, byte[] data) {
		...
	}
}
 
class HardDrive {
	public byte[] read(long lba, int size) {
		...
	}
}
 
/* Façade */
 
class Computer {
	public void startComputer() {
		cpu.freeze();
		memory.load(BOOT_ADDRESS, hardDrive.read(BOOT_SECTOR, SECTOR_SIZE));
		cpu.jump(BOOT_ADDRESS);
		cpu.execute();
	}
}
 
/* Client */
 
class You {
	public static void main(String[] args) {
		Computer facade = new Computer();
		facade.startComputer();
	}
}

[edit] C#

// Facade pattern -- Structural example  
 
using System;
 
namespace DoFactory.GangOfFour.Facade.Structural
{
 
  // Mainapp test application 
 
  class MainApp
  {
    public static void Main()
    {
      Facade facade = new Facade();
 
      facade.MethodA();
      facade.MethodB();
 
      // Wait for user 
      Console.Read();
    }
  }
 
  // "Subsystem ClassA" 
 
  class SubSystemOne
  {
    public void MethodOne()
    {
      Console.WriteLine(" SubSystemOne Method");
    }
  }
 
  // Subsystem ClassB" 
 
  class SubSystemTwo
  {
    public void MethodTwo()
    {
      Console.WriteLine(" SubSystemTwo Method");
    }
  }
 
  // Subsystem ClassC" 
 
  class SubSystemThree
  {
    public void MethodThree()
    {
      Console.WriteLine(" SubSystemThree Method");
    }
  }
 
  // Subsystem ClassD" 
 
  class SubSystemFour
  {
    public void MethodFour()
    {
      Console.WriteLine(" SubSystemFour Method");
    }
  }
 
  // "Facade" 
 
  class Facade
  {
    SubSystemOne one;
    SubSystemTwo two;
    SubSystemThree three;
    SubSystemFour four;
 
    public Facade()
    {
      one = new SubSystemOne();
      two = new SubSystemTwo();
      three = new SubSystemThree();
      four = new SubSystemFour();
    }
 
    public void MethodA()
    {
      Console.WriteLine("\nMethodA() ---- ");
      one.MethodOne();
      two.MethodTwo();
      four.MethodFour();
    }
 
    public void MethodB()
    {
      Console.WriteLine("\nMethodB() ---- ");
      two.MethodTwo();
      three.MethodThree();
    }
  }
}

[edit] External links

v • d • e
Design patterns in the book Design Patterns
Creational
Structural
Adapter · Bridge · Composite · Decorator · Facade · Flyweight · Proxy
Behavioral
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