外觀模式

外觀模式（Facade pattern），是軟件工程中常用的一種軟件設計模式，它為子系統中的一組介面提供一個統一的高層介面，使得子系統更容易使用。

結構

Facade: 這個外觀類為子系統中Packages 1、2、3提供一個共同的對外介面（接口）
Clients: 客戶對象通過一個外觀介面讀寫子系統中各介面的數據資源。
Packages: 客戶可以通過外觀介面讀取的内部庫。

示例

Java

這是一個抽象的範例。一個客戶“you”通過外觀介面“computer”獲取計算機内部複雜的系統信息。

/* 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();
	}
}

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();
    }
  }
}

C++

class CPU {
public:
	void freeze() { ... }
	void jump(long position) { ... }
	void execute() { ... }
}

class Memory {
public:
	void load(long position, char* data) {
		...
	}
}

class HardDrive {
public:
	char* 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:
	void start(String[] args) {
		Computer facade = new Computer();
		facade.startComputer();
	}
}

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