門面模式 Facade Pattern
一、定義：
門面模式（Facade Pattern），是指提供一個統一的介面去訪問多個子系統的多個不同的介面，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。
二、門面模式的作用：
參考文章： The facade pattern (also spelled façade) is a software-design pattern commonly used with object-oriented programming. Analogous to a facade in architecture, a facade is an object that serves as a front-facing interface masking more complex underlying or structural code. A facade can:
1、improve the readability and usability of a software library by masking interaction with more complex components behind a single (and often simplified) API； 2、provide a context-specific interface to more generic functionality (complete with context-specific input validation)； 3、serve as a launching point for a broader refactor of monolithic or tightly-coupled systems in favor of more loosely-coupled code；
Developers often use the facade design pattern when a system is very complex or difficult to understand because the system has a large number of interdependent classes or because its source code is unavailable. This pattern hides the complexities of the larger system and provides a simpler interface to the client. It typically involves a single wrapper class that contains a set of members required by the client. These members access the system on behalf of the facade client and hide the implementation details.
簡而言之就是門面模式主要有三個作用： 提高了可讀性和可用性，遮蔽系統內部複雜的邏輯； 提供了一個更通用的介面； 支援更多鬆散耦合程式碼；
三、UML圖：
客戶端的呼叫變得非常簡單明瞭。
四、設計模式使用例項：
1、設計組成一臺電腦的各個元件：
/*
* 電腦CPU
*/
public class CPU {
public void freeze() {
//...
}
public void jump(long position) {
//...
}
public void execute() {
//...}
}
}
/*
* 硬體驅動
*/
class HardDrive {
public byte[] read(long lba, int size) {
//...
}
}
/*
* 記憶體
*/
class Memory {
public void load(long position, byte[] data) { ... }
}
2、抽取一個門面介面，便於擴充套件（不僅僅在於電腦的啟動，手機和Pad亦然如此）
/*
* 抽取一個門面介面
*/
interface IComputerFacede{
public void start();
}
3、實現真正被客戶端呼叫的門面物件
/*
* 門面物件
*/
class ComputerFacade implements IComputerFacede {
private CPU cpu;
private Memory memory;
private HardDrive hardDrive;
public ComputerFacade() {
this.cpu = new CPU();
this.memory = new Memory();
this.hardDrive = new HardDrive();
}
public void start() {
cpu.freeze();
memory.load(BOOT_ADDRESS, hardDrive.read(BOOT_SECTOR, SECTOR_SIZE));
cpu.jump(BOOT_ADDRESS);
cpu.execute();
}
}
4、客戶端呼叫
/*
* 客戶端呼叫
*/
class Client {
/* 只要簡單的一個呼叫，就可以完成電腦的啟動，無需瞭解內部邏輯 */
public static void main(String[] args) {
ComputerFacade computer = new ComputerFacade();
computer.start();
}
}