GOF设计模式三: 外观模式 Facade
“现有系统”功能强大、复杂，开发“新系统”需要用到其中一部分，但又要增加一部 分新功能，该怎么办？
4.1 Facade Pattern: Key Features
Intent
You want to simplify how to use an existing system. You need to define your own interface.
现有系统的接口比较复杂，你希望利用原有的功能重新定义新的接口
Problem
You need to use only a subset of a complex system. Or you need to interact with the system in
a particular way.
如果只希望使用现有系统的部分功能、或者以一 种特殊的方式与现有系统交互
Solution
The Facade presents a new interface for the client of the existing system to use.
外观模式提供一套新的接口供客户使用现有系统
Participants and collaborators
It presents a simplified interface to the client that makes it easier to use.
它呈 现给客户一套简单地接口，易于使用。
Consequences
The Facade simplifies the use of the required subsystem. However, because the Facade is
not complete, certain functionality may be unavailable to the client.
现有系统的部分功能，客户可能会不可用
Implementation
Define a new class (or classes) that has the required interface. Have this new class use the
existing system.
新定义的类，使用了现有系统的功能 ，为客户提供所需要的接口
4.1 Facade Pattern: Key Features
动机 Motivation
最小化通信 Minimize communication
最小化依赖 Minimize dependencies
4.2 外观模式的比喻 Non-software examples
例如，贸易公司的客户通过目录下订单，就类似一个“外观模式
客户联系公司的客户联系代表
客户联系代表 就作为“外观”，为客户提供接口，完成下单、付费、运 输等公司内部复杂的功能
客户很轻松完成任务
4.3 外观模式与适配器模式的比较
外观定义了新的接口，适配器使用旧的接口
Facade defines a new interface, whereas Adapter uses an old interface
适配器使得两个不一致的接口协同工作，而不是定义一个新的
Adapter makes two existing interfaces work together as opposed to defining an entirely new one
外观模式的本意是产生一个轻便的接口，适配器的本意是把现有的接口转换 一下
The intent of Facade is to produce a simpler interface, and the intent of Adapter is to design to
an existing interface
一个外观接口可能包装了多个现有系统的对象、也可能增加了一些新功能， 而适配器只是包装一个对象 While Facade routinely wraps multiple objects and Adapter wraps a single object
多数情况下，外观是单实例的
Facade objects are often Singletons because only one Facade object is required
GRASP原则的外观控制器，就是外观模式
4.4 适用性 Applicability
简化复杂系统的使用 Provide a simple interface to a complex subsystem
You do not need to use all the functionality of a complex system and can create a new class
that contains all the rules for accessing that system
If this is a subset of the original system, as it usually is, the API that you create for the new
class should be much simpler than the original system's API
在使用复杂系统的某个功能时，可以加入新功能
You want to use the functionality of the original system and want to add some new functionality as well
为客户软件层提供一个接口，隐藏了现有系统的细节
Provide an interface to a software layer
You want to encapsulate or hide the original system
使客户与现有系统之间减少耦合、增加独立性、可移植性
节约成本
开发新的外观类的成本，要比每个人都去熟悉现有系统、了解如何使用现有系统的功能 要节省许多