设计模式之什么是设计模式

What is a Design Pattern?
什么是设计模式
    Christopher Alexander says, "Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice" [AIS+77, page x]. Even though Alexander was talking about patterns in buildings and towns, what he says is true about object-oriented design patterns. Our solutions are expressed in terms of objects and interfaces instead of walls and doors, but at the core of both kinds of patterns is a solution to a problem in a context.

   Christopher Alexander 说：每种模式描述了在我们周围不断重复发生的问题及他们的核心解决方案，你可以反反复复的使用这些方案，不用重复劳动。尽管 Alexander 讲述的是城市和建筑的模式，然而，这些思想也适用于面向对象的设计模式，只不过是我们的解决方案是用对象和接口代替了墙和门，这种模式的核心是提供了解决相关问题的方案。
   In general, a pattern has four essential elements:
   通常，一个模式有四个必要元素：
  1. The pattern name is a handle we can use to describe a design problem, its solutions, and consequences in a word or two. Naming a pattern immediately increases our design vocabulary. It lets us design at a higher level of abstraction. Having a vocabulary for patterns lets us talk about them with our colleagues, in our documentation, and even to ourselves. It makes it easier to think about designs and to communicate them and their trade-offs to others. Finding good names has been one of the hardest parts of developing our catalog.
  1.模式名称，模式名称是一个标识符，用一两个单词来描述设计问题，解决方案和结果。为新的模式起名称，能够增加我们的设计词汇量。它使我们的设计达到了一个抽象的高度。设计词汇表可以用来和同事交流，书写文档甚至有助于自己整理模式。模式名有助于我们思考，表达我们的设计思想以及和其他人交流我们的设计思想。找到一个好的名称是已经是我们设计模式编目的难点之一。
  2.The problem describes when to apply the pattern. It explains the problem and its context. It might describe specific design problems such as how to represent algorithms as objects. It might describe class or object structures that are symptomatic of an inflexible design. Sometimes the problem will include a list of conditions that must be met before it makes sense to apply the pattern.
  2.问题，问题描述了应用模式的场景。它解释了问题及他的背景。问题可以描述一个特殊的设计问题，例如,怎么用对象来表示运算法则。他也可能描术了产生不灵活设计的类或者结构。有时候，问题可能包括一系列应用这些模式的条件。
  3.The solution describes the elements that make up the design, their relationships, responsibilities, and collaborations. The solution doesn't describe a particular concrete design or implementation, because a pattern is like a template that can be applied in many different situations. Instead, the pattern provides an abstract description of a design problem and how a general arrangement of elements (classes and objects in our case) solves it.
  3.解决方案：解决方案描述了模式的组成成分及它们的相互关系，职责和协作。因为模式是一个模版，它被应用在许多不同的场景中，所以解决方案并不解决具体的设计或实现。而是对设计问题进行抽象描述和要具有一般意义的元素组合（类和对象的组合）。
  4.The consequences are the results and trade-offs of applying the pattern. Though consequences are often unvoiced when we describe design decisions, they are critical for evaluating design alternatives and for understanding the costs and benefits of applying the pattern. The consequences for software often concern space and time trade-offs. They may address language and implementation issues as well. Since reuse is often a factor in object-oriented design, the consequences of a pattern include its impact on a system's flexibility, extensibility, or portability. Listing these consequences explicitly helps you understand and evaluate them.
  效果 描述了模式应用的效果及使用模式应权衡的问题。尽管我们描述
设计决策时，并不总提到模式效果，但它们对于评价设计选择和理解使用模式的代价及好处具有重要意义。软件效果大多关注对时间和空间的衡量，它们也表述了语言和实现问题。因为复用是面向对象设计的要素之一，所以模式效果包括它对系统的灵活性、扩充性或可移植性的影响，显式地列出这些效果对理解和评价这些模式很有帮助。
    Point of view affects one's interpretation of what is and isn't a pattern. One person's pattern can be another person's primitive building block. For this book we have concentrated on patterns at a certain level of abstraction. Design patterns are not about designs such as linked lists and hash tables that can be encoded in classes and reused as is. Nor are they complex, domain-specific designs for an entire application or subsystem. The design patterns in this book are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context.
  出发点的不同会产生对什么是模式和什么不是模式的理解不同。一个人的模式对另一个人来说可能只是基本构造部件。本书中我们将在一定的抽象层次上讨论模式。《设计模式》并不描述链表和h a s h表那样的设计，尽管它们可以用类来封装，也可复用；也不包括那些复杂的、特定领域内的对整个应用或子系统的设计。本书中的设计模式是对被用来在特定场景下解决一般设计问题的类和相互通信的对象的描述。
   A design pattern names, abstracts, and identifies the key aspects of a common design structure that make it useful for creating a reusable object-oriented design. The design pattern identifies the participating classes and instances, their roles and collaborations, and the distribution of responsibilities. Each design pattern focuses on a particular object-oriented design problem or issue. It describes when it applies, whether it can be applied in view of other design constraints, and the consequences and trade-offs of its use. Since we must eventually implement our designs, a design pattern also provides sample C++ and (sometimes) Smalltalk code to illustrate an implementation.
   一个设计模式命名、抽象和确定了一个通用设计结构的主要方面，这些设计结构能被用来构造可复用的面向对象设计。设计模式确定了所包含的类和实例，它们的角色、协作方式以及职责分配。每一个设计模式都集中于一个特定的面向对象设计问题或设计要点，描述了什么时候使用它，在另一些设计约束条件下是否还能使用，以及使用的效果和如何取舍。既然我们最终要实现设计，设计模式还提供了C++ 和S m a l l t a l k示例代码来阐明其实现。