设计模式只是一把锤子

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"谈起《设计模式》，那是几乎无人不知，无人不晓，大名鼎鼎的GoF的惊世之作，真是“平生不识GoF，学尽设计也枉然”！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"然而，设计模式真的是软件设计的“瑞士军刀”，切、削、锯、钻样样精通吗？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"读过《设计模式》的读者估计不少，但真正注意过《设计模式》的副标题的估计很少，而这个副标题却是避免误解设计模式的关键。《设计模式》的副标题是：可复用面向对象软件的基础！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"不要小看了这短短的一句话，如果你没有看这句话，或者只是一扫而过并没有仔细体会，那么你很可能就认为设计模式是一把“瑞士军刀”，能够解决所有的设计问题；而实际上“设计模式只是一把锤子”，有句谚语叫作“如果你手里有一把锤子，那么所有的问题都变成了钉子”，如果你拿着设计模式这把锤子到处去敲，要么东西被敲坏，要么就不起作用。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为什么说设计模式只是一把锤子呢？我们还是从副标题来看。《设计模式》的副标题揭示了设计模式的两个主要约束。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"设计模式解决的是“可复用”的设计问题；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"设计模式应用的领域是“面向对象”。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"相信经过我这么一提醒，大家基本上都能够明白了为什么说“设计模式只是一把锤子”了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"设计模式只能解决“可复用”的设计问题，其他的例如性能设计、可靠性设计、安全性设计、可服务性设计等都不是设计模式能够解决的；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"设计模式只是在面向对象的语言中应用，如果是非面向对象的语言，那么就不怎么好用了。当然，你可以在C语言中应用设计模式，但毕竟要有不少折腾，用起来也不那么得心应手。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以，当你遇到一个问题就想到设计模式的时候，一定要注意“设计模式只是一把锤子”，不要拿着这把锤子到处去敲！"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"知易行难—设计模式应用的问题"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"形而上者谓之道，形而下者谓之器。—《易经·系辞》"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"正如很多流行的技术（面向对象、UML等）一样，几乎大部分人都会宣称自己“掌握”“熟练掌握”，甚至“精通”，然而，真正掌握或者精通的人实在是少之又少。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一个典型的现象是：很多人能够熟练背诵出所有的设计模式，能够快速画出各种设计模式的UML类图，也能够熟练地写出《设计模式》一书中各个模式的样例代码。但一到实际的项目设计和开发的时候，往往都会陷入迷茫：要么无从下手，不知道哪个地方要用设计模式；要么生搬硬套，胡乱使用设计模式，将方案和代码搞得一团乱麻。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这是什么原因呢？难道是设计模式不好用，或者设计模式根本就是一个噱头？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"答案不在于设计模式本身是否有用，而是在于我们没有掌握正确的学习和应用设计模式的方法。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"学习《设计模式》一书中的23个设计模式，只是掌握了设计模式的“器”，但并没有掌握设计模式的“道”。就像一个工匠，锯、钻、锤、刨样样精通，但如果不知道在什么地方该用锯，在什么地方该用钻，那么他肯定是一个不合格的工匠。为了能够更好地学习和应用设计模式，我们也需要掌握设计模式的“道”。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"设计模式的“道”就是用来指导我们什么时候用设计模式，为什么要用设计模式，23个设计模式只是告诉了我们How，而设计模式之道却可以告诉我们Why和Where！"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"拨云见日—寻找设计模式之道"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"熟悉《设计模式》一书内容的读者可能会想到：在《设计模式》中，不是每个模式都有“适用性”的说明吗？这个其实就是回答了Where和Why的问题啊！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"例如，Facade模式的“适用性”说明如下（摘自《设计模式》中文版）。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在遇到以下情况时使用Facade模式。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"当你要为一个复杂子系统提供一个简单接口时，子系统往往因为不断演化而变得越来越复杂。大多数模式使用时都会产生更多更小的类，这使得子系统更具可重用性，也更容易对子系统进行定制。但这也给那些不需要定制子系统的用户带来一些使用上的困难。Facade可以提供一个简单的默认视图，这一视图对大多数用户来说已经足够，而那些需要更多的可定制性的用户可以越过Facade层。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"客户程序与抽象类的实现部分之间存在着很大的依赖性，引入Facade将这个子系统与客户以及其他的子系统分离，可以提高子系统的独立性和可移植性。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"当你需要构建一个层次结构的子系统时，使用Facade模式定义子系统中每层的入口点。如果子系统之间是相互依赖的，则可以让它们仅通过Facade进行通信，从而简化了它们之间的依赖关系。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面这段文字，看起来回答了Where和Why的问题，但实际上我个人感觉作用并不大。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"首先，这段描述太长了：上面这段文字是否花了你几分钟的时间去阅读和理解？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其次，这段描述比较抽象：什么是复杂，什么叫作简单，什么叫作很大依赖性……可能每个人的理解都不一样。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"最后，23个模式，所有的“适应性”条款加起来估计有几十条，你能够记住吗？即使能够全部记住，你能够全部理解吗？即使能够全部理解，当你面对一个具体问题的时候，你知道几十条里面哪一条适应你的情况吗？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以，《设计模式》一书中关于“适用性”的描述，实际上还是太复杂、太多了，不具备很强的实践指导意义和可操作性。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们需要的是一个更简单的指导思想，“大道至简”，最好是一两句话就能够描述清楚！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"幸运的是，答案竟然就在《设计模式》一书中，但这个答案并不是那么明显！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"《设计模式》一书内容的侧重点是23个模式的详细阐述，大部分人可能都是直奔主题，逐一去研究每个模式，而对于本书开头部分第1章和第2章的内容并没有详细研读和思考，或者对于这两章只是简单地浏览，并未认真领会和思考，由此错过了最重要的内容。再加上GoF在这两章的内容中，既要引入一个全新的概念，又要提纲挈领地介绍各个模式，还要引入实例进行分析，以至于大量的内容反而将真正核心的内容给淹没或者冲淡了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"设计模式之道就隐藏在“2.6.2封装实现依赖关系”的最后一段，很简单的一句话："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"对变化的概念进行封装（encapsulate the concept that varies）。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"你看到这句话可能有点失望，前面分析了那么久，卖了那么多的关子，结果就这么简简单单一句话，这不是在忽悠吗？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"你可千万别小看了这句话，“大道至简”，设计模式之道也不例外，但“简”并不意味着没用；相反，正因为其“简”，每个人的理解才一致，也更好掌握，在实践中才更好应用。正所谓：“真传一句话，假传万卷书”。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"GoF在《设计模式》一书中最早提出这个原则，后来不断地有其他专家进行阐述，其中《设计模式精解》（"},{"type":"text","marks":[{"type":"italic"}],"text":"Design Pattern Explained"},{"type":"text","text":"）一书的阐述我认为是最精辟的：Find what varies and encapsulate it。翻译一下即“找到变化，封装变化”。虽然含义和GoF描述的基本一致，但其更加容易理解。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"正所谓：踏破铁鞋无觅处，得来全不费工夫！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"庖丁解牛—解析设计模式之道"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"现在，让我们来深入理解“找到变化，封装变化”的设计模式之道。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"首先，“找到变化”解决了“在哪里”使用设计模式的问题，即回答了“where”的问题。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"“找到变化”看起来是比较抽象的一句话，但在实践中非常好应用和操作，而且不同领域、不同行业的系统都可以完美地应用这句话。虽然不同领域、不同行业变化的因素、方式、时机等都不一样，但每个领域或者行业的需求分析人员、设计人员，对自己所处行业和领域的可能变化肯定是有比较深刻的理解的，什么会变化、会如何变化、什么时候会变化……肯定都能够自己判断，这种判断并不需要高深的技巧和知识水平，只需要一定的经验积累。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果我们刚接触一个行业或者领域，经验积累不够，那么怎么办呢？是否就无法“找到变化”了？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其实也不然，有一个万能的办法，只是要花费更多的精力了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这个万能的办法就是“唯一不变的是变化本身”，也就是说，如果你不知道什么会变化，那么就抱着怀疑一切的想法，一切都可能是变化的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但光有这条指导原则还不行，如果我们真的抱着“一切都是可能变化的”的想法，然后封装一切变化，那么就会陷入变化的旋涡中无法自拔，因为变化是会递归的，A可能变成B，B也可能继续变化，于是这样无穷无尽，系统是不可能做出来的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以我们需要一个终止条件，避免陷入无穷无尽的变化递归旋涡中。这个终止条件就是“有限时间内可能发生的变化”。这里的“有限时间”随行业和领域的不同而变化。例如（以下时间仅供参考）："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"互联网行业可以说，半年内可能发生的变化……"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"电信行业可以说，1年内可能发生的变化……"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"金融行业可以说，2年内可能发生的变化……"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"政府行业可以说，3年内可能发生的变化……"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"有了这个指导原则后，你可以这样去问有经验的前辈、“大牛”等：XXX在1年内会发生变化吗？会怎样变化？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"就这样，即使你是“菜鸟”，通过这么一招“借花献佛”，也能够轻松发现“变化”的地方。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"其次，“封装变化”解决了“为什么”使用设计模式的问题，即回答了“Why”的问题。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为什么我们要用设计模式？是因为我们要封装变化！但我们为什么要封装变化呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"答案很明显：变化不好！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"当然这个“不好”不是从业务的角度来说的，而是从系统的角度来说的。从业务的角度来说，“变化”是好的，变化意味着新的机会；但从系统的角度来说，变化并不好，因为变化必然要求系统改动，改动就意味着风险！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"par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