設計模式只是一把錘子

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"談起《設計模式》，那是幾乎無人不知，無人不曉，大名鼎鼎的GoF的驚世之作，真是“平生不識GoF，學盡設計也枉然”！"}]},{"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，設計模式真的是軟件設計的“瑞士軍刀”，切、削、鋸、鑽樣樣精通嗎？"}]},{"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":"讀過《設計模式》的讀者估計不少，但真正注意過《設計模式》的副標題的估計很少，而這個副標題卻是避免誤解設計模式的關鍵。《設計模式》的副標題是：可複用面向對象軟件的基礎！"}]},{"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":"不要小看了這短短的一句話，如果你沒有看這句話，或者只是一掃而過並沒有仔細體會，那麼你很可能就認爲設計模式是一把“瑞士軍刀”，能夠解決所有的設計問題；而實際上“設計模式只是一把錘子”，有句諺語叫作“如果你手裏有一把錘子，那麼所有的問題都變成了釘子”，如果你拿着設計模式這把錘子到處去敲，要麼東西被敲壞，要麼就不起作用。"}]},{"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":"爲什麼說設計模式只是一把錘子呢？我們還是從副標題來看。《設計模式》的副標題揭示了設計模式的兩個主要約束。"}]},{"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":"設計模式解決的是“可複用”的設計問題；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"設計模式應用的領域是“面向對象”。"}]}]}]},{"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":"相信經過我這麼一提醒，大家基本上都能夠明白了爲什麼說“設計模式只是一把錘子”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":"設計模式只能解決“可複用”的設計問題，其他的例如性能設計、可靠性設計、安全性設計、可服務性設計等都不是設計模式能夠解決的；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"設計模式只是在面向對象的語言中應用，如果是非面向對象的語言，那麼就不怎麼好用了。當然，你可以在C語言中應用設計模式，但畢竟要有不少折騰，用起來也不那麼得心應手。"}]}]}]},{"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，當你遇到一個問題就想到設計模式的時候，一定要注意“設計模式只是一把錘子”，不要拿着這把錘子到處去敲！"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"知易行難—設計模式應用的問題"}]},{"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":"形而上者謂之道，形而下者謂之器。—《易經·繫辭》"}]},{"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":"正如很多流行的技術（面向對象、UML等）一樣，幾乎大部分人都會宣稱自己“掌握”“熟練掌握”，甚至“精通”，然而，真正掌握或者精通的人實在是少之又少。"}]},{"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":"一個典型的現象是：很多人能夠熟練背誦出所有的設計模式，能夠快速畫出各種設計模式的UML類圖，也能夠熟練地寫出《設計模式》一書中各個模式的樣例代碼。但一到實際的項目設計和開發的時候，往往都會陷入迷茫：要麼無從下手，不知道哪個地方要用設計模式；要麼生搬硬套，胡亂使用設計模式，將方案和代碼搞得一團亂麻。"}]},{"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":"這是什麼原因呢？難道是設計模式不好用，或者設計模式根本就是一個噱頭？"}]},{"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":"答案不在於設計模式本身是否有用，而是在於我們沒有掌握正確的學習和應用設計模式的方法。"}]},{"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":"學習《設計模式》一書中的23個設計模式，只是掌握了設計模式的“器”，但並沒有掌握設計模式的“道”。就像一個工匠，鋸、鑽、錘、刨樣樣精通，但如果不知道在什麼地方該用鋸，在什麼地方該用鑽，那麼他肯定是一個不合格的工匠。爲了能夠更好地學習和應用設計模式，我們也需要掌握設計模式的“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":"設計模式的“道”就是用來指導我們什麼時候用設計模式，爲什麼要用設計模式，23個設計模式只是告訴了我們How，而設計模式之道卻可以告訴我們Why和Where！"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"撥雲見日—尋找設計模式之道"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"熟悉《設計模式》一書內容的讀者可能會想到：在《設計模式》中，不是每個模式都有“適用性”的說明嗎？這個其實就是回答了Where和Why的問題啊！"}]},{"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“適用性”說明如下（摘自《設計模式》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":"在遇到以下情況時使用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":"當你要爲一個複雜子系統提供一個簡單接口時，子系統往往因爲不斷演化而變得越來越複雜。大多數模式使用時都會產生更多更小的類，這使得子系統更具可重用性，也更容易對子系統進行定製。但這也給那些不需要定製子系統的用戶帶來一些使用上的困難。Facade可以提供一個簡單的默認視圖，這一視圖對大多數用戶來說已經足夠，而那些需要更多的可定製性的用戶可以越過Facade層。"}]}]}]},{"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":"客戶程序與抽象類的實現部分之間存在着很大的依賴性，引入Facade將這個子系統與客戶以及其他的子系統分離，可以提高子系統的獨立性和可移植性。"}]}]}]},{"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":"當你需要構建一個層次結構的子系統時，使用Facade模式定義子系統中每層的入口點。如果子系統之間是相互依賴的，則可以讓它們僅通過Facade進行通信，從而簡化了它們之間的依賴關係。"}]}]}]},{"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":"上面這段文字，看起來回答了Where和Why的問題，但實際上我個人感覺作用並不大。"}]},{"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，這段描述太長了：上面這段文字是否花了你幾分鐘的時間去閱讀和理解？"}]},{"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，這段描述比較抽象：什麼是複雜，什麼叫作簡單，什麼叫作很大依賴性……可能每個人的理解都不一樣。"}]},{"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":"最後，23個模式，所有的“適應性”條款加起來估計有幾十條，你能夠記住嗎？即使能夠全部記住，你能夠全部理解嗎？即使能夠全部理解，當你面對一個具體問題的時候，你知道幾十條裏面哪一條適應你的情況嗎？"}]},{"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，《設計模式》一書中關於“適用性”的描述，實際上還是太複雜、太多了，不具備很強的實踐指導意義和可操作性。"}]},{"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":"我們需要的是一個更簡單的指導思想，“大道至簡”，最好是一兩句話就能夠描述清楚！"}]},{"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":"幸運的是，答案竟然就在《設計模式》一書中，但這個答案並不是那麼明顯！"}]},{"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":"《設計模式》一書內容的側重點是23個模式的詳細闡述，大部分人可能都是直奔主題，逐一去研究每個模式，而對於本書開頭部分第1章和第2章的內容並沒有詳細研讀和思考，或者對於這兩章只是簡單地瀏覽，並未認真領會和思考，由此錯過了最重要的內容。再加上GoF在這兩章的內容中，既要引入一個全新的概念，又要提綱挈領地介紹各個模式，還要引入實例進行分析，以至於大量的內容反而將真正核心的內容給淹沒或者沖淡了。"}]},{"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":"設計模式之道就隱藏在“2.6.2封裝實現依賴關係”的最後一段，很簡單的一句話："}]},{"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":"對變化的概念進行封裝（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":"你看到這句話可能有點失望，前面分析了那麼久，賣了那麼多的關子，結果就這麼簡簡單單一句話，這不是在忽悠嗎？"}]},{"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":"你可千萬別小看了這句話，“大道至簡”，設計模式之道也不例外，但“簡”並不意味着沒用；相反，正因爲其“簡”，每個人的理解才一致，也更好掌握，在實踐中才更好應用。正所謂：“真傳一句話，假傳萬卷書”。"}]},{"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《設計模式》一書中最早提出這個原則，後來不斷地有其他專家進行闡述，其中《設計模式精解》（"},{"type":"text","marks":[{"type":"italic"}],"text":"Design Pattern Explained"},{"type":"text","text":"）一書的闡述我認爲是最精闢的：Find what varies and encapsulate 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