Introduction 介绍

Over the life time of an application, more time is spent adding to the existing codebase rather than constantly adding new features from scratch. As you are probably aware, this can be a tedious and frustrating process. Anytime you modify code, you risk introducing new bugs, or breaking old functionality completely. Ideally, we should be able to modify an existing codebase as quickly and easily as writing brand new code. If we correctly design our application according to the Open Closed principle, we can just do that!

在一个应用的生命周期里，大部分时间都花在了向现有代码库增加功能，而非一直从零开始写新功能。正像你所想的那样，这会是一个繁琐且令人痛苦的过程。当你修改代码的时候，你可能引入新的程序错误，或者将原来管用的功能搞坏掉。理想情况下，我们应该可以像写全新的代码一样，来快速且简单的修改现有的代码。只要采用开放封闭原则来正确的设计我们的应用程序，那么这是可以做到的！

Open Closed Principle 开放封闭原则

The Open Closed principle of SOLID design states that code is open for extension but closed for modification.

开放封闭原则规定代码对扩展是开放的，对修改是封闭的。