getter 和 setter 方法有什么意义？

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今天在看vue中使用axios的时候发现，axios提供了vue的版本。但是之前看了使用axios的一般做法是这样的，

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import axios from 'axios'
Vue.prototype.$ajax = axios

那么为什么还需要vue-axios呢？就要看看他的源码了。代码非常短

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(function () {

/**
 * Install plugin
 * @param Vue
 * @param axios
 */

function plugin(Vue, axios) {

  if (plugin.installed) {
    return
  }
  plugin.installed = true

  if (!axios) {
    console.error('You have to install axios')
    return
  }

  Vue.axios = axios

  Object.defineProperties(Vue.prototype, {

    axios: {
      get() {
        return axios
      }
    },

    $http: {
      get() {
        return axios
      }
    }

  })
}

if (typeof exports == "object") {
  module.exports = plugin
} else if (typeof define == "function" && define.amd) {
  define([], function(){ return plugin })
} else if (window.Vue && window.axios) {
  Vue.use(plugin, window.axios)
}

})();

输出不同类型的模块和挂载vue上的东东先不说，这个使用Object.defineProperties的getter的方式让我有些疑惑，最开始那种直接写入prototype的方式多简洁，为什么还要费劲吧啦的写个get呢？

使用“方法”来表达对象的能力，同使用“变量”相比，从宏观上来说，没有什么区别，只是形式上的不同。但使用“方法”来表达接口，更容易体现OOP的一个核心理念“隐藏细节”。 @Levski

下面直接上一个stackoverflow上的答案，

There are actually many good reasons to consider using accessors rather than directly exposing fields of a class - beyond just the argument of encapsulation and making future changes easier.

Here are the some of the reasons I am aware of:

  • Encapsulation of behavior associated with getting or setting the property - this allows additional functionality (like validation) to be added more easily later.
    方便增加额外功能（比如验证）
  • Hiding the internal representation of the property while exposing a property using an alternative representation.
    Insulating your public interface from change - allowing the public interface to remain constant while the implementation changes without affecting existing consumers.
    可以保持外部接口不变的情况下，修改内部存储方式和逻辑
  • Controlling the lifetime and memory management (disposal) semantics of the property - particularly important in non-managed memory environments (like C++ or Objective-C).
    任意管理变量的生命周期和内存存储方式
  • Providing a debugging interception point for when a property changes at runtime - debugging when and where a property changed to a particular value can be quite difficult without this in some languages.
    可以打断点
  • Improved interoperability with libraries that are designed to operate against property getter/setters - Mocking, Serialization, and WPF come to mind.
    能够和模拟对象、序列化乃至WPF库等融合
  • Allowing inheritors to change the semantics of how the property behaves and is exposed by overriding the getter/setter methods.
    允许继承者通过重写getter/setter改变语义和实现方式
  • Allowing the getter/setter to be passed around as lambda expressions rather than values.
    可以将getter、setter用于lambda表达式
  • Getters and setters can allow different access levels - for example the get may be public, but the set could be protected.
    允许getter/setter有不同级别的访问。比如get可以对外，set就是私有的