javascript ES6 Proxy

邱秋 • 2018年06月14日 • 阅读:1358 • javascript

proxy 的概念

proxy 英文原意是代理的意思,在 ES6 中,可以翻译为"代理器"。 它主要用于改变某些操作的默认行为,等同于在语言层面做出修改,所以属于一种“元编程”(meta programming),即对编程语言进行编程。

proxy 在目标对象的外层搭建了一层拦截,外界对目标对象的某些操作(后文会说明,有哪些操作可以拦截),必须通过这层拦截。

语法

var proxy = new Proxy(target, handler);

通过构造函数生成 proxy,target 参数是要拦截的目标对象,handler 参数也是一个对象,用来定制拦截行为。

例子

var obj = new Proxy(
  {},
  {
    get: function (target, key, receiver) {
      console.log(`getting ${key}!`);
      return Reflect.get(target, key, receiver);
    },
    set: function (target, key, value, receiver) {
      console.log(`setting ${key}!`);
      return Reflect.set(target, key, value, receiver);
    },
  }
);

一般将 handle 参数说成配置对象,在配置对象中,可以定义需要拦截的操作。如果配置对象为空,那么对 proxy 的操作将直通目标对象。

proxy 操作才有拦截效果,而不是目标对象。

Proxy 实例的方法

当读取不存在的属性时候,抛出错误而不是返回 undefined

var person = {
  name: "张三",
};

var proxy = new Proxy(person, {
  get: function (target, property) {
    if (property in target) {
      return target[property];
    } else {
      throw new ReferenceError('Property "' + property + '" does not exist.');
    }
  },
});

proxy.name; // "张三"
proxy.age; // 抛出一个错误

拦截读取继承属性

let proto = new Proxy(
  {},
  {
    get(target, propertyKey, receiver) {
      console.log("GET " + propertyKey);
      return target[propertyKey];
    },
  }
);

let obj = Object.create(proto);
obj.xxx; // "GET xxx"

数组读取负数索引(负数索引表示倒着取数)

function createArray(...elements) {
  let handler = {
    get(target, propKey, receiver) {
      let index = Number(propKey);
      if (index < 0) {
        propKey = String(target.length + index);
      }
      return Reflect.get(target, propKey, receiver);
    },
  };

  let target = [];
  target.push(...elements);
  return new Proxy(target, handler);
}

let arr = createArray("a", "b", "c");
arr[-1]; // c

实现数据的限制

let validator = {
  set: function (obj, prop, value) {
    if (prop === "age") {
      if (!Number.isInteger(value)) {
        throw new TypeError("The age is not an integer");
      }
      if (value > 200) {
        throw new RangeError("The age seems invalid");
      }
    }

    // 对于age以外的属性,直接保存
    obj[prop] = value;
  },
};

let person = new Proxy({}, validator);

person.age = 100;

person.age; // 100
person.age = "young"; // 报错
person.age = 300; // 报错

防止内部属性“\_”被外部读写(通常我们以下划线开头,表示其实内部属性)

var handler = {
  get(target, key) {
    invariant(key, "get");
    return target[key];
  },
  set(target, key, value) {
    invariant(key, "set");
    target[key] = value;
    return true;
  },
};
function invariant(key, action) {
  if (key[0] === "_") {
    throw new Error(`Invalid attempt to ${action} private "${key}" property`);
  }
}
var target = {};
var proxy = new Proxy(target, handler);
proxy._prop;
// Error: Invalid attempt to get private "_prop" property
proxy._prop = "c";
// Error: Invalid attempt to set private "_prop" property

拦截——函数调用、callapply 操作

var twice = {
  apply(target, ctx, args) {
    return Reflect.apply(...arguments) * 2;
  },
};
function sum(left, right) {
  return left + right;
}
var proxy = new Proxy(sum, twice);
proxy(1, 2); // 6
proxy.call(null, 5, 6); // 22
proxy.apply(null, [7, 8]); // 30

不对...in...循环生效

var handler = {
  has(target, key) {
    if (key[0] === "_") {
      return false;
    }
    return key in target;
  },
};
var target = { _prop: "foo", prop: "foo" };
var proxy = new Proxy(target, handler);
"_prop" in proxy; // false

不对 for...in...循环生效

let stu1 = { name: "张三", score: 59 };
let stu2 = { name: "李四", score: 99 };

let handler = {
  has(target, prop) {
    if (prop === "score" && target[prop] < 60) {
      console.log(`${target.name} 不及格`);
      return false;
    }
    return prop in target;
  },
};

let oproxy1 = new Proxy(stu1, handler);
let oproxy2 = new Proxy(stu2, handler);

"score" in oproxy1;
// 张三 不及格
// false

"score" in oproxy2;
// true

for (let a in oproxy1) {
  console.log(oproxy1[a]);
}
// 张三
// 59

for (let b in oproxy2) {
  console.log(oproxy2[b]);
}
// 李四
// 99

拦截 object.keys()方法

let target = {
  a: 1,
  b: 2,
  c: 3,
};

let handler = {
  ownKeys(target) {
    return ["a"];
  },
};

let proxy = new Proxy(target, handler);

Object.keys(proxy);
// [ 'a' ]

本文来源 RYF

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