vuejs中实现数据的劫持,v2中使用的是Object.defineProperty()来实现的，在大版本v3中彻底重写了这部分，使用了proxy这个数据代理的方式，来修复了v2中对数组和对象的劫持的遗留问题。
proxy是什么
Proxy 用于修改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 可以理解成对源对象进行一个封装，在操作源对象之前，做了一系列额外的操作，最终返回我们需要的新数据对象。
基础使用
let obj = new Proxy(
{},
{
get(target, prop, receiver) {
console.log("get", prop);
if (!target[prop]) target[prop] = 120;
return Reflect.get(target, prop, receiver);
},
set(target, prop, value, receiver) {
console.log("set", prop);
return Reflect.set(target, prop, value, receiver);
},
}
);
obj.count = 1;
obj.count;
obj.count;
obj.count;
console.log(obj.count);
obj.age;
console.log(obj.age);
proxy实例有两个参数，一个是目标对象，一个是操作方法的hash集合
取值函数get,赋值函数set。
对特定属性的劫持
const proxyObj = new Proxy(
{ name: "Tom", age: 18 },
{
get: function (target, prop) {
if (prop === "age") return target[prop] - 1;
return 35;
},
}
);
proxyObj.time;
console.log("🚀 ~ proxyObj.time:", proxyObj.time);
proxyObj.age;
console.log("🚀 ~ proxyObj.time:", proxyObj.age);
把实例方法封装在对象内部
const object = {
name: "Tom",
age: 18,
sayHi() {
console.log("sayHi");
},
proxy() {
return new Proxy(this, {
get(target, prop) {
console.log("🚀 ~ get ~ prop:", prop);
if (prop in target) {
return Reflect.get(target, prop);
} else {
return "no prop";
}
return Reflect.get(target, prop);
},
});
},
};
const newProxy = object.proxy();
// newObjj.age;
console.log("🚀 ~ newObjj.age;:", newProxy.age);
console.log("🚀 ~ newObjj.name;:", newProxy.sex);
对数组进行负值索引的操作
function createArray(...elements) {
let handler = {
get(target, prop, receiver) {
let index = Number(prop);
if (index < 0) {
prop = String(target.length + index);
}
return Reflect.get(target, prop, receiver);
},
};
let target = [];
target.push(...elements);
return new Proxy(target, handler);
}
let arr = createArray("a", "b", "c");
arr[-1];
console.log("🚀 ~ arr[-1]:", arr[-1]);
console.log("🚀 ~ arr[-1]:", arr[-2]);
console.log("🚀 ~ arr[-1]:", arr[-3]);
实现数据的链式调用
var double = (n) => n * 2;
var pow = (n) => Math.pow(n, 2);
var reverse = (n) => String(n).split("").reverse().join("");
const pipe = function (value) {
var funcStack = [];
var oProxy = new Proxy(
{},
{
get: function (target, key) {
console.log("🚀 ~ pipe ~ key:", key);
if (key === "get") {
return funcStack.reduce(function (val, func) {
return func(val);
}, value);
}
// 把方法存储到栈中
funcStack.push(window[key]);
console.log("🚀 ~ funcStack:", funcStack);
return oProxy;
},
}
);
return oProxy;
};
const data1 = pipe(3).double.pow.reverse.get;
console.log("🚀 ~ data:", data1);
注意：三个方法必须是var声明的，let/const都会报错
上面代码设置 Proxy 以后，达到了将函数名链式使用的效果。
利用get拦截，实现一个生成各种 DOM 节点的通用函数dom
const dom = new Proxy(
{},
{
get(target, prop) {
return function (arrts, ...children) {
const el = document.createElement(prop);
for (let prop of Object.keys(arrts)) {
el.setAttribute(prop, arrts[prop]);
}
for (let child of children) {
console.log("🚀 ~ get ~ child:", child);
if (typeof child === "string") {
child = document.createTextNode(child);
}
el.appendChild(child);
}
return el;
};
},
}
);
const el = dom.div(
{},
"Hello, my name is ",
dom.a({ href: "//example.com" }, "Mark"),
". I like:",
dom.ul(
{},
dom.li({}, "The web"),
dom.li({}, "Food"),
dom.li({}, "…actually that's it")
)
);
document.body.appendChild(el);
第三个参数，它总是指向原始的读操作所在的那个对象
const proxy = new Proxy(
{},
{
get: function (target, prop, receiver) {
console.log("🚀 ~ prop:", prop);
return receiver;
},
}
);
const isSame = proxy.getReceiver === proxy;
console.log("🚀 ~ isSame:", isSame);
const d = Object.create(proxy);
console.log("ddd", d.a === d);
如果一个属性不可配置（configurable）且不可写（writable），则 Proxy 不能修改该属性
const target = Object.defineProperties(
{},
{
foo: { value: "bar", enumerable: false, configurable: false },
}
);
const handler = {
get(target, prop) {
return "abc";
},
};
const proxy2 = new Proxy(target, handler);
proxy2.foo;
上面通过 Proxy 对象访问该属性会报错。
拦截方法的执行
上面的都是object对象的属性进行劫持，也可以作为方法调用时进行劫持。
var target = function () {
return "I am the target";
};
var handler = {
apply(target, thisArg, argumentsList) {
console.log("🚀 ~ apply ~ argumentsList:", argumentsList);
const res = target();
console.log("🚀 ~ apply ~ res:", res);
return "I am the proxy" + " " + argumentsList.join(",");
},
};
const p = new Proxy(target, handler);
const a = p("a", "b");
console.log("🚀 ~ a:", a);
变量p是 Proxy 的实例，当它作为函数调用时（p()），就会被apply方法拦截，返回一个字符串
function sum(left, right) {
return left + right;
}
var twice = {
apply(target, context, args) {
console.log("🚀 ~ apply ~ context:", context);
console.log("🚀 ~ apply ~ args:", args);
return Reflect.apply(target, context, args) * 2;
},
};
const proxy = new Proxy(sum, twice);
const data = proxy(1, 2);
console.log("🚀 ~ data:", data);
const data2 = proxy.call(null, 2, 5);
console.log("🚀 ~ data2:", data2);
const data3 = proxy.apply(null, [5, 5]);
console.log("🚀 ~ data3:", data3);
当执行proxy函数（直接调用或call和apply调用），就会被apply方法拦截。
get和set方法，实现内部属性的保护机制
const proxy = new Proxy(
{},
{
get(target, prop) {
invariant(prop, "get");
return Reflect.get(target, prop);
},
set(target, prop, value) {
invariant(prop, "set");
Reflect.set(target, prop, value);
return true;
},
}
);
function invariant(key, action) {
if (key[0] === "_") {
throw new Error(`Invalid attempt to ${action} private "${key}" property`);
}
}
// proxy._prop;
proxy._prop = "value";
拦截key in proxy的操作
var target = { _prop: "foo", prop: "foo" };
const proxy = new Proxy(target, {
has(target, key) {
if (key[0] === "_") {
console.log("false");
return false;
}
return key in target;
},
});
"_prop" in proxy; // false
deleteProperty删除属性的劫持
const handler = {
construct(target, args) {
console.log("called: " + args.join(","));
return new target(...args);
},
deleteProperty(target, prop) {
if (prop === "age") return false;
delete target[prop];
return true;
},
};
const P = new Proxy(function () {}, handler);
const p = new P(10);
P.value;
const p2 = new Proxy(
{
age: 20,
name: "John",
greet: () => console.log("hello"),
},
handler
);
delete p2.age;
delete p2.name;
