# Javascript 中的响应式与 Proxy

\>

```javascript
let data = { price: 5, quantity: 2 };
let target = null;
class Dep {
    constructor() {
        this.subscribers = [];
    }
    depend() {
        if (target && !this.subscribers.includes(target)) {
            this.subscribers.push(target);
        }
    }
    notify() {
        this.subscribers.forEach(sub => sub());
    }
}

let deps = new Map(); // 创建一个Map对象
Object.keys(data).forEach(key => {
    // 为每个属性都设置一个依赖实例 并放入 deps 中
    deps.set(key, new Dep());
});
let data_without_proxy = data; // 保存源对象
data = new Proxy(data_without_proxy, {
    // 重写数据以在中间创建一个代理
    get(obj, key) {
        deps.get(key).depend(); // 依旧为存储target
        return obj[key]; // 返回原始数据
    },
    set(obj, key, newVal) {
        obj[key] = newVal; // 将原始数据设置为新值
        deps.get(key).notify(); // 依旧为重新运行已存储的targets
        return true;
    }
});

// 模拟监听
function watcher(myFunc) {
    target = myFunc;
    target();
    target = null;
}

let total = 0
watcher(() => {
    total = data.price * data.quantity;
    console.log("total = ", total);
});
data.price = 20;
data.quantity = 10;

deps.set('discount', new Dep())
data.discount = 5;
let salePrice = 0;
watcher(() => {
    salePrice = data.price - data.discount;
    console.log("salePrice = ", salePrice);
});
data.discount = 7.5
```


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