vue实现双向数据绑定

function defineReactive(data, key, val){
    if(typeof val === 'object') new Observer(val)
    let dep = new Dep();
    Object.defineProperty(data, key, val) {
    enumerable: true,
    configurable: true,
    get: function () {
        dep.depend();
        return val;
    },
    set: function() {
        if(val === newVal) {
            return ;
        }
        val = newVal;
        dep.notify()
    }
}
}

export default class Dep {
    constructor() {
        this.subs = []
    }
    
    addSub() {
        this.subs.push(sub)
    }
    
    removeSub(sub) {
        remove(this.subs, sub)
    }
    
    depend() {
        if(window.target) {
            this.addSub(window.target)
        }
    }
    
    notify() {
        const subs = this.subs.slice()
        for(let i = 0, l = subs.length; i < l; i++) {
            subs[i].update()
        }
    }
}
 
function remove(arr, item) {
    if(arr.length) {
        const index = arr.indexOf(item)
        if(index > -1) {
            return arr.splice(index, 1)
        }
}
}

export default class Watcher {
    constructor(vm, expOrFn, cb) {
        this.vm = vm
        this.getter = parsePath(expOrFn)
        this.cb = cb
        this.value = this.get()
    }
    
    get() {
        window.target = this
        let value = this.getter.call(this.vm, this.vm)
        window.target = undefined
        return value
    }
    
    update() {
        const oldValue = this.value
        this.value = this.get()
        this.cb.call(this.vm, this.value, oldValue)
    }
}

export class Observer {
    constructor(value) {
        this.value = value;
        
        if(!Array.isArray(value)) {
            this.walk(value)
        }
    }
    walk(obj) {
        const keys = Object.keys(obj)
        for(let i = 0; i < keys.length; i++) {
            defineReactive(obj, keys[i], obj[keys[i]])
        }
    }
}

const arrayProto = Array.prototype
export const arrayMethods = Object.create(arrayProto);
//和数组原型一样的对象作为拦截器
['push','pop','shift','unshift','splice','sort','reverse'].forEach(function (method) {
    const original = arrayProto[method]
    Object.defineProperty(arrayMethods, method, {
        value: function mutator(...args) {
              return original.apply(this, args)
        },
        enumerable: false,
        writable: true,
        configurable: true
    })
})

value.__proto__ = arrayMethods

this.__ob__.dep.notify();

observeArray(items) {
    for(let i = 0; l = items.length; i < l; i++) {
        observe(items[i]);    
    }
}

if(inserted) ob.observeArray(inserted)