function observe(obj, vm) { 
    Object.keys(obj).forEach(function (key) { 
        defineReactive(vm, key, obj[key]) 
    }) 
}   

function defineReactive(obj, key, val) {
    var dep = new Dep();
    Object.defineProperty(obj, key, {
        get: function () {
            if (Dep.target) dep.addSub(Dep.target)
            return val
        },
        set: function (newValue) {
            if (newValue === val) return
            val = newValue
            console.log('值是这么变化的，正确的值是' + val)
            dep.notify()
        }
    })
}

function Dep() {
    this.subs = []
}
Dep.prototype = {
    addSub: function (sub) {
        this.subs.push(sub);
    },
    notify: function () {
        this.subs.forEach(function (sub) {
            sub.update();
        })
    }
}

function nodeToFragment(node, vm) {
    var flag = document.createDocumentFragment();
    var child;
    while (child = node.firstChild) {
        compile(child, vm);
        flag.appendChild(child)
    }
    return flag;
}

function compile(node, vm) {
    var reg = /\{\{(.*)\}\}/;
    if (node.nodeType === 1) {
        var attr = node.attributes;
        for(var i = 0; i < attr.length; i++) {
            if (attr[i].nodeName == 'v-model') {
                var name = attr[i].nodeValue;
                node.addEventListener('input', function (e) {
                    vm[name] = e.target.value
                })
                node.value = vm[name];
                node.removeAttribute('v-model');
            }
        }
    }
    if (node.nodeType === 3) {
        if (reg.test(node.nodeValue)) {
            var name = RegExp.$1;
            name = name.trim();
            // node.nodeValue = vm[name];
            new Watcher(vm, node, name)
        }
    }
}

function Watcher(vm, node, name) {
    Dep.target = this;
    this.name = name;
    this.node = node;
    this.vm = vm;
    this.update();
    Dep.target = null;
}
Watcher.prototype = {
    update: function () {
        this.get();
        this.node.nodeValue = this.value;
    },
    get: function () {
        this.value = this.vm[this.name];
    }
}

function Vue(options) {
    this.data = options.data;
    var data = this.data;
    observe(data, this);
    var id = options.el;
    var dom = nodeToFragment(document.getElementById(id), this);
    document.getElementById(id).appendChild(dom)
}

var vm = new Vue({
    el: 'app',
    data: {
        text: 'hello world',
        demo: 'ppp'
    }
});

<div id="app">
        <input type="text" v-model="text">
        {{text}}
    </div>