SOURCE

function myVue(options) {
        this._init(options);
    }
    myVue.prototype._init = function(options) {
        this.$options = options;
        this.$el = document.querySelector(options.el);
        this.$data = options.data;
        this.$methods = options.methods;
        this._binding = {};
        this._obverse(this.$data);
        this._complie(this.$el);
    }
    myVue.prototype._obverse = function(obj) {
        var value;
        for (key in obj) {
            if (obj.hasOwnProperty(key)) {
                this._binding[key] = {
                    _directives: []
                };
                value = obj[key];
                if (typeof value === 'object') {
                    this._obverse(value);
                }
                var binding = this._binding[key];
                Object.defineProperty(this.$data, key, {
                    enumerable: true,
                    configurable: true,
                    get: function() {
                        console.log(`获取$ {
                            value
                        }`);
                        return value;
                    },
                    set: function(newVal) {
                        console.log(`更新$ {
                            newVal
                        }`);
                        if (value !== newVal) {
                            value = newVal;
                            binding._directives.forEach(function(item) {
                                item.update();
                            })
                        }
                    }
                })
            }
        }
    }
    myVue.prototype._complie = function(root) {
        var _this = this;
        var nodes = root.children;
        for (var i = 0; i < nodes.length; i++) {
            var node = nodes[i];
            if (node.children.length) {
                this._complie(node);
            }
            if (node.hasAttribute('v-click')) {
                node.onclick = (function() {
                    var attrVal = nodes[i].getAttribute('v-click');
                    return _this.$methods[attrVal].bind(_this.$data);
                })();
            }
            if (node.hasAttribute('v-model') && (node.tagName == 'INPUT' || node.tagName == 'TEXTAREA')) {
                node.addEventListener('input', (function(key) {
                    var attrVal = node.getAttribute('v-model');
                    _this._binding[attrVal]._directives.push(new Watcher('input', node, _this, attrVal, 'value')) 
                    return  function() {
                        _this.$data[attrVal] = nodes[key].value;
                    }
                })(i));
            }
            if (node.hasAttribute('v-bind')) {
                var attrVal = node.getAttribute('v-bind');
                _this._binding[attrVal]._directives.push(new Watcher('text', node, _this, attrVal, 'innerHTML'))
            }
        }
    }
    function Watcher(name, el, vm, exp, attr) {
        this.name = name; //指令名称，例如文本节点，该值设为 "text" 
        this.el=el ; //指令对应的DOM元素 
        this.vm = vm; //指令所属myVue实例 
        this.exp=exp ; //指令对应的值，本例如 "number" 
        this.attr=attr ; //绑定的属性值，本例为 "innerHTML" 
        this.update();
    }
    Watcher.prototype.update = function() {
        this.el[this.attr] = this.vm.$data[this.exp];
    }
    window.onload = function() {
        var app = new myVue({
            el: '#app',
            data: {
                number: 0
            },
            methods: {
                increment: function() {
                    this.number++;
                },
            }
        })
    }

    <div id="app">
        <form>
            <input type="text" v-model="number">
            <button type="button" v-click="increment">增加</button>
        </form>
        <h3 v-bind="number"></h3>
    </div>