function MVVM(options) {

    this.$options = options || {};

    var data = this._data = this.$options.data;

    var me = this;

    // 数据代理

    // 实现 vm.xxx -> vm._data.xxx

    Object.keys(data).forEach(function(key) {

        me._proxyData(key);

    });

    this._initComputed();

    observe(data, this);

    this.$compile = new Compile(options.el || document.body, this)

}

MVVM.prototype = {

    $watch: function(key, cb, options) {

        new Watcher(this, key, cb);

    },

    _proxyData: function(key, setter, getter) {

        var me = this;

        setter = setter ||

        Object.defineProperty(me, key, {

            configurable: false,

            enumerable: true,

            get: function proxyGetter() {

                return me._data[key];

            },

            set: function proxySetter(newVal) {

                me._data[key] = newVal;

            }

        });

    },

    _initComputed: function() {

        var me = this;

        var computed = this.$options.computed;

        if (typeof computed === 'object') {

            Object.keys(computed).forEach(function(key) {

                Object.defineProperty(me, key, {

                    get: typeof computed[key] === 'function'

                            ? computed[key]

                            : computed[key].get,

                    set: function() {}

                });

            });

        }

    }

};