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);
  });
  // 代理计算属性
  // 同样通过Object.defineProperty进行劫持
  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);
  }
}

function Observer(data) {
  Object.keys(data).forEach(function() {
    defineReactive(data, key, data[key]);
  });
}
function defineReactive (data, key, val) {
  var dep = new Dep();
  var childObj = observe(val);

  Object.defineProperty(data, key, {
    enumerable: true, // 可枚举
    configurable: false, // 不能再define
    get: function() {
      if (Dep.target) {
        dep.depend();
      }
      return val;
    },
    set: function(newVal) {
      if (newVal === val) {
        return;
      }
      val = newVal;
      // 新的值是object的话，进行监听
      childObj = observe(newVal);
      // 通知订阅者
      dep.notify();
    }
  });
}

function Dep() {
  this.id = uid++;
  this.subs = [];
}
Dep.prototype = {
  addSub: function(sub) {
    this.subs.push(sub);
  },
  depend: function() {
    Dep.target.addDep(this);
  },
  notify: function() {
    this.subs.forEach(function(sub) {
      sub.update();
    });
  }
};

function compileElement (el) {
  var childNodes = el.childNodes,
    me = this;
  [].slice.call(childNodes).forEach(function(node) {
    var text = node.textContent;
    var reg = /\{\{(.*)\}\}/;
    // 解析元素节点
    if (me.isElementNode(node)) {
      me.compile(node);
    // {{}}替换变量
    } else if (me.isTextNode(node) && reg.test(text)) {
      me.compileText(node, RegExp.$1);
    }
    // 递归遍历子节点
    if (node.childNodes && node.childNodes.length) {
      me.compileElement(node);
    }
  });
}

compile: function(node) {
  var nodeAttrs = node.attributes,
    me = this;
  [].slice.call(nodeAttrs).forEach(function(attr) {
    // 指令以v-xxx命名
    // <span v-html="content"></span>
    var attrName = attr.name; // v-html
    if (me.isDirective(attrName)) {
      var exp = attr.value; // content
      var dir = attrName.substring(2);
      // 事件指令
      if (me.isEventDirective(dir)) {
        compileUtil.eventHandler(node, me.$vm, exp, dir);
        // 普通指令
      } else {
        compileUtil[dir] && compileUtil[dir](node, me.$vm, exp);
      }
      node.removeAttribute(attrName);
    }
  });
}

var compileUtil = {
  html: function(node, vm, exp) {
    this.bind(node, vm, exp, 'html');
  },
  
  bind: function(node, vm, exp, dir) {
    var updaterFn = updater[dir + 'Updater'];
    // 第一次初始化视图
    updaterFn && updaterFn(node, this._getVMVal(vm, exp));
    // 实例化Watcher，添加订阅者
    new Watcher(vm, exp, function(value, oldValue) {
      // 属性变化的视图更新函数
      updaterFn && updaterFn(node, value, oldValue);
    });
  },
}

var Updater = {
  htmlUpdater: function(node, value) {
    node.innerHTML = typeof value == 'undefined' ? '' : value;
  }
}

function Watcher(vm, expOrFn, cb) {
  this.cb = cb;
  this.vm = vm;
  this.expOrFn = expOrFn;
  this.value = this.get();
}

Watcher.prototype = {
  update: function() {
    this.run();
  },
  run: function() {
    var value = this.get();
    var oldVal = this.value;
    if (value !== oldVal) {
      this.value = value;
      this.cb.call(this.vm, value, oldVal);
    }
  },
  get: function() {
    Dep.target = this;
    var value = this.getter.call(this.vm, this.vm);
    Dep.target = null;
    return value;
  }
};