new Vue({
 el: '#container',
 data: {
  count: 100
 },
 ...
});

// 这里的options就是上面声明Vue对象的json对象
Vue.prototype._init = function (options) {
 ...
 var vm = this;
 ...
 initLifecycle(vm);
 initEvents(vm);
 initRender(vm);
 callHook(vm, 'beforeCreate');
 // 这里就是我们接下来要跟进的初始化Vue参数
 initState(vm);
 initInjections(vm);
 callHook(vm, 'created');
 ...
 };

function initState (vm) {
 vm._watchers = [];
 var opts = vm.$options;
 if (opts.props) { initProps(vm, opts.props); }
 if (opts.methods) { initMethods(vm, opts.methods); }
 // 我们的count在这里初始化
 if (opts.data) {
 initData(vm);
 } else {
 observe(vm._data = {}, true /* asRootData */);
 }
 if (opts.computed) { initComputed(vm, opts.computed); }
 if (opts.watch) { initWatch(vm, opts.watch); }
}

function initData (vm) {
 var data = vm.$options.data;
 data = vm._data = typeof data === 'function'
 ? data.call(vm)
 : data || {};
 if (!isPlainObject(data)) {
 data = {};
 }
 ...
 // observe data
 observe(data, true /* asRootData */);

/**
 * Attempt to create an observer instance for a value,
 * returns the new observer if successfully observed,
 * or the existing observer if the value already has one.
 */
function observe (value, asRootData) {
 if (!isObject(value)) {
 return
 }
 var ob;
 if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
 ob = value.__ob__;
 } else if (
 /* 为了防止value不是单纯的对象而是Regexp或者函数之类的，或者是vm实例再或者是不可扩展的 */
 observerState.shouldConvert &&
 !isServerRendering() &&
 (Array.isArray(value) || isPlainObject(value)) &&
 Object.isExtensible(value) &&
 !value._isVue
 ) {
 ob = new Observer(value);
 }
 if (asRootData && ob) {
 ob.vmCount++;
 }
 return ob
}

/**
 * Observer class that are attached to each observed
 * object. Once attached, the observer converts target
 * object's property keys into getter/setters that
 * collect dependencies and dispatches updates.
 */
var Observer = function Observer (value) {
 this.value = value;
 this.dep = new Dep();
 this.vmCount = 0;
 // def函数是defineProperty的简单封装
 def(value, '__ob__', this);
 if (Array.isArray(value)) {
 // 在es5及更低版本的js里，无法完美继承数组，这里检测并选取合适的函数
 // protoAugment函数使用原型链继承，copyAugment函数使用原型链定义（即对每个数组defineProperty）
 var augment = hasProto
  ? protoAugment
  : copyAugment;
 augment(value, arrayMethods, arrayKeys);
 this.observeArray(value);
 } else {
 this.walk(value);
 }
};

/**
 * Observe a list of Array items.
 */
Observer.prototype.observeArray = function observeArray (items) {
 for (var i = 0, l = items.length; i < l; i++) {
 observe(items[i]);
 }
};

/**
 * A dep is an observable that can have multiple
 * directives subscribing to it.
 */
var Dep = function Dep () {
 this.id = uid$1++;
 this.subs = [];
};

/**
 * Walk through each property and convert them into
 * getter/setters. This method should only be called when
 * value type is Object.
 */
Observer.prototype.walk = function walk (obj) {
 var keys = Object.keys(obj);
 for (var i = 0; i < keys.length; i++) {
 defineReactive$$1(obj, keys[i], obj[keys[i]]);
 }
};

/**
 * Define a reactive property on an Object.
 */
function defineReactive$$1 (obj, key, val, customSetter) {
 var dep = new Dep();

 var property = Object.getOwnPropertyDescriptor(obj, key);
 if (property && property.configurable === false) {
 return
 }

 // cater for pre-defined getter/setters
 var getter = property && property.get;
 var setter = property && property.set;

 var childOb = observe(val);
 Object.defineProperty(obj, key, {
 enumerable: true,
 configurable: true,
 get: function reactiveGetter () {
  var value = getter ? getter.call(obj) : val;
  if (Dep.target) {
  dep.depend();
  if (childOb) {
   childOb.dep.depend();
  }
  if (Array.isArray(value)) {
   dependArray(value);
  }
  }
  return value
 },
 set: function reactiveSetter (newVal) {
  var value = getter ? getter.call(obj) : val;
  // 脏检查，排除了NaN !== NaN的影响
  if (newVal === value || (newVal !== newVal && value !== value)) {
  return
  }
  if (setter) {
  setter.call(obj, newVal);
  } else {
  val = newVal;
  }
  childOb = observe(newVal);
  dep.notify();
 }
 });
}

// the current target watcher being evaluated.
// this is globally unique because there could be only one
// watcher being evaluated at any time.
Dep.target = null;

Dep.prototype.depend = function depend () {
 if (Dep.target) {
 Dep.target.addDep(this);
 }
};

Dep.prototype.notify = function notify () {
 // stablize the subscriber list first
 var subs = this.subs.slice();
 for (var i = 0, l = subs.length; i < l; i++) {
 subs[i].update();
 }
};

/**
 * Add a dependency to this directive.
 */
Watcher.prototype.addDep = function addDep (dep) {
 var id = dep.id;
 if (!this.newDepIds.has(id)) {
 this.newDepIds.add(id);
 this.newDeps.push(dep);
 if (!this.depIds.has(id)) {
  // 使用push()方法添加一个订阅者
  dep.addSub(this);
 }
 }
};

/**
 * Collect dependencies on array elements when the array is touched, since
 * we cannot intercept array element access like property getters.
 */
function dependArray (value) {
 for (var e = (void 0), i = 0, l = value.length; i < l; i++) {
 e = value[i];
 e && e.__ob__ && e.__ob__.dep.depend();
 if (Array.isArray(e)) {
  dependArray(e);
 }
 }
}

/*
 * not type checking this file because flow doesn't play well with
 * dynamically accessing methods on Array prototype
 */
var arrayProto = Array.prototype;
var arrayMethods = Object.create(arrayProto);
['push', 'pop', 'shift', 'unshift', 'splice', 'sort', 'reverse'].forEach(function (method) {
 // cache original method
 var original = arrayProto[method];
 def(arrayMethods, method, function mutator () {
 var arguments$1 = arguments;
 // avoid leaking arguments:
 // http://jsperf.com/closure-with-arguments
 var i = arguments.length;
 var args = new Array(i);
 while (i--) {
  args[i] = arguments$1[i];
 }
 var result = original.apply(this, args);
 var ob = this.__ob__;
 var inserted;
 switch (method) {
  case 'push':
  inserted = args;
  break
  case 'unshift':
  inserted = args;
  break
  case 'splice':
  inserted = args.slice(2);
  break
 }
 if (inserted) { ob.observeArray(inserted); }
 // notify change
 ob.dep.notify();
 return result
 });
});