{{message}}
    測試 
  
  

function initState (vm) {   //第3303行
 vm._watchers = [];
 var opts = vm.$options;
 if (opts.props) { initProps(vm, opts.props); }
 if (opts.methods) { initMethods(vm, opts.methods); }
 if (opts.data) {
  initData(vm);
 } else {
  observe(vm._data = {}, true /* asRootData */);
 }
 if (opts.computed) { initComputed(vm, opts.computed); }
 if (opts.watch && opts.watch !== nativeWatch) {      //如果傳入了watch 且 watch不等于nativeWatch(細節(jié)處理，在Firefox瀏覽器下Object的原型上含有一個watch函數(shù))
  initWatch(vm, opts.watch);                 //調(diào)用initWatch()函數(shù)初始化watch
 }
}

function initWatch (vm, watch) {  //第3541行
 for (var key in watch) {            //遍歷watch里的每個元素
  var handler = watch[key];
  if (Array.isArray(handler)) {          
   for (var i = 0; i < handler.length; i++) {
    createWatcher(vm, key, handler[i]);
   }
  } else {
   createWatcher(vm, key, handler);        //調(diào)用createWatcher
  }
 }
}

function createWatcher (             //創(chuàng)建用戶watcher
 vm,
 expOrFn,
 handler,
 options
) {
 if (isPlainObject(handler)) {           //如果handler是個對象，則將該對象的hanler屬性保存到handler里面 從這里看到值可以是個對象
  options = handler;
  handler = handler.handler;          
 }
 if (typeof handler === 'string') {        
  handler = vm[handler];
 }
 return vm.$watch(expOrFn, handler, options) 　　　 //最后創(chuàng)建一個用戶watch
}

Vue.prototype.$watch = function (   //第3596行
  expOrFn, 　　　　　　　　　　　　　　　　　　//監(jiān)聽的屬性，例如例子里的message
  cb, 　　　　　　　　　　　　　　　　　　　　 //對應(yīng)的函數(shù)
  options 　　　　　　　　　　　　　　　　　　 //選項
 ) {
  var vm = this;
  if (isPlainObject(cb)) {
   return createWatcher(vm, expOrFn, cb, options)
  }
  options = options || {};
  options.user = true;                   //設(shè)置options.user為true,表示這是一個用戶watch
  var watcher = new Watcher(vm, expOrFn, cb, options);   //創(chuàng)建一個Watcher對象
  if (options.immediate) { 　　　　　　　　　　　　　　　　　　　//如果有immediate選項，則直接運行
   cb.call(vm, watcher.value);
  }
  return function unwatchFn () {
   watcher.teardown();
  }
 };
}

var Watcher = function Watcher ( //第3082行
 vm,
 expOrFn,               //偵聽的屬性:message
 cb,                  //對應(yīng)的函數(shù)
 options,
 isRenderWatcher
) {
 this.vm = vm;
 if (isRenderWatcher) {
  vm._watcher = this;
 }
 vm._watchers.push(this);
 // options
 if (options) {
  this.deep = !!options.deep;
  this.user = !!options.user;               //用戶watch這里的user屬性為true
  this.lazy = !!options.lazy;
  this.sync = !!options.sync;
 } else {
  this.deep = this.user = this.lazy = this.sync = false;
 }
 this.cb = cb;
 this.id = ++uid$1; // uid for batching
 this.active = true;
 this.dirty = this.lazy; // for lazy watchers
 this.deps = [];
 this.newDeps = [];
 this.depIds = new _Set();
 this.newDepIds = new _Set();
 this.expression = expOrFn.toString();
 // parse expression for getter
 if (typeof expOrFn === 'function') {         
  this.getter = expOrFn; 
 } else {                         //偵聽器執(zhí)行到這里，
  this.getter = parsePath(expOrFn);            //get對應(yīng)的是parsePath()返回的匿名函數(shù)
  if (!this.getter) {
   this.getter = function () {};
   "development" !== 'production' && warn(
    "Failed watching path: \"" + expOrFn + "\" " +
    'Watcher only accepts simple dot-delimited paths. ' +
    'For full control, use a function instead.',
    vm
   );
  }
 }
 this.value = this.lazy
  ? undefined
  : this.get();                      //最后會執(zhí)行g(shù)et()方法
}; 
function parsePath (path) {       //解析路勁
 if (bailRE.test(path)) { 
  return
 }
 var segments = path.split('.');
 return function (obj) {        //返回一個函數(shù)，參數(shù)是一個對象
  for (var i = 0; i < segments.length; i++) {
   if (!obj) { return }
   obj = obj[segments[i]];
  }
  return obj
 }
}

Watcher.prototype.get = function get () {   //第3135行
 pushTarget(this);                 //將當前用戶watch保存到Dep.target總=中
 var value;
 var vm = this.vm;
 try {
  value = this.getter.call(vm, vm);        //執(zhí)行用戶wathcer的getter()方法，此方法會將當前用戶watcher作為訂閱者訂閱起來
 } catch (e) {
  if (this.user) {
   handleError(e, vm, ("getter for watcher \"" + (this.expression) + "\""));
  } else {
   throw e
  }
 } finally {
  // "touch" every property so they are all tracked as
  // dependencies for deep watching
  if (this.deep) {
   traverse(value);
  }
  popTarget();                  //恢復之前的watcher
  this.cleanupDeps();
 }
 return value
};

Watcher.prototype.update = function update () {  //第3200行 更新Watcher
 /* istanbul ignore else */
 if (this.lazy) {
  this.dirty = true;
 } else if (this.sync) {              //如果$this.sync為true，則直接運行this.run獲取結(jié)果
  this.run();                   
 } else {
  queueWatcher(this);               //否則調(diào)用queueWatcher()函數(shù)把所有要執(zhí)行update()的watch push到隊列中
 }
};

Watcher.prototype.run = function run () {   //第3215行 執(zhí)行，會調(diào)用get()獲取對應(yīng)的值 
 if (this.active) {    
  var value = this.get();
  if (
   value !== this.value ||
   // Deep watchers and watchers on Object/Arrays should fire even
   // when the value is the same, because the value may
   // have mutated.
   isObject(value) ||
   this.deep
  ) {
   // set new value
   var oldValue = this.value;
   this.value = value;
   if (this.user) {            //如果是個用戶 watcher
    try {
     this.cb.call(this.vm, value, oldValue);    //執(zhí)行這個回調(diào)函數(shù) vm作為上下文 參數(shù)1為新值 參數(shù)2為舊值 　　　　也就是最后我們自己定義的function(newval,val){ console.log(newval,val) }函數(shù)
    } catch (e) { 
     handleError(e, this.vm, ("callback for watcher \"" + (this.expression) + "\""));
    }
   } else {
    this.cb.call(this.vm, value, oldValue);
   }
  }
 }
};