vue中watch和computed为什么能监听到数据的改变以及不同之处

var app = new Vue({
 el: '#app',
 data: {
  message: 'Hello Vue!'
 }
})

//创建Vue构造函数
function Vue (options) {
 if (!(this instanceof Vue)
 ) {
  warn('Vue is a constructor and should be called with the `new` keyword');
 }
 this._init(options);
}
//_init方法，会初始化data,watch,computed等
Vue.prototype._init = function (options) {
 var vm = this;
 // a uid
 vm._uid = uid$3++;
 ......
 // expose real self
 vm._self = vm;
 initLifecycle(vm);
 initEvents(vm);
 initRender(vm);
 callHook(vm, 'beforeCreate');
 initInjections(vm); // resolve injections before data/props
 initState(vm);
 ......
};

function initState (vm) {
 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);//initData中也会调用observe方法
 } else {
  observe(vm._data = {}, true /* asRootData */);
 }
 if (opts.computed) { initComputed(vm, opts.computed); }
 if (opts.watch && opts.watch !== nativeWatch) {
  initWatch(vm, opts.watch);
 }
}

/**
 * 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.
 * 该函数在initState中有调用
 */
function observe (value, asRootData) {
 if (!isObject(value) || value instanceof VNode) {
  return
 }
 var ob;
 if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
  ob = value.__ob__;
 } else if (
  shouldObserve &&
  !isServerRendering() &&
  (Array.isArray(value) || isPlainObject(value)) &&
  Object.isExtensible(value) &&
  !value._isVue
 ) {
  ob = new Observer(value);
 }
 if (asRootData && ob) {
  ob.vmCount++;
 }
 re * Observer class that is attached to each observed
 * object. Once attached, the observer converts the target
 * object's property keys into getter/setters that
 * collect dependencies and dispatch updates.
 */
var Observer = function Observer (value) {
 this.value = value;
 this.dep = new Dep();
 this.vmCount = 0;
 def(value, '__ob__', this);
 if (Array.isArray(value)) {
  if (hasProto) {
   protoAugment(value, arrayMethods);
  } else {
   copyAugment(value, arrayMethods, arrayKeys);
  }
  this.observeArray(value);
 } else {
  this.walk(value);
 }
};
/**
 * Walk through all properties 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$$1(obj, keys[i]);
 }
};
/**
 * Define a reactive property on an Object.
 */
function defineReactive$$1 (
 obj,
 key,
 val,
 customSetter,
 shallow
) {
 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;
 if ((!getter || setter) && arguments.length === 2) {
  val = obj[key];
 }
 var childOb = !shallow && observe(val);
 Object.defineProperty(obj, key, {
  enumerable: true,
  configurable: true,
  get: function reactiveGetter () {
   var value = getter ? getter.call(obj) : val;
　　　　//Dep.target 全局变量指向的就是指向当前正在解析生成的 Watcher
　　　　//会执行到dep.addSub，将Watcher添加到Dep对象的Watcher数组中
   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;
   /* eslint-disable no-self-compare */
   if (newVal === value || (newVal !== newVal && value !== value)) {
    return
   }
   /* eslint-enable no-self-compare */
   if (customSetter) {
    customSetter();
   }
   // #7981: for accessor properties without setter
   if (getter && !setter) { return }
   if (setter) {
    setter.call(obj, newVal);
   } else {
    val = newVal;
   }
   childOb = !shallow && observe(newVal);
   dep.notify();//如果数据被重新赋值了, 调用 Dep 的 notify 方法, 通知所有的 Watcher
 } }); }

/**
 * A dep is an observable that can have multiple
 * directives subscribing to it.
 */
var Dep = function Dep () {
 this.id = uid++;
 this.subs = [];
};
//设置某个Watcher的依赖
//这里添加Dep.target，用来判断是不是Watcher的构造函数调用
//也就是其this.get调用
Dep.prototype.depend = function depend () {
 if (Dep.target) {
  Dep.target.addDep(this);
 }
};
//在该方法中会触发subs的update方法
Dep.prototype.notify = function notify () {
 // stabilize the subscriber list first
 var subs = this.subs.slice();
 if (!config.async) {
  // subs aren't sorted in scheduler if not running async
  // we need to sort them now to make sure they fire in correct
  // order
  subs.sort(function (a, b) { return a.id - b.id; });
 }
 for (var i = 0, l = subs.length; i < l; i++) {
  subs[i].update();
 }
};

function initWatch (vm, watch) {
 for (var key in watch) {
  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);
  }
 }
}
function createWatcher (
 vm,
 expOrFn,
 handler,
 options
) {
 if (isPlainObject(handler)) {
  options = handler;
  handler = handler.handler;
 }
 if (typeof handler === 'string') {
  handler = vm[handler];
 }
 return vm.$watch(expOrFn, handler, options)
}
Vue.prototype.$watch = function (
 expOrFn,
 cb,
 options
) {
 var vm = this;
 if (isPlainObject(cb)) {
  return createWatcher(vm, expOrFn, cb, options)
 }
 options = options || {};
 options.user = true;
 var watcher = new Watcher(vm, expOrFn, cb, options);
 if (options.immediate) {
  try {
   cb.call(vm, watcher.value);
  } catch (error) {
   handleError(error, vm, ("callback for immediate watcher \"" + (watcher.expression) + "\""));
  }
 }
 return function unwatchFn () {
  watcher.teardown();
 }
};
}

var computedWatcherOptions = { lazy: true }
function initComputed (vm, computed) {
 // $flow-disable-line
 var watchers = vm._computedWatchers = Object.create(null);
 // computed properties are just getters during SSR
 var isSSR = isServerRendering();
 for (var key in computed) {
  var userDef = computed[key];
  var getter = typeof userDef === 'function' ? userDef : userDef.get;
　　//getter也就是computed的函数
  if (getter == null) {
   warn(
    ("Getter is missing for computed property \"" + key + "\"."),
    vm
   );
  }
  if (!isSSR) {
   // create internal watcher for the computed property.
   watchers[key] = new Watcher(
    vm,
    getter || noop,
    noop,
    computedWatcherOptions
   );
  }
  //组件定义的计算属性已在
  //组件原型。我们只需要定义定义的计算属性
  //在这里实例化。
  if (!(key in vm)) {
   defineComputed(vm, key, userDef);
  } else {
   if (key in vm.$data) {
    warn(("The computed property \"" + key + "\" is already defined in data."), vm);
   } else if (vm.$options.props && key in vm.$options.props) {
    warn(("The computed property \"" + key + "\" is already defined as a prop."), vm);
   }
  }
 }
}
function defineComputed (
 target,
 key,
 userDef
) {
 var shouldCache = !isServerRendering();//true
 if (typeof userDef === 'function') {
  sharedPropertyDefinition.get = shouldCache
   ? createComputedGetter(key)
   : createGetterInvoker(userDef);
  sharedPropertyDefinition.set = noop;
 } else {
  sharedPropertyDefinition.get = userDef.get
   ? shouldCache && userDef.cache !== false
    ? createComputedGetter(key)
    : createGetterInvoker(userDef.get)
   : noop;
  sharedPropertyDefinition.set = userDef.set || noop;
 }
 if (sharedPropertyDefinition.set === noop) {
  sharedPropertyDefinition.set = function () {
   warn(
    ("Computed property \"" + key + "\" was assigned to but it has no setter."),
    this
   );
  };
 }
 Object.defineProperty(target, key, sharedPropertyDefinition);
}
//computed的getter函数，在模板获取对应computed数据时会调用
function createComputedGetter (key) {
 return function computedGetter () {
  var watcher = this._computedWatchers && this._computedWatchers[key];
  if (watcher) {
   if (watcher.dirty) {//true
    watcher.evaluate();//该方法会调用watcher.get方法，也就是computed对应的函数
   }
   if (Dep.target) {
    watcher.depend();
   }
   return watcher.value
  }
 }
}

/**
  *观察者解析表达式，收集依赖项，
  *并在表达式值更改时激发回调。
  *这用于$watch（）api和指令。
  */
 var Watcher = function Watcher (
  vm,
  expOrFn,
  cb,
  options,
  isRenderWatcher
 ) {
  this.vm = vm;
 ......
  this.cb = cb;//触发函数
  this.id = ++uid$2; // uid for batching
  this.active = true;
  this.dirty = this.lazy; // for lazy watchers
 ......
  this.value = this.lazy ? undefined ? this.get();//computed会返回undefined，而watch会执行Watcher.get
 };
 /**
  * Scheduler job interface.
  * Will be called by the scheduler.
  * 该方法会执行触发函数
  */
 Watcher.prototype.run = function run () {
  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) {
     try {
      this.cb.call(this.vm, value, oldValue);
     } catch (e) {
      handleError(e, this.vm, ("callback for watcher \"" + (this.expression) + "\""));
     }
    } else {
     this.cb.call(this.vm, value, oldValue);
    }
   }
  }
 };
 /**
  * Evaluate the getter, and re-collect dependencies.
  */
 Watcher.prototype.get = function get () {
  pushTarget(this);
  var value;
  var vm = this.vm;
  try {
   value = this.getter.call(vm, vm);
  } 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();
   this.cleanupDeps();
  }
  return value
 };
 /**
  * Subscriber interface.
  * Will be called when a dependency changes.
  * 在方法中调用Watcher的run方法
  */
 Watcher.prototype.update = function update () {
  /* istanbul ignore else */
  if (this.lazy) {
   this.dirty = true;
  } else if (this.sync) {
   this.run();
  } else {
   queueWatcher(this);//该方法最终也会调用run方法
  }
 };
 /**
  * Depend on all deps collected by this watcher.会调用new Dep的depend方法，dep的depend会调用Watcher的addDep方法
  */
 Watcher.prototype.depend = function depend () {
  var i = this.deps.length;
  while (i--) {
   this.deps[i].depend();
  }
 };
 /**
  * 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)) {
    dep.addSub(this);
   }
  }
 };