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);
      }
    }
  };