vue中watch和computed為什麼能監聽到數據的改變以及不同之處

先來個流程圖，水平有限，湊活看吧-_-|| 首先在創建一個Vue應用時： Vue構造函數源碼：在initState方法中會初始化data、watch和computed，並調用observe函數監聽data(Object.defineProperty)： 1、observe observe在init ...

先來個流程圖，水平有限，湊活看吧-_-||

首先在創建一個Vue應用時：

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

Vue構造函數源碼：

//創建Vue構造函數
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，會初始化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);
  ......
};

在initState方法中會初始化data、watch和computed，並調用observe函數監聽data(Object.defineProperty)：

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中也會調用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);
  }
}

1、observe

observe在initState 時被調用，為vue實例的data屬性值創建getter、setter函數，在setter中dep.depend會把watcher實例添加到Dep實例的subs屬性中，在getter中會調用dep.notify，調用watcher的update方法。

/**
 * 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.
 * 該函數在initState中有調用
 */
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 全局變數指向的就是指向當前正在解析生成的 Watcher
　　　　//會執行到dep.addSub，將Watcher添加到Dep對象的Watcher數組中
      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();//如果數據被重新賦值了, 調用 Dep 的 notify 方法, 通知所有的 Watcher

 } }); }

2、Dep

Watcher的update方法是在new Dep的notify的方法中被調用的

/**
 * A dep is an observable that can have multiple
 * directives subscribing to it.
 */
var Dep = function Dep () {
  this.id = uid++;
  this.subs = [];
};
//設置某個Watcher的依賴
//這裡添加Dep.target，用來判斷是不是Watcher的構造函數調用
//也就是其this.get調用
Dep.prototype.depend = function depend () {
  if (Dep.target) {
    Dep.target.addDep(this);
  }
};
//在該方法中會觸發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();
  }
};

3、watch

初始化watch，函數中會調用createWatcher，createWatcher會調用$watch，$watch調用new Watcher實例。

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();
  }
};
}

2、computed

初始化computed，調用new Watcher()，並通過defineComputed函數將計算屬性掛載到vue實例上，使計算屬性可以在模板中使用

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的函數
    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
      );
    }

    //組件定義的計算屬性已在
    //組件原型。我們只需要定義定義的計算屬性
    //在這裡實例化。
    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函數，在模板獲取對應computed數據時會調用
function createComputedGetter (key) {
  return function computedGetter () {
    var watcher = this._computedWatchers && this._computedWatchers[key];
    if (watcher) {
      if (watcher.dirty) {//true
        watcher.evaluate();//該方法會調用watcher.get方法，也就是computed對應的函數
      }
      if (Dep.target) {
        watcher.depend();
      }
      return watcher.value
    }
  }
}

通過以上代碼可以看到watch和computed都是通過new Watcher實例實現數據的監聽的，但是computed的options中lazy為true，這個參數導致它們走的是兩條不同路線。

computed：模板獲取數據時，觸發其getter函數，最終調用watcher.get，也就是調用對應回調函數。

watch：模板獲取數據時，觸發其getter函數，將watcher添加到對應的Dep.subs中，在之後setter被調用時，Dep.notify通知所有watcher進行update，最終調用watcher.cb,也就是調用對應回調函數。

3、Watcher

構造函數在是watch時，會最後調用this.get，會觸發屬性的getter函數，將該Watcher添加到Dep的subs中，用於通知數據變動時調用。

調用Watcher實例的update方法會觸發其run方法，run方法中會調用觸發函數。其depend方法會調用new Dep的depend方法，dep的depend會調用Watcher的addDep方法，最終會把該watcher實例添加到Dep的subs屬性中

/**
   *觀察者解析表達式，收集依賴項，
   *併在表達式值更改時激發回調。
   *這用於$watch（）api和指令。
   */
  var Watcher = function Watcher (
    vm,
    expOrFn,
    cb,
    options,
    isRenderWatcher
  ) {
    this.vm = vm;
  ......
    this.cb = cb;//觸發函數
    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會返回undefined，而watch會執行Watcher.get
  };

  /**
   * Scheduler job interface.
   * Will be called by the scheduler.
   * 該方法會執行觸發函數
   */
  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.
   * 在方法中調用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);//該方法最終也會調用run方法
    }
  };
  /**
   * Depend on all deps collected by this watcher.會調用new Dep的depend方法，dep的depend會調用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);
      }
    }
  };