Function Watcher

06.Router/basic/js/vue.js:3135–3166

* A watcher parses an expression, collects dependencies, * and fires callback when the expression value changes. * This is used for both the $watch() api and directives. * * @param {Vue} vm * @param {String|Function} expOrFn * @param {Function} cb * @param {Object} options *

(vm, expOrFn, cb, options)

   * @constructor
   */
  function Watcher(vm, expOrFn, cb, options) {
    // mix in options
    if (options) {
      extend(this, options);
    }
    var isFn = typeof expOrFn === 'function';
    this.vm = vm;
    vm._watchers.push(this);
    this.expression = expOrFn;
    this.cb = cb;
    this.id = ++uid$2; // 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.prevError = null; // for async error stacks
    // parse expression for getter/setter
    if (isFn) {
      this.getter = expOrFn;
      this.setter = undefined;
    } else {
      var res = parseExpression(expOrFn, this.twoWay);
      this.getter = res.get;
      this.setter = res.set;
    }
    this.value = this.lazy ? undefined : this.get();
    // state for avoiding false triggers for deep and Array
    // watchers during vm._digest()
    this.queued = this.shallow = false;
  }

  /**
   * Evaluate the getter, and re-collect dependencies.