text: /* @flow */
  
  import {
    warn,
    remove,
    isObject,
    parsePath,
    _Set as Set,
    handleError,
    noop
  } from '../util/index'
  
  import { traverse } from './traverse'
  import { queueWatcher } from './scheduler'
  import Dep, { pushTarget, popTarget } from './dep'
  
  import type { SimpleSet } from '../util/index'
  
  let uid = 0
  
  /**
   * 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.
   */
  export default class Watcher {
    vm: Component;
    expression: string;
    cb: Function;
    id: number;
    deep: boolean;
    user: boolean;
    lazy: boolean;
    sync: boolean;
    dirty: boolean;
    active: boolean;
    deps: Array<Dep>;
    newDeps: Array<Dep>;
    depIds: SimpleSet;
    newDepIds: SimpleSet;
    before: ?Function;
    getter: Function;
    value: any;
  
    constructor (
      vm: Component,
      expOrFn: string | Function,
      cb: Function,
      options?: ?Object,
      isRenderWatcher?: boolean
    ) {
      this.vm = vm
      if (isRenderWatcher) {
        vm._watcher = this
      }
      vm._watchers.push(this)
      // options
      if (options) {
        this.deep = !!options.deep
        this.user = !!options.user
        this.lazy = !!options.lazy
        this.sync = !!options.sync
        this.before = options.before
      } else {
        this.deep = this.user = this.lazy = this.sync = false
      }
      this.cb = cb
      this.id = ++uid // uid for batching
      this.active = true
      this.dirty = this.lazy // for lazy watchers如果是计算属性，那么dirty=true
      this.deps = []
      this.newDeps = []
      this.depIds = new Set()
      this.newDepIds = new Set()
      this.expression = process.env.NODE_ENV !== 'production'
        ? expOrFn.toString()
        : ''
      // parse expression for getter
      if (typeof expOrFn === 'function') {
        this.getter = expOrFn
      } else {
        this.getter = parsePath(expOrFn)
        if (!this.getter) {
          this.getter = noop
          process.env.NODE_ENV !== '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()
    }
  
    /**
     * Evaluate the getter, and re-collect dependencies.
     */
    get () {
      pushTarget(this)  // 将watcher放到全局上
      let value
      const vm = this.vm
      try {
        value = this.getter.call(vm, vm)  //  取值，会收集依赖
      } 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
    }
  
    /**
     * Add a dependency to this directive.
     */
    addDep (dep: Dep) {
      const id = dep.id
      if (!this.newDepIds.has(id)) {
        this.newDepIds.add(id)
        this.newDeps.push(dep)
        if (!this.depIds.has(id)) {
          dep.addSub(this)
        }
      }
    }
  
    /**
     * Clean up for dependency collection.
     */
    cleanupDeps () {
      let i = this.deps.length
      while (i--) {
        const dep = this.deps[i]
        if (!this.newDepIds.has(dep.id)) {
          dep.removeSub(this)
        }
      }
      let tmp = this.depIds
      this.depIds = this.newDepIds
      this.newDepIds = tmp
      this.newDepIds.clear()
      tmp = this.deps
      this.deps = this.newDeps
      this.newDeps = tmp
      this.newDeps.length = 0
    }
  
    /**
     * Subscriber interface.
     * Will be called when a dependency changes.
     */
    update () {
      /* istanbul ignore else */
      if (this.lazy) {
        this.dirty = true
      } else if (this.sync) {
        this.run()
      } else {
        queueWatcher(this)
      }
    }
  
    /**
     * Scheduler job interface.
     * Will be called by the scheduler.
     */
    run () {
      if (this.active) {
        const 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
          const 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 value of the watcher.
     * This only gets called for lazy watchers.
     */
    evaluate () {
      this.value = this.get()
      this.dirty = false
    }
  
    /**
     * Depend on all deps collected by this watcher.
     */
    depend () {
      let i = this.deps.length
      while (i--) {
        this.deps[i].depend()
      }
    }
  
    /**
     * Remove self from all dependencies' subscriber list.
     */
    teardown () {
      if (this.active) {
        // remove self from vm's watcher list
        // this is a somewhat expensive operation so we skip it
        // if the vm is being destroyed.
        if (!this.vm._isBeingDestroyed) {
          remove(this.vm._watchers, this)
        }
        let i = this.deps.length
        while (i--) {
          this.deps[i].removeSub(this)
        }
        this.active = false
      }
    }
  }
  