1. // 当前 Dep 的 id，唯一
    2. let uid = 0
    4. /**
    5.  * A dep is an observable that can have multiple
    6.  * directives subscribing to it.
    7.  */
    8. export default class Dep {
    9.   static target: ?Watcher;
    10.   id: number;
    11.   subs: Array<Watcher>;
    13.   constructor () {
    14.     // 当前 Dep 的 id，唯一
    15.     this.id = uid++
    16.     // 监听者列表
    17.     this.subs = []
    18.   }
    20.     // 添加监听者
    21.   addSub (sub: Watcher) {
    22.     this.subs.push(sub)
    23.   }
    25.     // 移除监听
    26.   removeSub (sub: Watcher) {
    27.     remove(this.subs, sub)
    28.   }
    30.     // 依赖收集，双向收集，目的是为了保持一致性
    31.   depend () {
    32.     if (Dep.target) {
    33.       Dep.target.addDep(this)
    34.     }
    35.   }
    37.     // 通知依赖项响应
    38.   notify () {
    39.     // stabilize the subscriber list first
    40.     // 浅拷贝，用于可能的数据处理
    41.     const subs = this.subs.slice()
    42.     if (process.env.NODE_ENV !== 'production' && !config.async) {
    43.       // subs aren't sorted in scheduler if not running async
    44.       // we need to sort them now to make sure they fire in correct
    45.       // order
    46.       subs.sort((a, b) => a.id - b.id)
    47.     }
    48.     // 通知监听者响应
    49.     for (let i = 0, l = subs.length; i < l; i++) {
    50.       subs[i].update()
    51.     }
    52.   }
    53. }
    55. // The current target watcher being evaluated.
    56. // This is globally unique because only one watcher
    57. // can be evaluated at a time.
    58. Dep.target = null
    59. const targetStack = []
    61. export function pushTarget (target: ?Watcher) {
    62.   targetStack.push(target)
    63.   Dep.target = target
    64. }
    66. export function popTarget () {
    67.   targetStack.pop()
    68.   Dep.target = targetStack[targetStack.length - 1]
    69. }