Dep

let uid = 0

export default class Dep {
  static target?: Watcher;
  id: number
  subs: Array<Watcher>
  
  constructor () {
    this.id = uid++
    this.subs = []
  }
  
  // 添加wathcer
  addSub (watcher: Watcher) {
    this.subs.push(watcher)
  }
  
  removeSub (sub: Wathcer) {
    remove(this.subs, sub)
  }
  
  // 向 wathcer 中添加 dep, 同时 dep 也会讲当前的 Dep.target 的 wathcer 添加进 subs
  depend () {
    if (Dep.target) {
      Dep.target.addDep(this)
    }
  }
  
  // 通过 subs 中的 wathcer.update() 方法
  notify () {
    // 拷贝
    const subs = this.subs.slice();
    if (process.env.NODE_ENV !== 'production' && !config.async) {
      // 如果 watcher 不是异步 (sync: true) 则需要进行排序再执行update
      subs.sort((a, b) => a.id - b.id)
    }
    for (let i = 0, l = subs.length; i< l; i++) {
      subs[i].update()
    }
  }
}

/**
* 保存当前正在执行的 wathcer
*/
Dep.target = null
const targetStack = [] // 栈
export function pushTarget (target?: Watcher) {
  targetStack.push(target)
  Dep.target = target
}

export function popTarget () {
  targetStack.pop()
  Dep.target = targetStack[targetStack.length -1]
}