关于Vue源码childob.dep.depend问题

李恒道

前文

我一直有一个疑问，就是vue的源码defineRective部分的childob.dep.depend这行代码到底有什么作用

export function defineReactive (
  obj: Object,
  key: string,
  val: any,
  customSetter?: ?Function,
  shallow?: boolean
) {
  const dep = new Dep()

  const property = Object.getOwnPropertyDescriptor(obj, key)
  if (property && property.configurable === false) {
    return
  }

  // cater for pre-defined getter/setters
  const getter = property && property.get
  const setter = property && property.set
  if ((!getter || setter) && arguments.length === 2) {
    val = obj[key]
  }

  let childOb = !shallow && observe(val)
  Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get: function reactiveGetter () {
      const value = getter ? getter.call(obj) : val
      if (Dep.target) {
        dep.depend()
        if (childOb) {
          childOb.dep.depend()
          if (Array.isArray(value)) {
            dependArray(value)
          }
        }
      }
      return value
    },
    set: function reactiveSetter (newVal) {
      const 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 (process.env.NODE_ENV !== 'production' && 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()
    }
  })
}

childob.dep.depend其主要目的就是在数组以及对象进行Vue.set的时候进行触发响应

那设置成获取属性的时候dep直接响应难道不可以么？

比如在获取childOb的时候

let childOb = !shallow && observe(val, dep)

将definedproperty的时候的闭包dep传入

这样就不需要再运行childOb.dep.depend收集依赖了

根据我个人的测试

认为这行代码更像是一种设计思想，在将源码修改成获取属性的封闭dep之后单测依然可以正常通过

HeadlessChrome 78.0.3882 (Windows 10.0.0): Executed 1248 of 1248 (1 FAILED) (15.102 secs / 14.846 secs)
TOTAL: 1 FAILED, 1247 SUCCESS

唯一的一个失败是

HeadlessChrome 78.0.3882 (Windows 10.0.0) Observer observing object prop change FAILED
        Expected 2 to be 3.
            at UserContext.<anonymous> (webpack:///test/unit/modules/observer/observer.spec.js:195:32 <- index.js:61131:33)
        Expected 2 to be 3.
            at UserContext.<anonymous> (webpack:///test/unit/modules/observer/observer.spec.js:200:41 <- index.js:61139:42)
        Expected 3 to be 4.
            at UserContext.<anonymous> (webpack:///test/unit/modules/observer/observer.spec.js:207:32 <- index.js:61145:33)

检测代码如下

    expect(watcher.deps.length).toBe(3) // obj.a + a + a.b
    obj.a.b = 3
    expect(watcher.update.calls.count()).toBe(1)
    // swap object
    obj.a = { b: 4 }
    expect(watcher.update.calls.count()).toBe(3)
    watcher.deps = []

    Dep.target = watcher
    obj.a.b
    obj.c
    Dep.target = null
    expect(watcher.deps.length).toBe(4)

这个我认为是完全不影响的

所以我更倾向于childob.dep.depend是一种设计思想，将其改为子继承父dep属性也算是完全可以实现的。

王一之

改了一行，然后全崩了？