vue.js 源代码学习笔记 ----- $watcher

/* @flow */

import { queueWatcher } from './scheduler'
import Dep, { pushTarget, popTarget } from './dep'

import {
warn,
remove,
isObject,
parsePath,
_Set as Set,
handleError
} 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.

   一个 watcher编译成 一个表达式 和 依赖集合, 还能出发回调当 v-model的值改变
    这也能用到wathcer api中, 如果 vm.$watch('abc', fn);
*/

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: Set;
newDepIds: Set;
getter: Function;
value: any;

constructor (
vm: Component,
expOrFn: string | Function,
cb: Function,
options?: Object
) {
    //这个watcher所属的vm
this.vm = vm

    // 一个vm 有多个watchers , vm.$watch(a,fn1)  vm.$watch(b,fn2) {{b.UpperCase()}} 等等
vm._watchers.push(this)

// options
if (options) {
      //是否深度检测依赖
this.deep = !!options.deep
this.user = !!options.user
this.lazy = !!options.lazy
this.sync = !!options.sync
} 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
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') {
      //监控函数
this.getter = expOrFn
} else {
      //监控表达式, 注意 这里也会返回一个路径的函数
this.getter = parsePath(expOrFn)

if (!this.getter) {

this.getter = function () {}

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 () {
    //Dep.target = this
pushTarget(this)

let value
const vm = this.vm

if (this.user) {
try {
        // 因为传入了 vm, 所以在methods中可以取得this.data.abc
        // 比如 this.getter 是 fucntion(){ return this.price * this.nums; }; 因为执行这个函数, 也就会去获取 this.price 和 this.nums的值
        // 就会分别触发这两个数据的get方法, 然后这个watcher就会加入这两个数据的dep, this.push(depq) this.push(dep2); 通过这一步就收集了这个函数的两个依赖
value = this.getter.call(vm, vm)
} catch (e) {
handleError(e, vm, `getter for watcher "${this.expression}"`)
}
} else {
value = this.getter.call(vm, vm)
}

// "touch" every property so they are all tracked as
// dependencies for deep watching
    //“触摸”每一个属性，这样它们都被跟踪为 深度观察依赖

if (this.deep) {
traverse(value)
}

popTarget()

this.cleanupDeps()
return value

}

/**
* Add a dependency to this directive.
     添加一个依赖到这个指令
*/
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.

     计算一个watcher的值, 这个只会被 延迟watchers调用
*/
evaluate () {
this.value = this.get()
this.dirty = false
}

/**
* Depend on all deps collected by this watcher.
     通过这个watcher, 收集所有的依赖?
*/
depend () {
let i = this.deps.length
while (i--) {
this.deps[i].depend()
}
}

/**
* Remove self from all dependencies' subscriber list.

     从所有的依赖订阅清单中移除自己
*/
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.
      // 移除依赖是一个昂贵的操作, 所有如果这个vm已经销毁le, 我们就跳一跳

if (!this.vm._isBeingDestroyed) {
remove(this.vm._watchers, this)
}

let i = this.deps.length

while (i--) {
this.deps[i].removeSub(this)
}

this.active = false
}
}
}

/**
* Recursively traverse an object to evoke all converted
* getters, so that every nested property inside the object
* is collected as a "deep" dependency.

   递归追踪一个对象去唤起所有的已经转换的getters,
   这样在一个对象的所以嵌套的熟悉都被收集成为一个深层的依赖
*/


const seenObjects = new Set()

function traverse (val: any) {
seenObjects.clear()
_traverse(val, seenObjects)
}

function _traverse (val: any, seen: Set) {
let i, keys
const isA = Array.isArray(val)
if ((!isA && !isObject(val)) || !Object.isExtensible(val)) {
return
}

if (val.__ob__) {
const depId = val.__ob__.dep.id

if (seen.has(depId)) {
return
}

seen.add(depId)
}

if (isA) {
i = val.length
while (i--) _traverse(val[i], seen)
} else {
    //for in 都不用了, 加快速度
keys = Object.keys(val)
i = keys.length
while (i--) _traverse(val[keys[i]], seen)
}
}