这两天突然想到之前遇到的一个问题,在组件中和混入文件的watch里侦听同一个变量,这时会发现变量改变时两个侦听函数都执行了,并且混入文件中的对应侦听函数先执行,这跟我的理解不太一样,我的理解是同一个变量的侦听组件中的会覆盖混入文件中的,即两个对象键名冲突时,取组件对象的键值对。现象如下图:
workflow.vue:
workFlowMixins.js:
这个时候还是去官网看下混入的介绍比较合适,在官网中有这么一句话:
当初看这段的时候我理所当然的以为watch的合并原则也是如此的,可事实啪啪打脸,正所谓不要以为你以为的就是你以为的,为了弄清混入的合并策略,这两天我对vue源码这一部分的内容作了个简单梳理。
1、mixin混入api
vue/src/core/global-api/mixin.js,这段代码中的mergeOptions是混入的关键,所以我们继续往下扒。
/* @flow */
import { mergeOptions } from '../util/index'
export function initMixin (Vue: GlobalAPI) {
Vue.mixin = function (mixin: Object) {
this.options = mergeOptions(this.options, mixin)
return this
}
}
2、mergeOptions
mergeOptions的主要逻辑是在vue/src/core/util/options.js文件中。
/**
* Merge two option objects into a new one.
* Core utility used in both instantiation and inheritance.
*/
export function mergeOptions (
parent: Object,
child: Object,
vm?: Component
): Object {
if (process.env.NODE_ENV !== 'production') {
checkComponents(child)
}
if (typeof child === 'function') {
child = child.options
}
normalizeProps(child, vm)
normalizeInject(child, vm)
normalizeDirectives(child)
// Apply extends and mixins on the child options,
// but only if it is a raw options object that isn't
// the result of another mergeOptions call.
// Only merged options has the _base property.
if (!child._base) {
if (child.extends) {
parent = mergeOptions(parent, child.extends, vm)
}
if (child.mixins) {
for (let i = 0, l = child.mixins.length; i < l; i++) {
parent = mergeOptions(parent, child.mixins[i], vm)
}
}
}
const options = {}
let key
for (key in parent) {
mergeField(key)
}
for (key in child) {
if (!hasOwn(parent, key)) {
mergeField(key)
}
}
function mergeField (key) {
const strat = strats[key] || defaultStrat
options[key] = strat(parent[key], child[key], vm, key)
}
return options
}
在此文件中我们可以找到mergeOptions函数,在此函数中最为关键的就是下面两行,这两行代码就是区分各选项所使用的合并策略的。
const strat = strats[key] || defaultStrat
options[key] = strat(parent[key], child[key], vm, key)
3、defaultStrat
defaultStrat是默认策略
/**
* Default strategy.
*/
const defaultStrat = function (parentVal: any, childVal: any): any {
return childVal === undefined
? parentVal
: childVal
}
defaultStrat传入两个参数parentVal、childVal,其中parentVal和childVal是每一次合并中的源和目标对象,比如:mixin.options.xx和vm.options.xx(请注意我书写的顺序,这很重要);采用的合并的策略为子组件选项不存在时返回父组件选项,否则返回子组件。
-
全局注册的混入最先完成混入,并按注册的顺序来逐个合并,先注册的先完成混入合并,依次类推
-
局部注册的混入次之,并按mixins数组里声明的顺序依次完成合并
-
每个混入也可以包含mixins局部混入数组,mixins先完成合并,本混入的options再进行合并
-
组件options最后完成混入合并
-
先合并的"优先级"低,后合并的"优先级"高,也就是组件的options合并优先级最高
-
不同的选项根据自身的混入策略合并方向不一样
4、strats[key]
strats[key]是不同的选项自身的混入策略,主要有:
1) el,propsData
/**
* Options with restrictions
*/
if (process.env.NODE_ENV !== 'production') {
strats.el = strats.propsData = function (parent, child, vm, key) {
if (!vm) {
warn(
`option "${key}" can only be used during instance ` +
'creation with the `new` keyword.'
)
}
return defaultStrat(parent, child)
}
}
通过上述代码可以看出:el和propsData的合并策略就是属于默认合并策略。
2)data
strats.data:
/**
* Helper that recursively merges two data objects together.
*/
function mergeData (to: Object, from: ?Object): Object {
if (!from) return to
let key, toVal, fromVal
const keys = hasSymbol
? Reflect.ownKeys(from)
: Object.keys(from)
for (let i = 0; i < keys.length; i++) {
key = keys[i]
// in case the object is already observed...
if (key === '__ob__') continue
toVal = to[key]
fromVal = from[key]
if (!hasOwn(to, key)) {
set(to, key, fromVal)
} else if (
toVal !== fromVal &&
isPlainObject(toVal) &&
isPlainObject(fromVal)
) {
mergeData(toVal, fromVal)
}
}
return to
}
/**
* Data
*/
export function mergeDataOrFn (
parentVal: any,
childVal: any,
vm?: Component
): ?Function {
if (!vm) {
// in a Vue.extend merge, both should be functions
if (!childVal) {
return parentVal
}
if (!parentVal) {
return childVal
}
// when parentVal & childVal are both present,
// we need to return a function that returns the
// merged result of both functions... no need to
// check if parentVal is a function here because
// it has to be a function to pass previous merges.
return function mergedDataFn () {
return mergeData(
typeof childVal === 'function' ? childVal.call(this, this) : childVal,
typeof parentVal === 'function' ? parentVal.call(this, this) : parentVal
)
}
} else {
return function mergedInstanceDataFn () {
// instance merge
const instanceData = typeof childVal === 'function'
? childVal.call(vm, vm)
: childVal
const defaultData = typeof parentVal === 'function'
? parentVal.call(vm, vm)
: parentVal
if (instanceData) {
return mergeData(instanceData, defaultData)
} else {
return defaultData
}
}
}
}
strats.data = function (
parentVal: any,
childVal: any,
vm?: Component
): ?Function {
if (!vm) {
if (childVal && typeof childVal !== 'function') {
process.env.NODE_ENV !== 'production' && warn(
'The "data" option should be a function ' +
'that returns a per-instance value in component ' +
'definitions.',
vm
)
return parentVal
}
return mergeDataOrFn(parentVal, childVal)
}
return mergeDataOrFn(parentVal, childVal, vm)
}
上述代码解读:如果传入参数vm,则表示组件为根实例。若子选项data存在但是不为function类型,直接返回父选项;否则进行合并策略。合并策略按照从父到子开始递归合并,以child为主,比较key规则如下:
-
若child无此key,parent有,直接合并此key
-
若child和parent都有此key,且非object类型,忽略不作为
-
若child和parent都有此key,且为object类型,则递归合并对象
3)生命周期钩子
/**
* Hooks and props are merged as arrays.
*/
function mergeHook (
parentVal: ?Array<Function>,
childVal: ?Function | ?Array<Function>
): ?Array<Function> {
const res = childVal
? parentVal
? parentVal.concat(childVal)
: Array.isArray(childVal)
? childVal
: [childVal]
: parentVal
return res
? dedupeHooks(res)
: res
}
function dedupeHooks (hooks) {
const res = []
for (let i = 0; i < hooks.length; i++) {
if (res.indexOf(hooks[i]) === -1) {
res.push(hooks[i])
}
}
return res
}
LIFECYCLE_HOOKS.forEach(hook => {
strats[hook] = mergeHook
})
其中LIFECYCLE_HOOKS如下:
//vue/src/shared/constants.js
export const LIFECYCLE_HOOKS = [
'beforeCreate',
'created',
'beforeMount',
'mounted',
'beforeUpdate',
'updated',
'beforeDestroy',
'destroyed',
'activated',
'deactivated',
'errorCaptured',
'serverPrefetch'
]
上述代码可以看出对LIFECYCLE_HOOKS遍历之后,生成每个生命周期的策略,通过mergeHook从父到子开始一步步链接合并成数组,父在前,子在后。在钩子触发时,按照数组从左至右按顺序调用触发。
4)component,directive,filter
/**
* Assets
*
* When a vm is present (instance creation), we need to do
* a three-way merge between constructor options, instance
* options and parent options.
*/
function mergeAssets (
parentVal: ?Object,
childVal: ?Object,
vm?: Component,
key: string
): Object {
const res = Object.create(parentVal || null)
if (childVal) {
process.env.NODE_ENV !== 'production' && assertObjectType(key, childVal, vm)
return extend(res, childVal)
} else {
return res
}
}
ASSET_TYPES.forEach(function (type) {
strats[type + 's'] = mergeAssets
})
其中
//vue/src/shared/constants.js
export const ASSET_TYPES = [
'component',
'directive',
'filter'
]
上述代码可以看出对ASSET_TYPES遍历之后,生成每个asset的合并策略,在mergeAssets中使用extend方法合并为一个对象,按照从子到父的顺序进行合并。const res = Object.create(parentVal || null)这边可以看出是将child合并到以parent为原型的对象上的,那么我们在使用的时候,在res中上查找(child),没有的再从原型上(parent)找,以此类推,所以child的优先级的更高。
5)watch
/**
* Watchers.
*
* Watchers hashes should not overwrite one
* another, so we merge them as arrays.
*/
strats.watch = function (
parentVal: ?Object,
childVal: ?Object,
vm?: Component,
key: string
): ?Object {
// work around Firefox's Object.prototype.watch...
if (parentVal === nativeWatch) parentVal = undefined
if (childVal === nativeWatch) childVal = undefined
/* istanbul ignore if */
if (!childVal) return Object.create(parentVal || null)
if (process.env.NODE_ENV !== 'production') {
assertObjectType(key, childVal, vm)
}
if (!parentVal) return childVal
const ret = {}
extend(ret, parentVal)
for (const key in childVal) {
let parent = ret[key]
const child = childVal[key]
if (parent && !Array.isArray(parent)) {
parent = [parent]
}
ret[key] = parent
? parent.concat(child)
: Array.isArray(child) ? child : [child]
}
return ret
}
上述代码可以看出watch会将每个watcher合并成一个数组,从父到子顺序合并,在同名wather属性触发时,按照数组从左至右的顺序调用触发。同样也是父在前,子在后。
6)props、methods、computed、inject
/**
* Other object hashes.
*/
strats.props =
strats.methods =
strats.inject =
strats.computed = function (
parentVal: ?Object,
childVal: ?Object,
vm?: Component,
key: string
): ?Object {
if (childVal && process.env.NODE_ENV !== 'production') {
assertObjectType(key, childVal, vm)
}
if (!parentVal) return childVal
const ret = Object.create(null)
extend(ret, parentVal)
if (childVal) extend(ret, childVal)
return ret
}
上述代码可以看出props,methods,computed,inject的合并策略和component,directive,filter比较相似,都是使用extend方法合并为一个对象,按照从子到父进行合并,所以child优先级更高。
7)provide
strats.provide = mergeDataOrFn
provide的合并策略和data类似,毕竟都是基于mergeDataOrFn实现的。
Time!