组件化是 Vue, React 等这些框架的一个核心思想，通过把页面拆成一个个高内聚、低耦合的组件，可以极大程度提高我们的代码复用度，同时也使得项目更加易于维护。所以，本文就来分析下组件的渲染流程。我们通过下面这个例子来进行分析：
<div id="demo">
<comp></comp>
</div>
<script>
Vue.component('comp', { template: '<div>I am comp</div>', }) const app = new Vue({ el: '#demo', })
</script>
这里我们分为两步来分析：组件声明、组件创建及渲染
组件声明
首先，我们看下 Vue.component 是什么东西，它的声明在 core/global-api/assets.js：
export function initAssetRegisters(Vue: GlobalAPI) {
// ASSET_TYPES是数组：['component','directive','filter']
ASSET_TYPES.forEach((type) => {
Vue[type] = function (
id: string, definition: Function | Object
): Function | Object | void {
if (!definition) {
return this.options[type + 's'][id]
} else {
/* istanbul ignore if */
if (process.env.NODE_ENV !== 'production' && type === 'component') {
validateComponentName(id)
}
// 组件声明相关代码
if (type === 'component' && isPlainObject(definition)) {
definition.name = definition.name || id
// _base是Vue
// Vue.extend({})返回组件构造函数
definition = this.options._base.extend(definition)
}
if (type === 'directive' && typeof definition === 'function') {
definition = {bind: definition, update: definition}
}
// 注册到components选项中去
// 在Vue原始选项上添加组件配置，将来其他组件继承，它们都有这些组件注册
this.options[type + 's'][id] = definition
return definition
}
}
})
}
这里 this.options._base.extend(definition) 调用的其实就是 Vue.extend(definition)：
Vue.extend = function (extendOptions: Object): Function {
extendOptions = extendOptions || {}
const Super = this
const SuperId = Super.cid
const cachedCtors = extendOptions._Ctor || (extendOptions._Ctor = {})
if (cachedCtors[SuperId]) {
return cachedCtors[SuperId]
}
const name = extendOptions.name || Super.options.name
if (process.env.NODE_ENV !== 'production' && name) {
validateComponentName(name)
}
const Sub = function VueComponent(options) {
this._init(options)
}
Sub.prototype = Object.create(Super.prototype)
Sub.prototype.constructor = Sub
Sub.cid = cid++
Sub.options = mergeOptions(Super.options, extendOptions)
Sub['super'] = Super
// For props and computed properties, we define the proxy getters on
// the Vue instances at extension time, on the extended prototype. This
// avoids Object.defineProperty calls for each instance created.
if (Sub.options.props) {
initProps(Sub)
}
if (Sub.options.computed) {
initComputed(Sub)
}
// allow further extension/mixin/plugin usage
Sub.extend = Super.extend
Sub.mixin = Super.mixin
Sub.use = Super.use
// create asset registers, so extended classes
// can have their private assets too.
ASSET_TYPES.forEach(function (type) {
Sub[type] = Super[type]
})
// enable recursive self-lookup
if (name) {
Sub.options.components[name] = Sub
}
// keep a reference to the super options at extension time.
// later at instantiation we can check if Super's options have
// been updated.
Sub.superOptions = Super.options
Sub.extendOptions = extendOptions
Sub.sealedOptions = extend({}, Sub.options)
// cache constructor
cachedCtors[SuperId] = Sub
return Sub
}
这里我们可以理解为返回了一个名叫 VueComponent 的构造函数且继承了 Vue。所以，这里的组件定义完成后 Vue 就会变成这样：
{
...
options: {
components: {
comp: function VueComponent() {}
}
}
..
}
组件创建及挂载
我们知道 Vue 中的模板最后会变编译成 render 函数，比如上面例子最终的 render 函数会如下所示：
render() {
with (this) {return _c('div',{attrs:{"id":"demo"}},[_c('comp')],1)}
}
这里 _c 的定义可以在 core/instance/render.js 中找到：
vm._c = (a, b, c, d) => createElement(vm, a, b, c, d, false)
所以 _c('comp') 最终还是调用了 createElement (core/vdom/create-element.js) 这个方法：参考 前端进阶面试题详细解答
export function createElement (
context: Component,
tag: any,
data: any,
children: any,
normalizationType: any,
alwaysNormalize: boolean
): VNode | Array<VNode> {
...
return _createElement(context, tag, data, children, normalizationType)
}
export function _createElement (
context: Component,
tag?: string | Class<Component> | Function | Object,
data?: VNodeData,
children?: any,
normalizationType?: number
): VNode | Array<VNode> {
...
} else if ((!data || !data.pre) && isDef(Ctor = resolveAsset(context.$options, 'components', tag))) {
// 自定义组件
vnode = createComponent(Ctor, data, context, children, tag)
} else {
// unknown or unlisted namespaced elements
// check at runtime because it may get assigned a namespace when its
// parent normalizes children
vnode = new VNode(
tag, data, children,
undefined, undefined, context
)
}
...
}
这里我们只看自定义组件的相关逻辑，发现最后调用了 createComponent (core/vdom/create-component.js)：
export function createComponent (
Ctor: Class<Component> | Function | Object | void,
data: ?VNodeData,
context: Component,
children: ?Array<VNode>,
tag?: string
): VNode | Array<VNode> | void {
...
// install component management hooks onto the placeholder node
// 安装组件管理钩子：未来会做组件初始化（实例创建、挂载）
installComponentHooks(data)
// return a placeholder vnode
const name = Ctor.options.name || tag
const vnode = new VNode(
`vue-component-${Ctor.cid}${name ? `-${name}` : ''}`,
data, undefined, undefined, undefined, context,
{ Ctor, propsData, listeners, tag, children },
asyncFactory
)
return vnode
}
这里我们跳过其他的代码，先看看 installComponentHooks：
function installComponentHooks(data: VNodeData) {
const hooks = data.hook || (data.hook = {})
for (let i = 0; i < hooksToMerge.length; i++) {
const key = hooksToMerge[i]
const existing = hooks[key]
const toMerge = componentVNodeHooks[key]
if (existing !== toMerge && !(existing && existing._merged)) {
hooks[key] = existing ? mergeHook(toMerge, existing) : toMerge
}
}
}
这里会在 data.hook 上挂载一些 hooks，如果用户也传了相同的 hooks 则会进行合并。这个 hooks 又是啥呢：
const componentVNodeHooks = {
// 实例化和挂载
init(vnode: VNodeWithData, hydrating: boolean): ?boolean {
if (
vnode.componentInstance && // 实例已经存在
!vnode.componentInstance._isDestroyed && // 未被销毁
vnode.data.keepAlive // 被标记为keepAlive
) {
// kept-alive components, treat as a patch
// 对于缓存组件，只需patch即可
const mountedNode: any = vnode // work around flow
componentVNodeHooks.prepatch(mountedNode, mountedNode)
} else {
// 创建组件实例
const child = (vnode.componentInstance = createComponentInstanceForVnode(
vnode,
activeInstance
))
// 子组件挂载
child.$mount(hydrating ? vnode.elm : undefined, hydrating)
}
},
prepatch(oldVnode: MountedComponentVNode, vnode: MountedComponentVNode) {
const options = vnode.componentOptions
const child = (vnode.componentInstance = oldVnode.componentInstance)
updateChildComponent(
child,
options.propsData, // updated props
options.listeners, // updated listeners
vnode, // new parent vnode
options.children // new children
)
},
insert(vnode: MountedComponentVNode) {
const {context, componentInstance} = vnode
if (!componentInstance._isMounted) {
componentInstance._isMounted = true
callHook(componentInstance, 'mounted')
}
if (vnode.data.keepAlive) {
if (context._isMounted) {
// vue-router#1212
// During updates, a kept-alive component's child components may
// change, so directly walking the tree here may call activated hooks
// on incorrect children. Instead we push them into a queue which will
// be processed after the whole patch process ended.
queueActivatedComponent(componentInstance)
} else {
activateChildComponent(componentInstance, true /* direct */)
}
}
},
destroy(vnode: MountedComponentVNode) {
const {componentInstance} = vnode
if (!componentInstance._isDestroyed) {
// 不是缓存组件直接销毁
if (!vnode.data.keepAlive) {
componentInstance.$destroy()
} else {
deactivateChildComponent(componentInstance, true /* direct */)
}
}
},
}
这里有四个 hooks ，看他们的名字就知道他们会在对应的操作去执行。比如 init 会在组件初始化的时候执行，这个后面碰到了再说。我们继续看 createComponent：
// return a placeholder vnode
const name = Ctor.options.name || tag
const vnode = new VNode(
`vue-component-${Ctor.cid}${name ? `-${name}` : ''}`,
data,
undefined,
undefined,
undefined,
context,
{Ctor, propsData, listeners, tag, children},
asyncFactory
)
return vnode
export default class VNode {
...
constructor(
tag?: string, data?: VNodeData, children?: ?Array<VNode>, text?: string, elm?: Node, context?: Component, componentOptions?: VNodeComponentOptions, asyncFactory?: Function
) {
...
this.componentOptions = componentOptions
}
// DEPRECATED: alias for componentInstance for backwards compat.
/* istanbul ignore next */
get child(): Component | void {
return this.componentInstance
}
}
这里初始化了一个 VNode 并进行了返回，到这里 _c('comp') 的任务就完成了。可以看到我们的自定义组件的构造函数在这一步并没有执行，仅仅只是挂载到了 componentOptions 属性上。那他什么时候执行呢？别急，我们接着往下走。
当根组件的 render 执行完后，会执行 vm._update 进行组件的更新，然后会调用 __patch__，我们顺藤摸瓜最终来到 core/vdom/patch.js：
return function patch(oldVnode, vnode, hydrating, removeOnly) {
...
// create new node
createElm(
vnode,
insertedVnodeQueue,
// extremely rare edge case: do not insert if old element is in a
// leaving transition. Only happens when combining transition +
// keep-alive + HOCs. (#4590)
oldElm._leaveCb ? null : parentElm,
nodeOps.nextSibling(oldElm)
)
...
return vnode.elm
}
然后会走到 createElm：
function createElm(
vnode,
insertedVnodeQueue,
parentElm,
refElm,
nested,
ownerArray,
index
) {
...
} else {
createChildren(vnode, children, insertedVnodeQueue);
if (isDef(data)) {
// 事件、属性等等初始化
invokeCreateHooks(vnode, insertedVnodeQueue);
}
// 插入节点
insert(parentElm, vnode.elm, refElm);
}
...
注意到这里的 vnode 是 <div id="demo"></div> 这个元素的，所以会走到 createChildren：
function createChildren(vnode, children, insertedVnodeQueue) {
if (Array.isArray(children)) {
if (process.env.NODE_ENV !== 'production') {
checkDuplicateKeys(children)
}
for (let i = 0; i < children.length; ++i) {
createElm(
children[i],
insertedVnodeQueue,
vnode.elm,
null,
true,
children,
i
)
}
} else if (isPrimitive(vnode.text)) {
nodeOps.appendChild(vnode.elm, nodeOps.createTextNode(String(vnode.text)))
}
}
这里最后又回到了 createElm，不过此时的 vnode 就是自定义组件了，会走到这里：
function createElm(
vnode, insertedVnodeQueue, parentElm, refElm, nested, ownerArray, index ) {
...
// 自定义组件创建
if (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {
return;
}
function createComponent(vnode, insertedVnodeQueue, parentElm, refElm) {
let i = vnode.data
if (isDef(i)) {
// 缓存的情况
const isReactivated = isDef(vnode.componentInstance) && i.keepAlive
// 前面安装的钩子在这里用到了，执行了 init，自定义组件实例化
if (isDef((i = i.hook)) && isDef((i = i.init))) {
i(vnode, false /* hydrating */)
}
// after calling the init hook, if the vnode is a child component
// it should've created a child instance and mounted it. the child
// component also has set the placeholder vnode's elm.
// in that case we can just return the element and be done.
// 假如上面创建过程已完成，组件实例已存在
if (isDef(vnode.componentInstance)) {
// 初始化组件：组件上面事件、属性等
initComponent(vnode, insertedVnodeQueue)
// 插入dom
insert(parentElm, vnode.elm, refElm)
if (isTrue(isReactivated)) {
reactivateComponent(vnode, insertedVnodeQueue, parentElm, refElm)
}
return true
}
}
}
注意到这里会执行 i.init 方法，该方法上文已经说过，会实例化组件对象，然后进行 $mount。而执行 $mount 最终又会走到 patch 方法，并最终执行 createElm：
function patch(oldVnode, vnode, hydrating, removeOnly) {
...
if (isUndef(oldVnode)) {
// empty mount (likely as component), create new root element
isInitialPatch = true;
createElm(vnode, insertedVnodeQueue);
}
...
}
执行该方法又会递归的将自定义组件内的 vnode 渲染成真实的 dom，最后通过 insert 方法将整颗 dom 树插入到父元素之中。到这里自定义组件的渲染过程就结束了