- 之前分析过 createElement 的实现，它最终会调用 _createElement 方法，其中有一段逻辑是对参数 tag 的判断，如果是一个普通的 HTML 标签，则会实例化一个普通的 VNode 节点，否则通过 createComponent 方法创建一个组件 VNode。
代码在 src/core/vdom/crate-element.js 文件里:
if (typeof tag === 'string') {
let Ctor
ns = (context.$vnode && context.$vnode.ns) || config.getTagNamespace(tag)
if (config.isReservedTag(tag)) {
// platform built-in elements
vnode = new VNode(
config.parsePlatformTagName(tag), data, children,
undefined, undefined, context
)
} else if (isDef(Ctor = resolveAsset(context.$options, 'components', tag))) {
// component
vnode = createComponent(Ctor, data, context, children, tag)
} else {
// unknown or unlisted namespaced elements
// check at runtime because it may get assigned a namespace when its
// parent normalizes children
vnode = new VNode(
tag, data, children,
undefined, undefined, context
)
}
} else {
// direct component options / constructor
vnode = createComponent(tag, data, context, children)
}
- 由于是要看创建组件的方法，传入的是 App 对象，它本质上是一个 Component 类型，那么就会走 👆 代码中的 else 逻辑，直接调用 createComponent 方法来创建 VNode。所以接下来看一下 createComponent 方法的实现，代码定义在 src/core/vdom/create-component.js 文件中:
export function createComponent (
Ctor: Class<Component> | Function | Object | void,
data: ?VNodeData,
context: Component,
children: ?Array<VNode>,
tag?: string
): VNode | Array<VNode> | void {
if (isUndef(Ctor)) {
return
}
const baseCtor = context.$options._base
// plain options object: turn it into a constructor
if (isObject(Ctor)) {
Ctor = baseCtor.extend(Ctor)
}
// if at this stage it's not a constructor or an async component factory,
// reject.
if (typeof Ctor !== 'function') {
if (process.env.NODE_ENV !== 'production') {
warn(`Invalid Component definition: ${String(Ctor)}`, context)
}
return
}
// async component
let asyncFactory
if (isUndef(Ctor.cid)) {
asyncFactory = Ctor
Ctor = resolveAsyncComponent(asyncFactory, baseCtor, context)
if (Ctor === undefined) {
// return a placeholder node for async component, which is rendered
// as a comment node but preserves all the raw information for the node.
// the information will be used for async server-rendering and hydration.
return createAsyncPlaceholder(
asyncFactory,
data,
context,
children,
tag
)
}
}
data = data || {}
// resolve constructor options in case global mixins are applied after
// component constructor creation
resolveConstructorOptions(Ctor)
// transform component v-model data into props & events
if (isDef(data.model)) {
transformModel(Ctor.options, data)
}
// extract props
const propsData = extractPropsFromVNodeData(data, Ctor, tag)
// functional component
if (isTrue(Ctor.options.functional)) {
return createFunctionalComponent(Ctor, propsData, data, context, children)
}
// extract listeners, since these needs to be treated as
// child component listeners instead of DOM listeners
const listeners = data.on
// replace with listeners with .native modifier
// so it gets processed during parent component patch.
data.on = data.nativeOn
if (isTrue(Ctor.options.abstract)) {
// abstract components do not keep anything
// other than props & listeners & slot
// work around flow
const slot = data.slot
data = {}
if (slot) {
data.slot = slot
}
}
// install component management hooks onto the placeholder node
installComponentHooks(data)
// return a placeholder vnode
const name = Ctor.options.name || tag
const vnode = new VNode(
`vue-component-${Ctor.cid}${name ? `-${name}` : ''}`,
data, undefined, undefined, undefined, context,
{ Ctor, propsData, listeners, tag, children },
asyncFactory
)
// Weex specific: invoke recycle-list optimized @render function for
// extracting cell-slot template.
// https://github.com/Hanks10100/weex-native-directive/tree/master/component
/* istanbul ignore if */
if (__WEEX__ && isRecyclableComponent(vnode)) {
return renderRecyclableComponentTemplate(vnode)
}
return vnode
}
- createComponent 的逻辑有些复杂，但是分析源码还是主要只分析核心流程，分支流程以后可以有针对性的看，现在就针对组件渲染这个 case 主要看 3 个关键步骤:
- 构造子类构造函数
- 安装组件钩子函数
- 实例化 VNode。
构造子类构造函数
const baseCtor = context.$options._base
// plain options object: turn it into a constructor
if (isObject(Ctor)) {
Ctor = baseCtor.extend(Ctor)
}
- 通常我们在编写一个组件的时候，都是创建一个普通对象，还以 App.vue 为例:
import HelloWorld from './components/HelloWord'
export default {
name: 'app',
components: {
HelloWorld
}
}
- 这里 export 是一个对象，所以 createComponent 里面的代码逻辑会执行到 baseCtor.extend(Ctor)，在这里 baseCtor 实际上就是 Vue，这个的定义是在最开始初始化 Vue 的阶段，在 src/core/global-api/index.js 中的 initGlobalAPI 函数有这么一段逻辑:
// this is used to identify the "base" constructor to extend all plain-object
// components with in Weex's multi-instance scenarios.
Vue.options._base = Vue
- 这里会发现，在这定义的是 Vue.options，而 createComponent 取的是 context.$options，实际上在 src/core/instance/init.js 里 Vue 原型上 _init 函数中有这么一段逻辑:
vm.$options = mergeOptions(
resolveConstructorOptions(vm.constructor),
options || {},
vm
)
- 这样就把 Vue 上的一些 options 扩展到了 vm.
options._base 拿到 Vue 这个钩子函数了。mergeOptions 这里只需要理解它的功能就是把 Vue 构造函数的 options 和用户传入的 options 做一层合并到 vm.$options 上。
先开一下 Vue.extend 函数的定义，在 src/core/global-api/extend.js
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
}
Vue.extend 的作用就是构造一个 Vue 的子类，它使用 Object.create 的原型继承的方式把一个纯对象转换一个继承于 Vue 的构造器 Sub 并返回，然后对 Sub 这个对象本身扩展了一些属性，如扩展 options，添加全局 API 等；并对配置中的 props 和 computed 做了初始化的工作；最后对于这个 sub 构造函数做了缓存，避免了多次执行 Vue.extend 的时候对同一个子组件重复构造。
这样当去实例化 Sub 的时候，就会执行 this._init 逻辑再次走到 Vue 实例的初始化逻辑。
const Sub = function VueComponent (options) {
this._init(options)
}
安装组件钩子函数
// install component management hooks onto the placeholder node
installComponentHooks(data)
- 我们都知道 Vue.js 使用的 Virtual DOM 参考的开源库 snabbdom，它的一个特点是在 VNode 的 patch 流程中对外暴露了各种时机的钩子函数，方便做一些额外的事情，Vue.js 也是充分利用这一点，在初始化一个 Component 类型的 VNode 的过程中实现了几个钩子函数:
const componentVNodeHooks = {
init (
vnode: VNodeWithData,
hydrating: boolean,
parentElm: ?Node,
refElm: ?Node
): ?boolean {
if (
vnode.componentInstance &&
!vnode.componentInstance._isDestroyed &&
vnode.data.keepAlive
) {
// kept-alive components, treat as a patch
const mountedNode: any = vnode // work around flow
componentVNodeHooks.prepatch(mountedNode, mountedNode)
} else {
const child = vnode.componentInstance = createComponentInstanceForVnode(
vnode,
activeInstance,
parentElm,
refElm
)
child.$mount(hydrating ? vnode.elm : undefined, hydrating)
}
},
prepatch (oldVnode: MountedComponentVNode, vnode: MountedComponentVNode) {
const options = vnode.componentOptions
const child = vnode.componentInstance = oldVnode.componentInstance
updateChildComponent(
child,
options.propsData, // updated props
options.listeners, // updated listeners
vnode, // new parent vnode
options.children // new children
)
},
insert (vnode: MountedComponentVNode) {
const { context, componentInstance } = vnode
if (!componentInstance._isMounted) {
componentInstance._isMounted = true
callHook(componentInstance, 'mounted')
}
if (vnode.data.keepAlive) {
if (context._isMounted) {
// vue-router#1212
// During updates, a kept-alive component's child components may
// change, so directly walking the tree here may call activated hooks
// on incorrect children. Instead we push them into a queue which will
// be processed after the whole patch process ended.
queueActivatedComponent(componentInstance)
} else {
activateChildComponent(componentInstance, true /* direct */)
}
}
},
destroy (vnode: MountedComponentVNode) {
const { componentInstance } = vnode
if (!componentInstance._isDestroyed) {
if (!vnode.data.keepAlive) {
componentInstance.$destroy()
} else {
deactivateChildComponent(componentInstance, true /* direct */)
}
}
}
}
const hooksToMerge = Object.keys(componentVnodeHooks)
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
}
}
}
function mergeHook (f1: any, f2: any): Function {
const merged = (a, b) => {
// flow complains about extra args which is why we use any
f1(a, b)
f2(a, b)
}
merged._merged = true
return merged
}
- 整个 installComponentHooks 的过程既是把 componentVNodeHooks 的钩子函数合并到 data.hook 中，VNode 执行 patch 的过程中执行相关的钩子函数，具体的执行会在 patch 过程中介绍到。这里只要注意的是合并策略，在合并过程中，如果某个钩子已经存在 data.hook 中，那么通过执行 mergeHook 函数做合并，最后执行的时候，依次执行这两个钩子函数。
实例化 VNode
// 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
)
// Weex specific: invoke recycle-list optimized @render function for
// extracting cell-slot template.
// https://github.com/Hanks10100/weex-native-directive/tree/master/component
/* istanbul ignore if */
if (__WEEX__ && isRecyclableComponent(vnode)) {
return renderRecyclableComponentTemplate(vnode)
}
return vnode
- 通过 new VNode 实例化一个 vnode 并返回，这里需要注意的是和普通元素节点 vnode 不同，组件的 vnode 是没有 children 的这点很关键。
从上面了解到 createComponent 它在渲染一个组件的时候的 3 个关键逻辑: 构造子类构造函数，安装组件钩子函数和实例化 vnode。createComponent 后返回的是组件 vnode，它也一样会走 vm._update 方法，从而执行了 patch 函数。
