Vue
的其中一个核心思想为组件化,将页面拆分成不同的组件,独立了资源,利于开发和维护。前面讲了整个 Vue
的实例和挂载,但并没有详细记录子组件是怎么开始一轮生命周期的。
createComponent
回顾一下 vnode
的创建的过程:
createElement
_createElement
createComponent
// 在 src/core/vdom/create-component.js 中:
export function createComponent (
Ctor: Class<Component> | Function | Object | void,
data: ?VNodeData,
context: Component,
children: ?Array<VNode>,
tag?: string
): VNode | Array<VNode> | void {
const baseCtor = context.$options._base
// plain options object: turn it into a constructor
if (isObject(Ctor)) {Ctor = baseCtor.extend(Ctor)
}
// extract props
const propsData = extractPropsFromVNodeData(data, Ctor, tag)
// install component management hooks onto the placeholder node
installComponentHooks(data)
// return a placeholder vnode
const name = Ctor.options.name || tag
const vnode = new VNode(`vue-component-${Ctor.cid}${name ? `-${name}` : ''}`,
data, undefined, undefined, undefined, context,
{Ctor, propsData, listeners, tag, children},
asyncFactory
)
return vnode
}
核心流程大致分为 4 步:
- 创建
Ctor(Vue 子类构造函数)
- 提取
props
- 安装组件钩子函数
- 实例化
vnode
Ctor
const baseCtor = context.$options._base
// plain options object: turn it into a constructor
if (isObject(Ctor)) {Ctor = baseCtor.extend(Ctor)
}
这里的关键在于弄清 baseCtor
是什么:
- 在
initGlobalAPI
中,我们定义了Vue.options._base = Vue
。 -
在
Vue.prototype._init
中,将Vue.options
与options
进行了合并。vm.$options = mergeOptions(resolveConstructorOptions(vm.constructor), options || {}, vm )
所以这里的 baseCtor
就是 Vue
本身,而这里相当于执行了Vue.extend(Ctor)
Vue.extend
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
}
}
这里用了经典原型继承的方式,构造了一个 Vue
的子类 Sub
,当实例化Sub
的时候,就会调用 _init
方法,重新走到组件初始化创建的逻辑。
installComponentHooks
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
}
}
}
遍历 hooksToMerge
,不断向data.hook
插入 componentVNodeHooks
对象中对应的钩子函数,包括 init
、prepatch
、insert
、destory
。
这一步就是安装组件钩子函数,等待 patch
过程时去执行。
const componentVNodeHooks = {init (vnode: VNodeWithData, hydrating: boolean): ?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
)
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 */)
}
}
}
}
实例化vnode
const name = Ctor.options.name || tag
const vnode = new VNode(`vue-component-${Ctor.cid}${name ? `-${name}` : ''}`,
data, undefined, undefined, undefined, context,
{Ctor, propsData, listeners, tag, children},
asyncFactory
)
return vnode
最终生成的 vnode
对象。
组件的patch
在 createElm
的实现中,有下面这个判断:
function createElm (
vnode,
insertedVnodeQueue,
parentElm,
refElm,
nested,
ownerArray,
index
) {
// ...
if (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {return}
// ...
}
function createComponent (vnode, insertedVnodeQueue, parentElm, refElm) {
let i = vnode.data
if (isDef(i)) {const isReactivated = isDef(vnode.componentInstance) && i.keepAlive
if (isDef(i = i.hook) && isDef(i = i.init)) {i(vnode, false /* hydrating */)
}
// after calling the init hook, if the vnode is a child component
// it should've created a child instance and mounted it. the child
// component also has set the placeholder vnode's elm.
// in that case we can just return the element and be done.
if (isDef(vnode.componentInstance)) {initComponent(vnode, insertedVnodeQueue)
insert(parentElm, vnode.elm, refElm)
if (isTrue(isReactivated)) {reactivateComponent(vnode, insertedVnodeQueue, parentElm, refElm)
}
return true
}
}
}
let i = vnode.data
如果 i
有定义,则说明 vnode
是一个组件,最后 i
经过一系列的赋值指向了 data.hook.init
,然后执行 i(vnode, false)
,也就是执行了上面提到过的init
钩子函数。
init (vnode: VNodeWithData, hydrating: boolean): ?boolean {
if (
vnode.componentInstance &&
!vnode.componentInstance._isDestroyed &&
vnode.data.keepAlive
) {
// kept-alive components, treat as a patch
const mountedNode: any = vnode // work around flow
componentVNodeHooks.prepatch(mountedNode, mountedNode)
} else {
const child = vnode.componentInstance = createComponentInstanceForVnode(
vnode,
activeInstance
)
child.$mount(hydrating ? vnode.elm : undefined, hydrating)
}
},
这里的逻辑就是通过 createComponentInstanceForVnode
创建子组件实例,然后通过 $mount
挂载子组件。
export function createComponentInstanceForVnode (
vnode: any,
parent: any,
): Component {
const options: InternalComponentOptions = {
_isComponent: true,
_parentVnode: vnode,
parent
}
// ...
return new vnode.componentOptions.Ctor(options)
}
首先看 vnode.componentOptions
,它是在new VNode()
实例化 vnode
时,将 Ctor
作为参数传入的,上面也提到了,它其实就是 Vue
的子类构造器 Sub
,所以这里相当于在new Sub()
创建子组件实例。这里用 _isComponent
标识为一个组件,它的用处是在 _init()
的时候会采取不同方式处理options
。
Vue.prototype._init = function (options?: Object) {
const vm: Component = this
// merge options
if (options && options._isComponent) {initInternalComponent(vm, options)
} else {
vm.$options = mergeOptions(resolveConstructorOptions(vm.constructor),
options || {},
vm
)
}
}
在前面关于 Vue 实例化的文章也提到过,Vue
的合并策略是很慢的,当注册一个内部组件的时候不需要做特殊的处理,所以可以直接初始化内部组件提升性能。
可以注意到 child.$mount(hydrating ? vnode.elm : undefined, hydrating)
接管了子组件的挂载,又开始新的一轮 render
、update
、patch
,不断的递归,只到整颗树挂载完毕为止。由于这种递归的关系,在进行 insert(parentElm, vnode.elm, refElm)
的时候,插入的顺序是先子后父。所以其实也可以得知了父子的生命周期是 父 beforeCreate -> 父 created -> 父 beforeMount -> 子 beforeCreate -> 子 created -> 子 beforeMount -> 子 mounted -> 父 mounted
这样的顺序。