前言
上一篇文章介绍了@vue/composition-api是什么,以及为什么要用,现在来系统地解析一下 @vue/composition-api 的实现原理,希望可以加深对其工作机制的理解。
老规矩先分享下AI评价:对@vue/composition-api实现原理的介绍整体上非常详细和准确,展示了核心代码以及关键逻辑,这为理解其工作机制提供了很好的分析。
核心代码分析
install
引入@vue/composition-api时,需要手动通过Vue.use(),来调用该插件的install,所以我们看看安装时到底做了什么:
export function install(Vue: VueConstructor) {setVueConstructor(Vue)mixin(Vue)
}
这里最重要的其实是mixin函数,定义了在beforeCreate钩子中去执行初始化,
构建props和上下文ctx对象,执行setup函数:
export function mixin(Vue: VueConstructor) {Vue.mixin({beforeCreate: functionApiInit,mounted(this: ComponentInstance) {afterRender(this)},beforeUpdate() {updateVmAttrs(this as ComponentInstance)},updated(this: ComponentInstance) {afterRender(this)},})function functionApiInit(this: ComponentInstance) {const vm = thisconst $options = vm.$optionsconst { setup, render } = $optionsconst { data } = $options// wrapper the data option, so we can invoke setup before data get resolved$options.data = function wrappedData() {// 在data周期时,进行setup函数的执行initSetup(vm, vm.$props)return data || {}}}function initSetup(vm: ComponentInstance, props: Record<any, any> = {}) {const setup = vm.$options.setup!// 构造ctx对象,有以下key:// slots: 组件的插槽,默认为 {}// root: 组件的根实例// parent: 组件的父实例// refs: 组件的 ref 引用// listeners: 组件的事件监听器// isServer: 是否是服务端渲染// ssrContext: 服务端渲染的上下文// emit: 组件的自定义事件触发函数const ctx = createSetupContext(vm)const instance = toVue3ComponentInstance(vm)instance.setupContext = ctx// 通过Vue.observable对props进行响应式监听def(props, '__ob__', createObserver())}
}
ref
ref函数可以把一个普通的值转成响应式的数据。它返回一个可变的ref对象,对象上挂载了一个.value属性,我们可以通过这个.value属性读取或者修改ref的值。
其本质还是基于reactive来实现的响应式,只不过做了一个前置的get、set封装,源码如下:
export function ref(raw?: unknown) {// ref的本质其实还是调用的reactive// 然后通过 get/set 方法操作这个对象的值来实现对 ref 值的跟踪和响应const value = reactive({ [RefKey]: raw })return createRef({get: () => value[RefKey] as any,set: (v) => ((value[RefKey] as any) = v),})
}
createRef最后还是基于Object.defineProperty:
export function proxy(target: any,key: string,{ get, set }: { get?: Function; set?: Function }
) {Object.defineProperty(target, key, {enumerable: true,configurable: true,get: get || noopFn,set: set || noopFn,})
}
reactive
底层基于Vue.observable来建立响应式监听,Vue.observable 在 Vue2 和 Vue3 中的实现方式有所区别而已:
export function reactive<T extends object>(obj: T): UnwrapRef<T> {// 基于Vue.observableconst observed = observe(obj)setupAccessControl(observed)return observed as UnwrapRef<T>
}
computed
computed函数用来创建一个计算属性,它根据依赖进行缓存和懒执行。
构建get、set函数,通过vue的Watcher对象,进行监听绑定。
export function computed<T>(getterOrOptions: ComputedGetter<T> | WritableComputedOptions<T>
): ComputedRef<T> | WritableComputedRef<T> {const vm = getCurrentScopeVM()let getter: ComputedGetter<T>let setter: ComputedSetter<T> | undefinedgetter = getterOrOptions.getsetter = getterOrOptions.setlet computedSetterlet computedGetterconst { Watcher, Dep } = getVueInternalClasses()let watcher: anycomputedGetter = () => {if (!watcher) {watcher = new Watcher(vm, getter, noopFn, { lazy: true })}if (watcher.dirty) {watcher.evaluate()}if (Dep.target) {watcher.depend()}return watcher.value}computedSetter = (v: T) => {if (setter) {setter(v)}}return createRef<T>({get: computedGetter,set: computedSetter,},!setter,true) as WritableComputedRef<T> | ComputedRef<T>
}
watch
watch函数用于侦听特定的数据源,并在回调函数中执行副作用。
底层是通过Vue.$watch来实现的:
// 底层是通过Vue.$watch来实现
function createVueWatcher(vm: ComponentInstance,getter: () => any,callback: (n: any, o: any) => any,options: {deep: booleansync: booleanimmediateInvokeCallback?: booleannoRun?: booleanbefore?: () => void}
): VueWatcher {const index = vm._watchers.length// @ts-ignore: use undocumented optionsvm.$watch(getter, callback, {immediate: options.immediateInvokeCallback,deep: options.deep,lazy: options.noRun,sync: options.sync,before: options.before,})return vm._watchers[index]
}
toRefs
toRefs 可以把一个 reactive 对象的属性都转成 ref 形式。
底层是一个个遍历key去调用toRef,然后基于createRef来保留响应式能力:
export function toRef<T extends object, K extends keyof T>(object: T,key: K
): Ref<T[K]> {if (!(key in object)) set(object, key, undefined)const v = object[key]if (isRef<T[K]>(v)) return vreturn createRef({get: () => object[key],set: (v) => (object[key] = v),})
}
生命周期
生命周期其实是一个临时覆盖:
// 利用 Vue 的选项合并策略(option merge strategies),
// 通过给组件实例的 $options 注入自定义的生命周期hook函数,来override原有的生命周期选项。// 生命周期hook函数需要通过 getCurrentInstance() 获取当前活跃的组件实例,
// 并在执行回调前通过 setCurrentInstance()绑定实例,在回调执行完毕后恢复之前的实例。export const onBeforeMount = createLifeCycle('beforeMount')
export const onMounted = createLifeCycle('mounted')
export const onBeforeUpdate = createLifeCycle('beforeUpdate')
export const onUpdated = createLifeCycle('updated')
export const onBeforeUnmount = createLifeCycle('beforeDestroy')
export const onUnmounted = createLifeCycle('destroyed')
export const onErrorCaptured = createLifeCycle('errorCaptured')
export const onActivated = createLifeCycle('activated')
export const onDeactivated = createLifeCycle('deactivated')
export const onServerPrefetch = createLifeCycle('serverPrefetch')