export function initState (vm: Component) {
vm._watchers = []
const opts = vm.$options
if (opts.props) initProps(vm, opts.props)
if (opts.methods) initMethods(vm, opts.methods)
if (opts.data) {
initData(vm)
} else {
observe(vm._data = {}, true /* asRootData */)
}
if (opts.computed) initComputed(vm, opts.computed)
if (opts.watch && opts.watch !== nativeWatch) {
initWatch(vm, opts.watch)
}
}
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在初始化computed之前,props、methods、data等已初始化完成。我们主要介绍computed,所以其他的不做介绍。上面代码中
if (opts.computed) initComputed(vm, opts.computed)
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初始化computed,判断opts.computed(我们传人的computed对象)是否存在,如果存在会进入initComputed(vm, opts.computed)方法
function initComputed (vm: Component, computed: Object) {
// $flow-disable-line
// 创建一个空对象,赋值给vm._computedWatchers与watchers
const watchers = vm._computedWatchers = Object.create(null)
// computed properties are just getters during SSR
// 是否为服务端渲染
const isSSR = isServerRendering()
// 循环遍历computed
for (const key in computed) {
const userDef = computed[key]
// 判断userDef类型是否为function,true 返回 userDef; false 返回userDef.get方法
const getter = typeof userDef === 'function' ? userDef : userDef.get
// getter 为null 则抛出警告
if (process.env.NODE_ENV !== 'production' && getter == null) {
warn(
`Getter is missing for computed property "${key}".`,
vm
)
}
//
if (!isSSR) {
// create internal watcher for the computed property.
// 为每一个watchers[key]创建一个Watcher实例对象
watchers[key] = new Watcher(
vm,
getter || noop,
noop,
computedWatcherOptions
)
}
// component-defined computed properties are already defined on the
// component prototype. We only need to define computed properties defined
// at instantiation here.
// 查找vm对象中是否存在当前key,如果存在则提示警告,否则调用defineComputed()
if (!(key in vm)) {
defineComputed(vm, key, userDef)
} else if (process.env.NODE_ENV !== 'production') {
if (key in vm.$data) {
warn(`The computed property "${key}" is already defined in data.`, vm)
} else if (vm.$options.props && key in vm.$options.props) {
warn(`The computed property "${key}" is already defined as a prop.`, vm)
}
}
}
}
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上面代码的意思是创建一个_computedWatchers空对象,循环遍历computed,如果不是服务端渲染则在_computedWatchers添加key为当前computed[key]的Watcher实例对象。然后,查找vm对象中是否存在当前key,如果存在则提示警告,否则调用defineComputed()
export function defineComputed (
target: any,
key: string,
userDef: Object | Function
) {
const shouldCache = !isServerRendering()
if (typeof userDef === 'function') {
sharedPropertyDefinition.get = shouldCache
? createComputedGetter(key)
: userDef
sharedPropertyDefinition.set = noop
} else {
sharedPropertyDefinition.get = userDef.get
? shouldCache && userDef.cache !== false
? createComputedGetter(key)
: userDef.get
: noop
sharedPropertyDefinition.set = userDef.set
? userDef.set
: noop
}
if (process.env.NODE_ENV !== 'production' &&
sharedPropertyDefinition.set === noop) {
sharedPropertyDefinition.set = function () {
warn(
`Computed property "${key}" was assigned to but it has no setter.`,
this
)
}
}
Object.defineProperty(target, key, sharedPropertyDefinition)
}
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defineComputed()函数还是比较容易理解的,简单说就是实现响应式,通过 Object.defineProperty设置get、set方法。如果userDef类型为函数并且不是服务端渲染,则get方法为createComputedGetter(key)的返回值,否则为传入userDef,set方法设置为noop(空函数);如果userDef类型不是函数并且不是服务端渲染没有缓存时,则get方法为createComputedGetter(key)的返回值,否则为传入userDef.get,set方法会首先判断是否有userDef.set,如果有则返回userDef.set,否则为noop。具体createComputedGetter函数实现如下:
function createComputedGetter (key) {
return function computedGetter () {
const watcher = this._computedWatchers && this._computedWatchers[key]
if (watcher) {
// ---1----
if (watcher.dirty) {
watcher.evaluate()
}
if (Dep.target) {
watcher.depend()
}
// ----1---
return watcher.value
}
}
}
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上面我们已经介绍了_computedWatchers中在初始化时存储了当前key指定的watcher对象的实例。这里我们会判断_computedWatchers中是否存在指定的watcher对象的实例,如果存在则返回watcher对象的value。 如上1注释,如果dirty(是否延迟)为true,则调用处理watcher.evaluate()。如果存在Dep.target,则进行依赖收集,这样在某个值改变时,可以通过update方法,更新所有watcher对象。
以上为自己理解,如有错误欢迎大家指导!!!!!
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