vue中computed/method/watch的区别
摘要：本文通过官方文档结合源码来分析computed/method/watch的区别。
Tips：本文分析的源码版本是v2.6.11，文章中牵涉到vue响应式系统原理部分，如果不是很了解，建议先阅读上一篇文章《深入解析vue响应式原理》。
computed
首先来看官网的解释：计算属性是基于响应式依赖进行缓存的，只在相关响应式依赖发生改变时它们才会重新求值。
下面通过源码来分析computed是怎么实现响应式缓存的：
initComputed
function initComputed (vm: Component, computed: Object) {
// $flow-disable-line
const watchers = vm._computedWatchers = Object.create(null)
// computed properties are just getters during SSR
const isSSR = isServerRendering()
for (const key in computed) {
const userDef = computed[key]
const getter = typeof userDef === 'function' ? userDef : userDef.get
if (process.env.NODE_ENV !== 'production' && getter == null) {
warn(
`Getter is missing for computed property "${key}".`,
vm
)
}
if (!isSSR) {
// create internal watcher for the computed property.
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.
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 "${key}" is already defined as a prop.`, vm)
}
}
}
}
- 首先创建一个computedWatchers挂到vm上；
- 遍历computed属性，依次将单个computed属性的get方法作为参数创建Watcher实例保存到computedWatchers中；
- 再将单个computed属性作为参数传入defineComputed方法。
defineComputed
export function defineComputed (
target: any,
key: string,
userDef: Object | Function
) {
const shouldCache = !isServerRendering()
if (typeof userDef === 'function') {
sharedPropertyDefinition.get = shouldCache
? createComputedGetter(key)
: createGetterInvoker(userDef)
sharedPropertyDefinition.set = noop
} else {
sharedPropertyDefinition.get = userDef.get
? shouldCache && userDef.cache !== false
? createComputedGetter(key)
: createGetterInvoker(userDef.get)
: noop
sharedPropertyDefinition.set = userDef.set || noop
}
if (process.env.NODE_ENV !== 'production' &&
sharedPropertyDefinition.set === noop) {
sharedPropertyDefinition.set = function () {
warn(
`Computed property "${key}" was assigned to but it has no setter.`,
this
)
}
}
Object.defineProperty(target, key, sharedPropertyDefinition)
}
function createComputedGetter (key) {
return function computedGetter () {
const watcher = this._computedWatchers && this._computedWatchers[key]
if (watcher) {
if (watcher.dirty) {
watcher.evaluate()
}
if (Dep.target) {
watcher.depend()
}
return watcher.value
}
}
}
function createGetterInvoker(fn) {
return function computedGetter () {
return fn.call(this, this)
}
}
将计算属性挂到vm上，定义getter属性方法，方法会执行计算属性获取新值（新值会存储到Watcher.value中）及收集依赖该computed属性的视图。
总结：
- 页面初始渲染时，读取computed属性值，computed属性值的getter函数读取data数据，触发data的getter方法，将computed属性对应的Watcher绑定到data的依赖收集器Dep中；
- computed属性getter方法中，还会调用Watcher.depend方法，将上层视图的观察者也添加到data的依赖收集器Dep中；
- data属性值变更后，将会调用Dep.notify方法，通知所有依赖的Watcher进行update方法；
- 首先触发computed关联Watcher的update方法，由于lazy为true，将会设置dirty为true，表示computed属性依赖的data值已经变更，但不会调用Watcher的get方法获取新值。
- 然后触发视图关联Watcher的update方法，在更新页面时会调用computed属性值，触发定义的getter函数。由于当前dirty为true，会执行关联Watcher.get方法获取新值，更新Watcher.value的值，并返回新值，完成页面的重新渲染。
method
function initMethods (vm: Component, methods: Object) {
const props = vm.$options.props
for (const key in methods) {
if (process.env.NODE_ENV !== 'production') {
if (typeof methods[key] !== 'function') {
warn(
`Method "${key}" has type "${typeof methods[key]}" in the component definition. ` +
`Did you reference the function correctly?`,
vm
)
}
if (props && hasOwn(props, key)) {
warn(
`Method "${key}" has already been defined as a prop.`,
vm
)
}
if ((key in vm) && isReserved(key)) {
warn(
`Method "${key}" conflicts with an existing Vue instance method. ` +
`Avoid defining component methods that start with _ or $.`
)
}
}
vm[key] = typeof methods[key] !== 'function' ? noop : bind(methods[key], vm)
}
}
method的源码很简单，只是绑定了所有method的this为vm。
watch
function initWatch (vm: Component, watch: Object) {
for (const key in watch) {
const handler = watch[key]
if (Array.isArray(handler)) {
for (let i = 0; i < handler.length; i++) {
createWatcher(vm, key, handler[i])
}
} else {
createWatcher(vm, key, handler)
}
}
}
function createWatcher (
vm: Component,
expOrFn: string | Function,
handler: any,
options?: Object
) {
if (isPlainObject(handler)) {
options = handler
handler = handler.handler
}
if (typeof handler === 'string') {
handler = vm[handler]
}
return vm.$watch(expOrFn, handler, options)
}
Vue.prototype.$watch = function (
expOrFn: string | Function,
cb: any,
options?: Object
): Function {
const vm: Component = this
if (isPlainObject(cb)) {
return createWatcher(vm, expOrFn, cb, options)
}
options = options || {}
options.user = true
const watcher = new Watcher(vm, expOrFn, cb, options)
if (options.immediate) {
try {
cb.call(vm, watcher.value)
} catch (error) {
handleError(error, vm, `callback for immediate watcher "${watcher.expression}"`)
}
}
return function unwatchFn () {
watcher.teardown()
}
}
Tips：Watcher对象源码在上文中《深入解析vue响应式原理》分析过，这里就不再赘述。
经过分析watch的源码可以发现，实际上每一个watch属性对应生成了一个Watcher对象，通过获取data属性值将Watcher添加到依赖收集器Dep中，当data数据更新时，就会调用Dep.notify通知Watcher。
总结：
- computed属性是根据依赖的data属性（有可能多个）进行更新标记，等视图获取该computed属性数据时才执行更新计算，并将值缓存到对应的Watcher实例中。且只有当data数据变化后，视图读取关联computed属性才会重新计算结果。
- method属性是根据视图需要即时计算获得，不具有缓存性质，当关联data数据没有更新时也会重新计算。
- watch属性是有且只能依赖单个data属性，当data数据变化后，会立即触发Watcher.update，调用对应watch定义的方法执行。不具有缓存性质。
