默认computed也是一个watcher，具备缓存，只有当依赖的属性发生变化才会更新视图。
原理图：
流程：computed watcher在defineReactive的get中订阅属性的变化（4），在defineReactive的set时触发notify（4），notify调用每个订阅了改属性变化的watcher的update（3），update中将computed watcher 的dirty赋值为true（2），notify中其中一个watcher为渲染watcher，此时执行渲染，渲染时因为模板中使用了computed watcher对应的计算属性，故而触发计算属性的get方法createComputedGetter，在createComputedGetter中调用computed watcher 的evaluate方法（1），evaluate方法调用computed watcher的this.get（5），进而调用用户在计算属性上定义的方法进行计算。
源码：
1、定义computed 的getter方法（在dirty为true时，使用缓存数据）（src\core\instance\state.js）：
const computedWatcherOptions = { lazy: true }
function initComputed (vm: Component, computed: Object) {
...
if (!isSSR) {
// create internal watcher for the computed property.
watchers[key] = new Watcher(
vm,
getter || noop,//将用户定义传到watcher中
noop,
computedWatcherOptions//lazy:true懒watcher
)
}
// 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 "${key}" is already defined as a prop.`, vm)
}
}
}
}
export function defineComputed (
target: any,
key: string,
userDef: Object | Function
) {
const shouldCache = !isServerRendering()
if (typeof userDef === ‘function‘) {
sharedPropertyDefinition.get = shouldCache
? createComputedGetter(key)//创建计算属性的getter，不是用用户传的
: createGetterInvoker(userDef)
sharedPropertyDefinition.set = noop
} else {
sharedPropertyDefinition.get = userDef.get
? shouldCache && userDef.cache !== false
? createComputedGetter(key)
: createGetterInvoker(userDef.get)
: noop
sharedPropertyDefinition.set = userDef.set || noop
}
if (process.env.NODE_ENV !== ‘production‘ &&
sharedPropertyDefinition.set === noop) {
sharedPropertyDefinition.set = function () {
warn(
`Computed property "${key}" was assigned to but it has no setter.`,
this
)
}
}
Object.defineProperty(target, key, sharedPropertyDefinition)
}
function createComputedGetter (key) {
return function computedGetter () {//用户取值的时候会调用此方法
const watcher = this._computedWatchers && this._computedWatchers[key]
if (watcher) {
if (watcher.dirty) {//dirty为true会去进行求值，这儿的dirty起到了缓存的作用
watcher.evaluate()
}
if (Dep.target) {
watcher.depend()
}
return watcher.value
}
}
}
2、computed的getter方法中dirty的赋值时机（src\core\observer\watcher.js）：
update () { /* istanbul ignore else */ if (this.lazy) { this.dirty = true } else if (this.sync) { this.run() } else { queueWatcher(this) } }
3、update的调用时机（调用当前属性的订阅中每一个watcher的update）（src\core\observer\dep.js）：
notify () { ... for (let i = 0, l = subs.length; i < l; i++) { subs[i].update() } }
4、notify的调用时机(src\core\observer\index.js中 defineReactive的set)以及watcher在defineReactive的get订阅属性的变化（其中dep.target的赋值是在new Watcher时的this.get）：
export function defineReactive ( obj: Object, key: string, val: any, customSetter?: ?Function, shallow?: boolean ) { ... Object.defineProperty(obj, key, { enumerable: true, configurable: true, get: function reactiveGetter () { const value = getter ? getter.call(obj) : val if (Dep.target) { dep.depend() if (childOb) { childOb.dep.depend() if (Array.isArray(value)) { dependArray(value) } } } return value }, set: function reactiveSetter (newVal) { ... dep.notify() } }) }
dep.target的赋值是在new Watcher时调用this.get（src\core\observer\watcher.js）：
constructor ( vm: Component, expOrFn: string | Function, cb: Function, options?: ?Object, isRenderWatcher?: boolean ) { ... this.value = this.lazy ? undefined : this.get() }
get () { pushTarget(this) let value const vm = this.vm try { value = this.getter.call(vm, vm) } catch (e) { if (this.user) { handleError(e, vm, `getter for watcher "${this.expression}"`) } else { throw e } } finally { // "touch" every property so they are all tracked as // dependencies for deep watching if (this.deep) { traverse(value) } popTarget() this.cleanupDeps() } return value }
5、使用用户定义的计算属性的getter方法计算值（src\core\observer\watcher.js）：
get () { pushTarget(this) let value const vm = this.vm try { value = this.getter.call(vm, vm) } catch (e) { if (this.user) { handleError(e, vm, `getter for watcher "${this.expression}"`) } else { throw e } } finally { // "touch" every property so they are all tracked as // dependencies for deep watching if (this.deep) { traverse(value) } popTarget() this.cleanupDeps() } return value }
原文：https://www.cnblogs.com/vickylinj/p/14034645.html