Vue中computed分析
<!DOCTYPE html>
<html>
<head>
<title>Vue</title>
</head>
<body>
<div id="app"></div>
</body>
<script src="https://cdn.bootcss.com/vue/2.4.2/vue.js"></script>
<script type="text/javascript">
var vm = new Vue({
el: "#app",data: {
a: 1,b: 2
},template:`
<div>
<div>{{multiplication}}</div>
<div>{{multiplication}}</div>
<div>{{multiplication}}</div>
<div>{{multiplicationArrow}}</div>
<button @click="updateSetting">updateSetting</button>
</div>
`,computed:{
multiplication: function(){
console.log("a * b"); // 初始只打印一次 返回值被缓存
return this.a * this.b;
},multiplicationArrow: vm => vm.a * vm.b * 3,// 箭头函数可以通过传入的参数获取当前实例
setting: {
get: function(){
console.log("a * b * 6");
return this.a * this.b * 6;
},set: function(v){
console.log(`${v} -> a`);
this.a = v;
}
}
},methods:{
updateSetting: function(){ // 点击按钮后
console.log(this.setting); // 12
this.setting = 3; // 3 -> a
console.log(this.setting); // 36
}
},})
</script>
</html>
-
Observer: 这里的主要工作是递归地监听对象上的所有属性，在属性值改变的时候，触发相应的Watcher。
-
Watcher: 观察者，当监听的数据值修改时，执行响应的回调函数，在Vue里面的更新模板内容。
-
Dep: 链接Observer和Watcher的桥梁，每一个Observer对应一个Dep，它内部维护一个数组，保存与该Observer相关的Watcher。
// dev/src/core/instance/state.js line 47
export function initState (vm: Component) {
vm._watchers = []
const opts = vm.$options // 获取组件定义的选项
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) // 定义computed属性则进行初始化
if (opts.watch && opts.watch !== nativeWatch) {
initWatch(vm,opts.watch)
}
}
// dev/src/core/instance/state.js line 169
function initComputed (vm: Component,computed: Object) {
// $flow-disable-line
const watchers = vm._computedWatchers = Object.create(null) // 创建一个没有原型链指向的对象
// computed properties are just getters during SSR
const isSSR = isServerRendering()
for (const key in computed) {
const userDef = computed[key] // 获取计算属性的key值定义
const getter = typeof userDef === 'function' ? userDef : userDef.get // 由于计算属性接受两种类型的参数 此处判断用以获取getter
if (process.env.NODE_ENV !== 'production' && getter == null) {
warn(
`Getter is missing for computed property "${key}".`,vm
)
}
if (!isSSR) {
// create internal watcher for the computed property.
// 生成computed watcher(vm,getter,noop,{ lazy: true })
watchers[key] = new Watcher( // 计算属性创建观察者watcher和消息订阅器dep
vm,getter || 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)) { // 检查重名属性
defineComputed(vm,key,userDef) // 定义属性
} 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 "${key}" is already defined as a prop.`,vm)
}
}
}
}
// dev/src/core/instance/state.js line 31
const sharedPropertyDefinition = {
enumerable: true,configurable: true,get: noop,set: noop
}
// dev/src/core/instance/state.js line 210
export function defineComputed (
target: any,key: string,userDef: Object | Function
) {
const shouldCache = !isServerRendering()
if (typeof userDef === 'function') {
sharedPropertyDefinition.get = shouldCache
? createComputedGetter(key)
: createGetterInvoker(userDef)
sharedPropertyDefinition.set = noop
} else {
sharedPropertyDefinition.get = userDef.get
? shouldCache && userDef.cache !== false
? createComputedGetter(key)
: createGetterInvoker(userDef.get)
: noop
sharedPropertyDefinition.set = userDef.set || noop
}
if (process.env.NODE_ENV !== 'production' &&
sharedPropertyDefinition.set === noop) {
sharedPropertyDefinition.set = function () {
warn(
`Computed property "${key}" was assigned to but it has no setter.`,this
)
}
}
Object.defineProperty(target,sharedPropertyDefinition)
}
/**
经过重写之后的属性描述符在某条件分支大致呈现如下
sharedPropertyDefinition = {
enumerable: true,get: function computedGetter () {
const watcher = this._computedWatchers && this._computedWatchers[key]
if (watcher) {
if (watcher.dirty) {
watcher.evaluate()
}
if (Dep.target) {
watcher.depend()
}
return watcher.value
}
},set: userDef.set || noop
}
当计算属性被调用时便会执行 get 访问函数，从而关联上观察者对象 watcher 然后执行 wather.depend() 收集依赖和 watcher.evaluate() 计算求值。
*/
https://github.com/WindrunnerMax/EveryDay
https://cn.vuejs.org/v2/api/#computed
https://juejin.im/post/6844903678533451783
https://juejin.im/post/6844903873925087239
https://cn.vuejs.org/v2/guide/computed.html
https://zheyaoa.github.io/2019/09/07/computed/
https://www.cnblogs.com/tugenhua0707/p/11760466.html
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