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在 Vue 中，子组件为何不可以修改父组件传递的 Prop，如果修改了，Vue 是如何监控到属性的修改并给出警告的。
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正文回答
- 子组件为何不可以修改父组件传递的
Prop？
- 一个父组件下不只有你一个子组件。同样，使用这份 prop 数据的也不只有你一个子组件。如果每个子组件都能修改 prop 的话，将会导致修改数据的源头不止一处。
- 单向数据流，易于监测数据的流动，出现了错误可以更加迅速的定位到错误发生的位置。
- 如果修改了，Vue 是如何监控到属性的修改并给出警告的。
// 在initProps的时候，在defineReactive时通过判断是否在开发环境
// 如果是开发环境，会在触发set的时候判断是否此key是否处于updatingChildren中被修改
// 如果不是，说明此修改来自子组件，触发warning提示
if (process.env.NODE_ENV !== "production") {
var hyphenatedKey = hyphenate(key);
if (
isReservedAttribute(hyphenatedKey) ||
config.isReservedAttr(hyphenatedKey)
) {
warn(
'"' +
hyphenatedKey +
'" is a reserved attribute and cannot be used as component prop.',
vm
);
}
defineReactive$$1(props, key, value, function () {
if (!isRoot && !isUpdatingChildComponent) {
warn(
"Avoid mutating a prop directly since the value will be " +
"overwritten whenever the parent component re-renders. " +
"Instead, use a data or computed property based on the prop's " +
'value. Prop being mutated: "' +
key +
'"',
vm
);
}
});
}
需要特别注意的是，当你从子组件修改的
prop属于基础类型时会触发提示。这种情况下，你是无法修改父组件的数据源的， 因为基础类型赋值时是值拷贝。你直接将另一个非基础类型（Object, array）赋值到此 key 时也会触发提示(但实际上不会影响父组件的数据源)， 当你修改object的属性时不会触发提示，并且会修改父组件数据源的数据。
vue 源码
// src/core/instance/state.js 源码路径
function initProps(vm: Component, propsOptions: Object) {
const propsData = vm.$options.propsData || {};
const props = (vm._props = {});
// cache prop keys so that future props updates can iterate using Array
// instead of dynamic object key enumeration.
const keys = (vm.$options._propKeys = []);
const isRoot = !vm.$parent;
// root instance props should be converted
if (!isRoot) {
toggleObserving(false);
}
for (const key in propsOptions) {
keys.push(key);
const value = validateProp(key, propsOptions, propsData, vm);
/* istanbul ignore else */
if (process.env.NODE_ENV !== "production") {
const hyphenatedKey = hyphenate(key);
if (
isReservedAttribute(hyphenatedKey) ||
config.isReservedAttr(hyphenatedKey)
) {
warn(
`"${hyphenatedKey}" is a reserved attribute and cannot be used as component prop.`,
vm
);
}
defineReactive(props, key, value, () => {
if (!isRoot && !isUpdatingChildComponent) {
warn(
`Avoid mutating a prop directly since the value will be ` +
`overwritten whenever the parent component re-renders. ` +
`Instead, use a data or computed property based on the prop's ` +
`value. Prop being mutated: "${key}"`,
vm
);
}
});
} else {
defineReactive(props, key, value);
}
// static props are already proxied on the component's prototype
// during Vue.extend(). We only need to proxy props defined at
// instantiation here.
if (!(key in vm)) {
proxy(vm, `_props`, key);
}
}
toggleObserving(true);
}
// src/core/observer/index.js
/**
* Define a reactive property on an Object.
*/
export function defineReactive(
obj: Object,
key: string,
val: any,
customSetter?: ?Function,
shallow?: boolean
) {
const dep = new Dep();
const property = Object.getOwnPropertyDescriptor(obj, key);
if (property && property.configurable === false) {
return;
}
// cater for pre-defined getter/setters
const getter = property && property.get;
const setter = property && property.set;
if ((!getter || setter) && arguments.length === 2) {
val = obj[key];
}
let childOb = !shallow && observe(val);
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) {
const value = getter ? getter.call(obj) : val;
/* eslint-disable no-self-compare */
if (newVal === value || (newVal !== newVal && value !== value)) {
return;
}
/* eslint-enable no-self-compare */
if (process.env.NODE_ENV !== "production" && customSetter) {
customSetter();
}
// #7981: for accessor properties without setter
if (getter && !setter) return;
if (setter) {
setter.call(obj, newVal);
} else {
val = newVal;
}
childOb = !shallow && observe(newVal);
dep.notify();
},
});
}