React和Vue中监听变量变化的方法

Component.prototype.setState = function (partialState, callback) {
 !(typeof partialState === 'object' || typeof partialState === 'function' || partialState == null) "htmlcode">

function warnNoop(publicInstance, callerName) {
 {
 // 实例的构造体
 var _constructor = publicInstance.constructor;
 var componentName = _constructor && (_constructor.displayName || _constructor.name) || 'ReactClass';
 // 组成一个key 组件名称+方法名（列如setState）
 var warningKey = componentName + '.' + callerName;
 // 如果已经输出过警告了就不会再输出
 if (didWarnStateUpdateForUnmountedComponent[warningKey]) {
 return;
 }
 // 在开发者工具的终端里输出警告日志 不能直接使用 component.setState来调用 
 warningWithoutStack$1(false, "Can't call %s on a component that is not yet mounted. " + 'This is a no-op, but it might indicate a bug in your application. ' + 'Instead, assign to `this.state` directly or define a `state = {};` ' + 'class property with the desired state in the %s component.', callerName, componentName);
 didWarnStateUpdateForUnmountedComponent[warningKey] = true;
 }
}


看来 ReactNoopUpdateQueue 是一个抽象类，实际的方法并不是在这里实现的，同时我们看下最初 updater 赋值的地方，初始化 Component 时，会传入实际的 updater


function Component(props, context, updater) {
 this.props = props;
 this.context = context;
 // If a component has string refs, we will assign a different object later.
 this.refs = emptyObject;
 // We initialize the default updater but the real one gets injected by the
 // renderer.
 this.updater = updater || ReactNoopUpdateQueue;
}


我们在组件的构造方法当中将 this 进行打印


class App extends Component {
 constructor(props) {
 super(props);
 //..省略

 console.log('constructor', this);
 }
} 


方法指向的是，在 react-dom.development.js 的 classComponentUpdater


var classComponentUpdater = {
 // 是否渲染
 isMounted: isMounted,
 enqueueSetState: function(inst, payload, callback) {
 // inst 是fiber
 inst = inst._reactInternalFiber;
 // 获取时间
 var currentTime = requestCurrentTime();
 currentTime = computeExpirationForFiber(currentTime, inst);
 // 根据更新时间初始化一个标识对象
 var update = createUpdate(currentTime);
 update.payload = payload;
 void 0 !== callback && null !== callback && (update.callback = callback);
 // 排队更新 将更新任务加入队列当中
 enqueueUpdate(inst, update);
 //
 scheduleWork(inst, currentTime);
 },
 // ..省略
}
enqueueUpdate


就是将更新任务加入队列当中


function enqueueUpdate(fiber, update) {
 var alternate = fiber.alternate;
 // 如果alternat为空并且更新队列为空则创建更新队列
 if (null === alternate) {
 var queue1 = fiber.updateQueue;
 var queue2 = null;
 null === queue1 &&
 (queue1 = fiber.updateQueue = createUpdateQueue(fiber.memoizedState));
 } else

 (queue1 = fiber.updateQueue),
 (queue2 = alternate.updateQueue),
 null === queue1
 "htmlcode">

function scheduleWork(fiber, expirationTime) {
 // 获取根 node
 var root = scheduleWorkToRoot(fiber, expirationTime);
 null !== root &&
 (!isWorking &&
 0 !== nextRenderExpirationTime &&
 expirationTime < nextRenderExpirationTime &&
 ((interruptedBy = fiber), resetStack()),
 markPendingPriorityLevel(root, expirationTime),
 (isWorking && !isCommitting$1 && nextRoot === root) ||
 requestWork(root, root.expirationTime),
 nestedUpdateCount > NESTED_UPDATE_LIMIT &&
 ((nestedUpdateCount = 0), reactProdInvariant("185")));
}
function requestWork(root, expirationTime) {
 // 将需要渲染的root进行记录
 addRootToSchedule(root, expirationTime);
 if (isRendering) {
 // Prevent reentrancy. Remaining work will be scheduled at the end of
 // the currently rendering batch.
 return;
 }

 if (isBatchingUpdates) {
 // Flush work at the end of the batch.
 if (isUnbatchingUpdates) {
 // ...unless we're inside unbatchedUpdates, in which case we should
 // flush it now.
 nextFlushedRoot = root;
 nextFlushedExpirationTime = Sync;
 performWorkOnRoot(root, Sync, true);
 }
 // 执行到这边直接return，此时setState()这个过程已经结束
 return;
 }

 // TODO: Get rid of Sync and use current time"htmlcode">

performSyncWork => performWorkOnRoot => renderRoot => workLoop => performUnitOfWork => beginWork => applyDerivedStateFromProps


最终方法是执行


function applyDerivedStateFromProps(
 workInProgress,
 ctor,
 getDerivedStateFromProps,
 nextProps
) {
 var prevState = workInProgress.memoizedState;
 {
 if (debugRenderPhaseSideEffects || debugRenderPhaseSideEffectsForStrictMode && workInProgress.mode & StrictMode) {
  // Invoke the function an extra time to help detect side-effects.
  getDerivedStateFromProps(nextProps, prevState);
 }
 }
 // 获取改变的state
 var partialState = getDerivedStateFromProps(nextProps, prevState);
 {
 // 对一些错误格式进行警告
 warnOnUndefinedDerivedState(ctor, partialState);
 } // Merge the partial state and the previous state.
 // 判断getDerivedStateFromProps返回的格式是否为空，如果不为空则将由原的state和它的返回值合并
 var memoizedState = partialState === null || partialState === undefined "color: #ff0000">Vue

vue监听变量变化依靠的是 watch ，因此我们先从源码中看看， watch 是在哪里触发的。
Watch触发条件
在 src/core/instance 中有 initState()
/core/instance/state.js
在数据初始化时 initData() ，会将每vue的data注册到 objerserver 中


function initData (vm: Component) {
 // ...省略部分代码
 
 // observe data
 observe(data, true /* asRootData */)
}
/**
 * Attempt to create an observer instance for a value,
 * returns the new observer if successfully observed,
 * or the existing observer if the value already has one.
 */
export function observe (value: any, asRootData: "htmlcode">

constructor (value: any) {
 this.value = value
 this.dep = new Dep()
 this.vmCount = 0
 def(value, '__ob__', this)
 if (Array.isArray(value)) {
 const augment = hasProto
 "htmlcode">

/**
* Walk through each property and convert them into
* getter/setters. This method should only be called when
* value type is Object.
*/
walk (obj: Object) {
 const keys = Object.keys(obj)
 for (let i = 0; i < keys.length; i++) {
 defineReactive(obj, keys[i])
 }
}
/core/observer/index.js 
defineReactive 方法最为核心，它将set和get方法改写，如果我们重新对变量进行赋值，那么会判断变量的新值是否等于旧值，如果不相等，则会触发 dep.notify() 从而回调watch中的方法。
/**
 * Define a reactive property on an Object.
 */
export function defineReactive (
 obj: Object,
 key: string,
 val: any,
 customSetter"color: #ff0000">小程序

自定义Watch
小程序的data本身是不支持watch的，但是我们可以自行添加，我们参照 Vue 的写法自己写一个。
watcher.js


export function defineReactive (obj, key, callbackObj, val) {
 const property = Object.getOwnPropertyDescriptor(obj, key);
 console.log(property);
 const getter = property && property.get;
 const setter = property && property.set;
 val = obj[key]
 const callback = callbackObj[key];
 Object.defineProperty(obj, key, {
 enumerable: true,
 get: function reactiveGetter () {
 const value = getter "color: #ff0000">使用

我们在执行watch回调前没有对新老赋值进行比较，原因是微信当中对data中的变量赋值，即使给引用变量赋值还是相同的值，也会因为引用地址不同，判断不相等。如果想对新老值进行比较就不能使用 === ，可以先对obj或者array转换为json字符串再比较。


//index.js
//获取应用实例
const app = getApp()
import {watch} from '../../utils/watcher';
Page({
 data: {
 motto: 'hello world',
 userInfo: {},
 hasUserInfo: false,
 canIUse: wx.canIUse('button.open-type.getUserInfo'),
 tableData: []
 },
 onLoad: function () {
 this.initWatcher();
 },
 initWatcher () {
 watch(this, {
 motto(newVal, oldVal) {
 console.log('newVal', newVal, 'oldVal', oldVal);
 },

 userInfo(newVal, oldVal) {
 console.log('newVal', newVal, 'oldVal', oldVal);
 },

 tableData(newVal, oldVal) {
 console.log('newVal', newVal, 'oldVal', oldVal);
 }
 }); 
 },
 onClickChangeStringData() {
 this.setData({
 motto: 'hello'
 });
 },
 onClickChangeObjData() {
 this.setData({
 userInfo: {
 name: 'helo'
 }
 });
 },
 onClickChangeArrayDataA() {
 const tableData = [];
 this.setData({
 tableData
 });
 }
})


参考
如何阅读React源码


React 16.3 ~ React 16.5 一些比较重要的改动
总结
以上所述是小编给大家介绍的React和Vue中监听变量变化的方法，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对网站的支持！

        

            
上一篇：Puppeteer 爬取动态生成的网页实战
            
下一篇：详解jQuery获取特殊属性的值以及设置内容