connect简介
前方高能预警,有耐心才能看完文章!!
react-redux仅有2个API,Provider和connect,Provider提供的是一个顶层容器的作用,实现store的上下文传递。
connect方法比较复杂,虽然代码只有368行,但是为redux中常用的功能实现了和react连接的建立。
一个基础的connect方法如下:
connect(mapStateToProps, mapDispatchToProps, mergeProps, options = {})
为什么我们需要react-redux?
熟悉redux的人可能知道,redux是数据存储和管理的工具,但是想要在react中使用redux,并不能直接将store、action和react组件建立连接,所以就需要react-redux来结合react和redux。
react-redux文件体积非常小,你完全不需要担心给你的项目带来太多的垃圾代码。
从何处开始解析react-redux源码?
1、在JavaScript中,读懂别人的代码文件,你首先应该看的是函数的入口。
2、找到函数入口,然后看有哪些参数。
3、看看导入了哪些额外的插件,每个插件的作用大概预测一下。
4、进入函数体进行解读。在react插件中解读函数有一个好处,就是react插件大部分都是采用了react组件的写法,你可以在react插件中看到很多react组件的影子。而不是像jQuery那样到处都是扩展性的方法,每个方法都有自己的设计模式,没有统一的规律可循。
react-redux使用场景
下面这个官方例子展示了mapStateToProps和mapDispatchToProps的使用方法。
import * as todoActionCreators from './todoActionCreators'
import * as counterActionCreators from './counterActionCreators'
import { bindActionCreators } from 'redux'
function mapStateToProps(state) {
return { todos: state.todos }
}
function mapDispatchToProps(dispatch) {
return {
todoActions: bindActionCreators(todoActionCreators, dispatch),
counterActions: bindActionCreators(counterActionCreators, dispatch)
}
}
export default connect(mapStateToProps, mapDispatchToProps)(TodoApp)
mergeProps的用法:
import * as actionCreators from './actionCreators'
function mapStateToProps(state) {
return { todos: state.todos }
}
function mergeProps(stateProps, dispatchProps, ownProps) {
return Object.assign({}, ownProps, {
todos: stateProps.todos[ownProps.userId],
addTodo: (text) => dispatchProps.addTodo(ownProps.userId, text)
})
}
export default connect(mapStateToProps, actionCreators, mergeProps)(TodoApp)
connect源码解析
源码有点长,你可以选择性的查看:
import { Component, createElement } from 'react'
import storeShape from '../utils/storeShape'
import shallowEqual from '../utils/shallowEqual'
import wrapActionCreators from '../utils/wrapActionCreators'
import warning from '../utils/warning'
import isPlainObject from 'lodash/isPlainObject'
import hoistStatics from 'hoist-non-react-statics'
import invariant from 'invariant'
const defaultMapStateToProps = state => ({}) // eslint-disable-line no-unused-vars
const defaultMapDispatchToProps = dispatch => ({ dispatch })
const defaultMergeProps = (stateProps, dispatchProps, parentProps) => ({
...parentProps,
...stateProps,
...dispatchProps
})
function getDisplayName(WrappedComponent) {
return WrappedComponent.displayName || WrappedComponent.name || 'Component'
}
let errorObject = { value: null }
function tryCatch(fn, ctx) {
try {
return fn.apply(ctx)
} catch (e) {
errorObject.value = e
return errorObject
}
}
// Helps track hot reloading.
let nextVersion = 0
export default function connect(mapStateToProps, mapDispatchToProps, mergeProps, options = {}) {
const shouldSubscribe = Boolean(mapStateToProps)
const mapState = mapStateToProps || defaultMapStateToProps
let mapDispatch
if (typeof mapDispatchToProps === 'function') {
mapDispatch = mapDispatchToProps
} else if (!mapDispatchToProps) {
mapDispatch = defaultMapDispatchToProps
} else {
mapDispatch = wrapActionCreators(mapDispatchToProps)
}
const finalMergeProps = mergeProps || defaultMergeProps
const { pure = true, withRef = false } = options
const checkMergedEquals = pure && finalMergeProps !== defaultMergeProps
// Helps track hot reloading.
const version = nextVersion++
return function wrapWithConnect(WrappedComponent) {
const connectDisplayName = `Connect(${getDisplayName(WrappedComponent)})`
function checkStateShape(props, methodName) {
if (!isPlainObject(props)) {
warning(
`${methodName}() in ${connectDisplayName} must return a plain object. ` +
`Instead received ${props}.`
)
}
}
function computeMergedProps(stateProps, dispatchProps, parentProps) {
const mergedProps = finalMergeProps(stateProps, dispatchProps, parentProps)
if (process.env.NODE_ENV !== 'production') {
checkStateShape(mergedProps, 'mergeProps')
}
return mergedProps
}
class Connect extends Component {
shouldComponentUpdate() {
return !pure || this.haveOwnPropsChanged || this.hasStoreStateChanged
}
constructor(props, context) {
super(props, context)
this.version = version
this.store = props.store || context.store
invariant(this.store,
`Could not find "store" in either the context or ` +
`props of "${connectDisplayName}". ` +
`Either wrap the root component in a <Provider>, ` +
`or explicitly pass "store" as a prop to "${connectDisplayName}".`
)
const storeState = this.store.getState()
this.state = { storeState }
this.clearCache()
}
computeStateProps(store, props) {
if (!this.finalMapStateToProps) {
return this.configureFinalMapState(store, props)
}
const state = store.getState()
const stateProps = this.doStatePropsDependOnOwnProps ?
this.finalMapStateToProps(state, props) :
this.finalMapStateToProps(state)
if (process.env.NODE_ENV !== 'production') {
checkStateShape(stateProps, 'mapStateToProps')
}
return stateProps
}
configureFinalMapState(store, props) {
const mappedState = mapState(store.getState(), props)
const isFactory = typeof mappedState === 'function'
this.finalMapStateToProps = isFactory ? mappedState : mapState
this.doStatePropsDependOnOwnProps = this.finalMapStateToProps.length !== 1
if (isFactory) {
return this.computeStateProps(store, props)
}
if (process.env.NODE_ENV !== 'production') {
checkStateShape(mappedState, 'mapStateToProps')
}
return mappedState
}
computeDispatchProps(store, props) {
if (!this.finalMapDispatchToProps) {
return this.configureFinalMapDispatch(store, props)
}
const { dispatch } = store
const dispatchProps = this.doDispatchPropsDependOnOwnProps ?
this.finalMapDispatchToProps(dispatch, props) :
this.finalMapDispatchToProps(dispatch)
if (process.env.NODE_ENV !== 'production') {
checkStateShape(dispatchProps, 'mapDispatchToProps')
}
return dispatchProps
}
configureFinalMapDispatch(store, props) {
const mappedDispatch = mapDispatch(store.dispatch, props)
const isFactory = typeof mappedDispatch === 'function'
this.finalMapDispatchToProps = isFactory ? mappedDispatch : mapDispatch
this.doDispatchPropsDependOnOwnProps = this.finalMapDispatchToProps.length !== 1
if (isFactory) {
return this.computeDispatchProps(store, props)
}
if (process.env.NODE_ENV !== 'production') {
checkStateShape(mappedDispatch, 'mapDispatchToProps')
}
return mappedDispatch
}
updateStatePropsIfNeeded() {
const nextStateProps = this.computeStateProps(this.store, this.props)
if (this.stateProps && shallowEqual(nextStateProps, this.stateProps)) {
return false
}
this.stateProps = nextStateProps
return true
}
updateDispatchPropsIfNeeded() {
const nextDispatchProps = this.computeDispatchProps(this.store, this.props)
if (this.dispatchProps && shallowEqual(nextDispatchProps, this.dispatchProps)) {
return false
}
this.dispatchProps = nextDispatchProps
return true
}
updateMergedPropsIfNeeded() {
const nextMergedProps = computeMergedProps(this.stateProps, this.dispatchProps, this.props)
if (this.mergedProps && checkMergedEquals && shallowEqual(nextMergedProps, this.mergedProps)) {
return false
}
this.mergedProps = nextMergedProps
return true
}
isSubscribed() {
return typeof this.unsubscribe === 'function'
}
trySubscribe() {
if (shouldSubscribe && !this.unsubscribe) {
this.unsubscribe = this.store.subscribe(this.handleChange.bind(this))
this.handleChange()
}
}
tryUnsubscribe() {
if (this.unsubscribe) {
this.unsubscribe()
this.unsubscribe = null
}
}
componentDidMount() {
this.trySubscribe()
}
componentWillReceiveProps(nextProps) {
if (!pure || !shallowEqual(nextProps, this.props)) {
this.haveOwnPropsChanged = true
}
}
componentWillUnmount() {
this.tryUnsubscribe()
this.clearCache()
}
clearCache() {
this.dispatchProps = null
this.stateProps = null
this.mergedProps = null
this.haveOwnPropsChanged = true
this.hasStoreStateChanged = true
this.haveStatePropsBeenPrecalculated = false
this.statePropsPrecalculationError = null
this.renderedElement = null
this.finalMapDispatchToProps = null
this.finalMapStateToProps = null
}
handleChange() {
if (!this.unsubscribe) {
return
}
const storeState = this.store.getState()
const prevStoreState = this.state.storeState
if (pure && prevStoreState === storeState) {
return
}
if (pure && !this.doStatePropsDependOnOwnProps) {
const haveStatePropsChanged = tryCatch(this.updateStatePropsIfNeeded, this)
if (!haveStatePropsChanged) {
return
}
if (haveStatePropsChanged === errorObject) {
this.statePropsPrecalculationError = errorObject.value
}
this.haveStatePropsBeenPrecalculated = true
}
this.hasStoreStateChanged = true
this.setState({ storeState })
}
getWrappedInstance() {
invariant(withRef,
`To access the wrapped instance, you need to specify ` +
`{ withRef: true } as the fourth argument of the connect() call.`
)
return this.refs.wrappedInstance
}
render() {
const {
haveOwnPropsChanged,
hasStoreStateChanged,
haveStatePropsBeenPrecalculated,
statePropsPrecalculationError,
renderedElement
} = this
this.haveOwnPropsChanged = false
this.hasStoreStateChanged = false
this.haveStatePropsBeenPrecalculated = false
this.statePropsPrecalculationError = null
if (statePropsPrecalculationError) {
throw statePropsPrecalculationError
}
let shouldUpdateStateProps = true
let shouldUpdateDispatchProps = true
if (pure && renderedElement) {
shouldUpdateStateProps = hasStoreStateChanged || (
haveOwnPropsChanged && this.doStatePropsDependOnOwnProps
)
shouldUpdateDispatchProps =
haveOwnPropsChanged && this.doDispatchPropsDependOnOwnProps
}
let haveStatePropsChanged = false
let haveDispatchPropsChanged = false
if (haveStatePropsBeenPrecalculated) {
haveStatePropsChanged = true
} else if (shouldUpdateStateProps) {
haveStatePropsChanged = this.updateStatePropsIfNeeded()
}
if (shouldUpdateDispatchProps) {
haveDispatchPropsChanged = this.updateDispatchPropsIfNeeded()
}
let haveMergedPropsChanged = true
if (
haveStatePropsChanged ||
haveDispatchPropsChanged ||
haveOwnPropsChanged
) {
haveMergedPropsChanged = this.updateMergedPropsIfNeeded()
} else {
haveMergedPropsChanged = false
}
if (!haveMergedPropsChanged && renderedElement) {
return renderedElement
}
if (withRef) {
this.renderedElement = createElement(WrappedComponent, {
...this.mergedProps,
ref: 'wrappedInstance'
})
} else {
this.renderedElement = createElement(WrappedComponent,
this.mergedProps
)
}
return this.renderedElement
}
}
Connect.displayName = connectDisplayName
Connect.WrappedComponent = WrappedComponent
Connect.contextTypes = {
store: storeShape
}
Connect.propTypes = {
store: storeShape
}
if (process.env.NODE_ENV !== 'production') {
Connect.prototype.componentWillUpdate = function componentWillUpdate() {
if (this.version === version) {
return
}
// We are hot reloading!
this.version = version
this.trySubscribe()
this.clearCache()
}
}
return hoistStatics(Connect, WrappedComponent)
}
}
我们按照上面介绍的解析步骤来一步步有序的分析源码。
1、查看函数入口,以及需要传入的参数。
如果只是看这样一个函数体,我们无法得知每个参数到底是什么?有什么作用?但是,我们可以先结合使用的demo初步了解各个参数的作用。
export default function connect(mapStateToProps, mapDispatchToProps, mergeProps, options = {}) {}
mapStateToProps:传入所有state,返回指定的state数据。
function mapStateToProps(state) {
return { todos: state.todos }
}
mapDispatchToProps:传入dispatch,返回使用bindActionCreators()绑定的action方法。我们不再这里讨论bindActionCreators的用法,这个知识将会放到redux解析的文章中。
function mapDispatchToProps(dispatch) {
return bindActionCreators(Object.assign({}, todoActionCreators, counterActionCreators), dispatch)
}
mergeProps:mergeProps如果不指定,则默认返回 Object.assign({}, ownProps, stateProps, dispatchProps),顾名思义,mergeProps是合并的意思,将state合并后传递给组件。
function mergeProps(stateProps, dispatchProps, ownProps) {
return Object.assign({}, ownProps, {
todos: stateProps.todos[ownProps.userId],
addTodo: (text) => dispatchProps.addTodo(ownProps.userId, text)
})
}
options:通过配置项可以更加详细的定义connect的行为,通常只需要执行默认值。
2、查看导入了哪些插件
import { Component, createElement } from 'react'
import storeShape from '../utils/storeShape'
import shallowEqual from '../utils/shallowEqual'
import wrapActionCreators from '../utils/wrapActionCreators'
import warning from '../utils/warning'
import isPlainObject from 'lodash/isPlainObject'
import hoistStatics from 'hoist-non-react-statics'
import invariant from 'invariant'
react:使用到了react组件,那么我们可以猜测connect和Provider类似,需要创建一个Connect组件。
storeShape:通过了redux常用API的类型验证。
import PropTypes from 'prop-types'
export default PropTypes.shape({
subscribe: PropTypes.func.isRequired,
dispatch: PropTypes.func.isRequired,
getState: PropTypes.func.isRequired
})
shallowEqual:这个文件的作用是传入2个对象,首先比较对象是否一致,如果一致,则返回true,如果不一致,则获取2个对象的key数组,判断2个对象key数组的长度是否相等,如果不相等,返回false,如果相等,最后用for循环遍历A对象的key,如果当前的遍历值不存在于B的key中或者A对象的当前key的value不等于B对象的当前key的value,则返回false,如果不属于上面的任何情况,则返回true。(如果认为我这段讲的迷迷糊糊,你也可以自己理解下面的代码。)
export default function shallowEqual(objA, objB) {
if (objA === objB) {
return true
}
const keysA = Object.keys(objA)
const keysB = Object.keys(objB)
if (keysA.length !== keysB.length) {
return false
}
// 测试A对象的key和B对象的key不一致
const hasOwn = Object.prototype.hasOwnProperty
for (let i = 0; i < keysA.length; i++) {
if (!hasOwn.call(objB, keysA[i]) || objA[keysA[i]] !== objB[keysA[i]]) {
return false
}
}
return true
}
hasOwn的作用是判断对象里面是否包含某个属性。这段代码的实际用途是判断下一个props和当前的props是否一致。
shallowEqual(nextStateProps, this.stateProps)
wrapActionCreators:实现了bindActionCreators方法绑定action到组件的操作。
import { bindActionCreators } from 'redux'
export default function wrapActionCreators(actionCreators) {
return dispatch => bindActionCreators(actionCreators, dispatch)
}
函数使用方法
wrapActionCreators(mapDispatchToProps)
warning:在控制台打印warning信息
export default function warning(message) {
if (typeof console !== 'undefined' && typeof console.error === 'function') {
console.error(message)
}
try {
throw new Error(message)
} catch (e) {}
}
lodash/isPlainObject:检查传入的值是不是纯对象,如果是,返回true,否则返回false。方法详情查看 lodash之isPlainObject
function isPlainObject(value) {
if (!isObjectLike(value) || baseGetTag(value) != objectTag) {
return false;
}
var proto = getPrototype(value);
if (proto === null) {
return true;
}
var Ctor = hasOwnProperty.call(proto, 'constructor') && proto.constructor;
return typeof Ctor == 'function' && Ctor instanceof Ctor &&
funcToString.call(Ctor) == objectCtorString;
}
hoist-non-react-statics:这段代码有点神奇,REACT_STATICS是一堆react的常用方法,KNOWN_STATICS是函数的一些属性。
var REACT_STATICS = {
childContextTypes: true,
contextTypes: true,
defaultProps: true,
displayName: true,
getDefaultProps: true,
mixins: true,
propTypes: true,
type: true
};
var KNOWN_STATICS = {
name: true,
length: true,
prototype: true,
caller: true,
arguments: true,
arity: true
};
var isGetOwnPropertySymbolsAvailable = typeof Object.getOwnPropertySymbols === 'function';
module.exports = function hoistNonReactStatics(targetComponent, sourceComponent, customStatics) {
if (typeof sourceComponent !== 'string') { // don't hoist over string (html) components
var keys = Object.getOwnPropertyNames(sourceComponent);
if (isGetOwnPropertySymbolsAvailable) {
keys = keys.concat(Object.getOwnPropertySymbols(sourceComponent));
}
for (var i = 0; i < keys.length; ++i) {
if (!REACT_STATICS[keys[i]] && !KNOWN_STATICS[keys[i]] && (!customStatics || !customStatics[keys[i]])) {
try {
targetComponent[keys[i]] = sourceComponent[keys[i]];
} catch (error) {
}
}
}
}
return targetComponent;
};
我们首先从函数入口解读,入口传入了3个参数,targetComponent, sourceComponent, customStatics,首先判断sourceComponent的类型不是一个字符串,然后使用getOwnPropertyNames获取sourceComponent对象的key,返回值是key组成的数组keys。接着判断isGetOwnPropertySymbolsAvailable(肯定是true),如果为true,执行下面的语句:
keys = keys.concat(Object.getOwnPropertySymbols(sourceComponent));
getOwnPropertySymbols和getOwnPropertyNames作用类似,但是getOwnPropertyNames只是返回字符串类型的key,而getOwnPropertySymbols可以返回Symbol类型的key。然后我们再把2种情况下的key拼接到一个数组里面返回新的keys。
然后执行for语句,遍历keys,如果不包含REACT_STATICS中的react的静态方法,同时不包含KNOWN_STATICS中的属性,同时不存在customStatics(传入函数的第三个参数不存在)或者存在但没有sourceComponent的key,就执行:
//将sourceComponent的方法写入targetComponent中
targetComponent[keys[i]] = sourceComponent[keys[i]];
最后返回targetComponent:
return targetComponent
该方法在connect中的实际作用是:将WrappedComponent内的react静态方法绑定到Connect组件上。
hoistStatics(Connect, WrappedComponent)
invariant:我们看到invariant传入了好几个参数,第一个if语句表示如果不是生产环境,并且format没有定义,就抛出异常。第二个if表示如果condition未定义,同时format未定义,就抛出error,如果condition不存在但format存在,抛出另外的错误。(总结就是一个错误检查机制)
var NODE_ENV = process.env.NODE_ENV;
var invariant = function(condition, format, a, b, c, d, e, f) {
if (NODE_ENV !== 'production') {
if (format === undefined) {
throw new Error('invariant requires an error message argument');
}
}
if (!condition) {
var error;
if (format === undefined) {
error = new Error(
'Minified exception occurred; use the non-minified dev environment ' +
'for the full error message and additional helpful warnings.'
);
} else {
var args = [a, b, c, d, e, f];
var argIndex = 0;
error = new Error(
format.replace(/%s/g, function() { return args[argIndex++]; })
);
error.name = 'Invariant Violation';
}
error.framesToPop = 1; // we don't care about invariant's own frame
throw error;
}
};
module.exports = invariant;
该方法实际用途:检查store是否存在
invariant(this.store,
`Could not find "store" in either the context or ` +
`props of "${connectDisplayName}". ` +
`Either wrap the root component in a <Provider>, ` +
`or explicitly pass "store" as a prop to "${connectDisplayName}".`
)
3、定义几个参数默认值常量
当你没有给组件绑定state和dispatch的时候,就执行默认的配置。
defaultMapStateToProps:传入state,返回空对象
defaultMapDispatchToProps: 传入dispatch,返回dispatch对象
defaultMergeProps:传入stateProps, dispatchProps, parentProps,返回当前传入的对象。
const defaultMapStateToProps = state => ({})
const defaultMapDispatchToProps = dispatch => ({ dispatch })
const defaultMergeProps = (stateProps, dispatchProps, parentProps) => ({
...parentProps,
...stateProps,
...dispatchProps
})
4、getDisplayName方法
返回当前传入的组件名
function getDisplayName(WrappedComponent) {
return WrappedComponent.displayName || WrappedComponent.name || 'Component'
}
5、tryCatch方法
给fn函数指定上下文。
let errorObject = { value: null }
function tryCatch(fn, ctx) {
try {
return fn.apply(ctx)
} catch (e) {
errorObject.value = e
return errorObject
}
}
使用场景:在connect内调用tryCatch给updateStatePropsIfNeeded方法指定当前的上下文
tryCatch(this.updateStatePropsIfNeeded, this)
如果你不明白上面的代码,可以看下面比较简单的例子:
let b = {
a: 1,
e: function() {
console.log(this.a)
},
c: function() {
tryCatch(this.e, this)
}
}
b.c() // 1
6、connect函数解析思路
connect函数是核心,我们需要大概了解函数做的事情,才能更好的读懂源码。
既然是函数,那就有返回值,connect()返回值是Connect组件(请注意大小写的区别)。
通俗点理解,使用connect可以把state和dispatch绑定到react组件,使得组件可以访问到redux的数据。
常看到下面这种写法:
export default connect(mapStateToProps)(TodoApp)
我把connect的核心实现简化提取出来,是下面这种形式:WrappedComponent参数对应的就是TodoApp。函数最终返回的是将state和dispatch绑定到Connect之后的新组件。
funtion connect(mapStateToProps) {
return function wrapWithConnect(WrappedComponent) {
class Connect extends Component {
}
return hoistStatics(Connect, WrappedComponent)
}
}
7、Connect组件执行
既然已经知道connect函数返回的是Connect组件,而Connect组件继承于react,我们就可以按照react的生命周期来阅读代码。
Connect组件方法组成:方法虽然很多,但是我们只需要紧跟react生命周期函数去了解代码,而其他方法都是在生命周期函数中调用的。
class Connect extends Component {
shouldComponentUpdate() {}
constructor(props, context) {}
computeStateProps(store, props) {}
configureFinalMapState(store, props) {}
computeDispatchProps(store, props) {}
configureFinalMapDispatch(store, props) {}
updateStatePropsIfNeeded() {}
updateDispatchPropsIfNeeded() {}
updateMergedPropsIfNeeded() {}
isSubscribed() {}
trySubscribe() {}
tryUnsubscribe() {}
componentDidMount() {}
componentWillReceiveProps(nextProps) {}
componentWillUnmount() {}
clearCache() {}
handleChange() {}
getWrappedInstance() {}
render() {}
}
简单了解react生命周期的函数执行顺序:
初次渲染:render => componentDidMount
当state更新时:componentWillReceiveProps => shouldComponentUpdate => render
render:进入Connect组件执行的时候,先进入render方法。
render() {
const {haveOwnPropsChanged, hasStoreStateChanged, haveStatePropsBeenPrecalculated, statePropsPrecalculationError, renderedElement} = this
this.haveOwnPropsChanged = false
this.hasStoreStateChanged = false
this.haveStatePropsBeenPrecalculated = false
this.statePropsPrecalculationError = null
if (statePropsPrecalculationError) {
throw statePropsPrecalculationError
}
let shouldUpdateStateProps = true
let shouldUpdateDispatchProps = true
if (pure && renderedElement) {
shouldUpdateStateProps = hasStoreStateChanged || (
haveOwnPropsChanged && this.doStatePropsDependOnOwnProps
)
shouldUpdateDispatchProps =
haveOwnPropsChanged && this.doDispatchPropsDependOnOwnProps
}
let haveStatePropsChanged = false
let haveDispatchPropsChanged = false
if (haveStatePropsBeenPrecalculated) {
haveStatePropsChanged = true
} else if (shouldUpdateStateProps) {
haveStatePropsChanged = this.updateStatePropsIfNeeded()
}
if (shouldUpdateDispatchProps) {
haveDispatchPropsChanged = this.updateDispatchPropsIfNeeded()
}
let haveMergedPropsChanged = true
if (haveStatePropsChanged || haveDispatchPropsChanged || haveOwnPropsChanged) {
haveMergedPropsChanged = this.updateMergedPropsIfNeeded()
} else {
haveMergedPropsChanged = false
}
if (!haveMergedPropsChanged && renderedElement) {
return renderedElement
}
if (withRef) {
this.renderedElement = createElement(WrappedComponent, {
...this.mergedProps,
ref: 'wrappedInstance'
})
} else {
this.renderedElement = createElement(WrappedComponent,
this.mergedProps
)
}
a、首先定义了5个成员变量,在Connect组件内部的任意函数位置可以访问到this定义的成员变量。
const {haveOwnPropsChanged, hasStoreStateChanged, haveStatePropsBeenPrecalculated, statePropsPrecalculationError, renderedElement} = this
//上面的代码等于下面的写法,this指当前的组件对象。
//判断新传入的props和当前的是否相等,是bool值
var haveOwnPropsChanged = this.haveOwnPropsChanged;
//当state更新时,改变hasStoreStateChanged的状态,是bool值
var hasStoreStateChanged = this.hasStoreStateChanged;
//表示state和props已经提前计算改变,也是bool值
var haveStatePropsBeenPrecalculated = this.haveStatePropsBeenPrecalculated;
//如果state和props更新时出现错误,则抛出statePropsPrecalculationError异常
var statePropsPrecalculationError = this.statePropsPrecalculationError;
//将要渲染的react组件
var renderedElement = this.renderedElement;
b、给成员变量设置默认值。默认值要么是false,要么是null。
this.haveOwnPropsChanged = false
this.hasStoreStateChanged = false
this.haveStatePropsBeenPrecalculated = false
this.statePropsPrecalculationError = null
c、抛出异常:初次渲染时,statePropsPrecalculationError为null,不会抛出异常,当执行state和props更新出现异常时,会抛出错误。
if (statePropsPrecalculationError) {
throw statePropsPrecalculationError
}
我们追踪到statePropsPrecalculationError的赋值是在handleChange()里面执行的,受到haveStatePropsChanged的结果影响。当haveStatePropsChanged出现错误时,就把报错内容赋值给statePropsPrecalculationError。
if (haveStatePropsChanged === errorObject) {
this.statePropsPrecalculationError = errorObject.value
}
d、定义shouldUpdateStateProps和shouldUpdateDispatchProps:默认为true前者表示默认允许更新state和props,后者表示默认允许更新dispatch。
pure:options的配置项,初始值为true。
shouldUpdateStateProps:我们看到 || 符号,只要左右2边满足一个为true,则返回true,如果2个都是false,则返回false。
shouldUpdateDispatchProps:同时满足haveOwnPropsChanged、doDispatchPropsDependOnOwnProps为true,则返回true,否则返回false。
let shouldUpdateStateProps = true
let shouldUpdateDispatchProps = true
if (pure && renderedElement) {
shouldUpdateStateProps = hasStoreStateChanged ||
(haveOwnPropsChanged && this.doStatePropsDependOnOwnProps)
shouldUpdateDispatchProps = haveOwnPropsChanged && this.doDispatchPropsDependOnOwnProps
}
e、上面几个步骤都是定义state和props的各种状态的变量,目的是为了判断render方法返回怎样的renderedElement。
//如果haveMergedPropsChanged为false,并且renderedElement不为null,则返回renderedElement
//这段代码在初次渲染是不会执行,只有在更新state和props的时候执行
if (!haveMergedPropsChanged && renderedElement) {
return renderedElement
}
//haveMergedPropsChanged由updateMergedPropsIfNeeded方法的返回值控制,如果mergedProps等于nextMergedProps,返回false,不相等则返回true,表示应该更新state和props
updateMergedPropsIfNeeded() {
const nextMergedProps = computeMergedProps(this.stateProps, this.dispatchProps, this.props)
if (this.mergedProps && checkMergedEquals && shallowEqual(nextMergedProps, this.mergedProps)) {
return false
}
this.mergedProps = nextMergedProps
return true
}
初次进入组件最先渲染的返回值是下面这段:
if (withRef) {
this.renderedElement = createElement(WrappedComponent, {
...this.mergedProps,
ref: 'wrappedInstance'
})
} else {
this.renderedElement = createElement(WrappedComponent,
this.mergedProps
)
}
connect渲染结果:在你绑定的组件外层包裹了Connect组件,看下面的图你应该能更加清晰的了解connect做的事情。
componentWillReceiveProps:组件接收到新的state。如果pure为false,并且nextProps和this.props不相等,则设置this.haveOwnPropsChanged为true。
componentWillReceiveProps(nextProps) {
if (!pure || !shallowEqual(nextProps, this.props)) {
this.haveOwnPropsChanged = true
}
}
shouldComponentUpdate():判断组件是否允许更新。
shouldComponentUpdate() {
return !pure || this.haveOwnPropsChanged || this.hasStoreStateChanged
}
componentDidMount():组件初次渲染完成,执行订阅更新
componentDidMount() {
this.trySubscribe()
}
componentWillUnmount():组件卸载时恢复状态。
componentWillUnmount() {
this.tryUnsubscribe()
this.clearCache()
}
clearCache() {
this.dispatchProps = null
this.stateProps = null
this.mergedProps = null
this.haveOwnPropsChanged = true
this.hasStoreStateChanged = true
this.haveStatePropsBeenPrecalculated = false
this.statePropsPrecalculationError = null
this.renderedElement = null
this.finalMapDispatchToProps = null
this.finalMapStateToProps = null
}
8、总结
如果看到这里,你还没有理清思路,那么可以看完总结再回过头去理解源码。
connect方法做的事情是将state和dispatch绑定到Connect组件的参数上,然后Connect组件将你当前的App组件封装起来,使得App组件可以通过props获取到父组件Connect传递的state和props。
这也就是为什么你可以在自己写的组件上面直接通过this.props访问到state和action。有的人是通过store去读取state和dispatch action,也是一样的道理。
从connect方法的实现,我们看到了非常多react组件的影子,生命周期,props传递,context上下文。
对比Provider组件:
Provider是顶层组件的作用,将store作为上下文提供给全局共享,而Connect组件是局部组件,将某个react组件包装起来,传递指定的state和props给该组件访问。