react-redux源码

总结一下看完react-redux的源码后的一些收获

ps： react-redux版本为5.0.5

关键代码

 class Connect extends Component {
      constructor(props, context) {
        super(props, context)

        this.version = version
        this.state = {}
        this.renderCount = 0
        this.store = props[storeKey] || context[storeKey]
        this.propsMode = Boolean(props[storeKey])
        this.setWrappedInstance = this.setWrappedInstance.bind(this)

        invariant(this.store,
          `Could not find "${storeKey}" in either the context or props of ` +
          `"${displayName}". Either wrap the root component in a <Provider>, ` +
          `or explicitly pass "${storeKey}" as a prop to "${displayName}".`
        )

        this.initSelector()
        this.initSubscription()
      }

      getChildContext() {
        // If this component received store from props, its subscription should be transparent
        // to any descendants receiving store+subscription from context; it passes along
        // subscription passed to it. Otherwise, it shadows the parent subscription, which allows
        // Connect to control ordering of notifications to flow top-down.
        const subscription = this.propsMode ? null : this.subscription
        return { [subscriptionKey]: subscription || this.context[subscriptionKey] }
      }

      componentDidMount() {
        if (!shouldHandleStateChanges) return

        // componentWillMount fires during server side rendering, but componentDidMount and
        // componentWillUnmount do not. Because of this, trySubscribe happens during ...didMount.
        // Otherwise, unsubscription would never take place during SSR, causing a memory leak.
        // To handle the case where a child component may have triggered a state change by
        // dispatching an action in its componentWillMount, we have to re-run the select and maybe
        // re-render.
        this.subscription.trySubscribe()
        this.selector.run(this.props)
        if (this.selector.shouldComponentUpdate) this.forceUpdate()
      }

      componentWillReceiveProps(nextProps) {
        this.selector.run(nextProps)
      }

      shouldComponentUpdate() {
        return this.selector.shouldComponentUpdate
      }




    

      initSubscription() {
        if (!shouldHandleStateChanges) return

        // parentSub's source should match where store came from: props vs. context. A component
        // connected to the store via props shouldn't use subscription from context, or vice versa.
        const parentSub = (this.propsMode ? this.props : this.context)[subscriptionKey]
        this.subscription = new Subscription(this.store, parentSub, this.onStateChange.bind(this))

        // `notifyNestedSubs` is duplicated to handle the case where the component is  unmounted in
        // the middle of the notification loop, where `this.subscription` will then be null. An
        // extra null check every change can be avoided by copying the method onto `this` and then
        // replacing it with a no-op on unmount. This can probably be avoided if Subscription's
        // listeners logic is changed to not call listeners that have been unsubscribed in the
        // middle of the notification loop.
        this.notifyNestedSubs = this.subscription.notifyNestedSubs.bind(this.subscription)
      }

      onStateChange() {
        this.selector.run(this.props)
        
        if (!this.selector.shouldComponentUpdate) {
          this.notifyNestedSubs()
        } else {
          this.componentDidUpdate = this.notifyNestedSubsOnComponentDidUpdate
          this.setState(dummyState)
        }
      }

      notifyNestedSubsOnComponentDidUpdate() {
        // `componentDidUpdate` is conditionally implemented when `onStateChange` determines it
        // needs to notify nested subs. Once called, it unimplements itself until further state
        // changes occur. Doing it this way vs having a permanent `componentDidMount` that does
        // a boolean check every time avoids an extra method call most of the time, resulting
        // in some perf boost.
        this.componentDidUpdate = undefined
        this.notifyNestedSubs()
      }

      

  

      render() {
        const selector = this.selector
        selector.shouldComponentUpdate = false

        if (selector.error) {
          throw selector.error
        } else {
          return createElement(WrappedComponent, this.addExtraProps(selector.props))
        }
      }
    }

总结：

shouldComponentUpdate

connect组件存在shouldComponentUpdate，所以使用react-redux在利用hoc进行组件绑定的时候，是存在性能优化的，避免不必要的更新。即便存在嵌套的connect组件，父节点更新，而子节点如果没有变化，也不会进行更新。

判断是否更新的逻辑

主要是简单的进行浅相等判断,判断下一次更新的props和之前的时候一直，如果一直就不更新，避免多于更新。

function makeSelectorStateful(sourceSelector, store) {
  // wrap the selector in an object that tracks its results between runs.
  const selector = {
    run: function runComponentSelector(props) {
      try {
        const nextProps = sourceSelector(store.getState(), props) 
        if (nextProps !== selector.props || selector.error) {
          selector.shouldComponentUpdate = true
          selector.props = nextProps
          selector.error = null
        }
      } catch (error) {
        selector.shouldComponentUpdate = true
        selector.error = error
      }
    }
  }

  return selector
}

connect 绑定组件的方式：

const mapStateToProps = (state) => {
  return {
    topic: state.topic,
  }
}

const mapDispatchToProps = dispatch => (
  {
    actions: bindActionCreators(Actions, dispatch)
  }
)

export default connect(mapStateToProps, mapDispatchToProps)(MpNewsPage)

所以优化的方式就是减少mapStateToProps中涉及的props，如果放入多余的状态，可能会引起页面的多余刷新。