react-redux 如何在子组件里访问store对象

1. 要在constructor-super里接收context对象

2. 给组件(class / pure function)指定contextType属性，用来接收store对象

以下代码模拟了connect(类似react-redux里connect的功能)高阶组件的实现：

function connect(mapStateToProps=doNothing, mapDispatchToProps=doNothing){
  return function( WrappedComponent ){
    class HOC extends React.Component{
     constructor(props, context){ super(props, context); }

      render(){
        const store = this.context.store;
        const stateProps = mapStateToProps(store.getState());// deliver state
        const dispatchProps = mapDispatchToProps(store.dispatch);// deliver dispatch
        const props = {...stateProps, ...dispatchProps};
        return <WrappedComponent {...props}/>;
      }
    }
   HOC.contextTypes = { // get store from context
      store: PropTypes.object
    }
    return HOC;
  }
}

export {connect};