我无法在使用 redux 概念实现的构造函数中获取props。

容器组件代码

class UpdateItem extends Component{
    constructor(props) {
    super(props);
    console.log(this.props.item.itemTitle) // output: undefined
    this.state = {
        itemTitle: this.props.item.itemTitle,
        errors: {}
    };

    this.handleChange = this.handleChange.bind(this);
}

handleChange(e) {
    //If the input fields were directly within this
    //this component, we could use this.refs.[FIELD].value
    //Instead, we want to save the data for when the form is submitted
    let state = {};
    state[e.target.name] = e.target.value.trim();
    this.setState(state);
}

    handleSubmit(e) {
    //we don't want the form to submit, so we pritem the default behavior
    e.preventDefault();

    let errors = {};
    errors = this._validate();
    if(Object.keys(errors).length != 0) {
        this.setState({
            errors: errors
        });
        return;
    }
    let itemData = new FormData();
    itemData.append('itemTitle',this.state.itemTitle)

    this.props.onSubmit(itemData);
}

    componentDidMount(){
        this.props.getItemByID();
    }

    componentWillReceiveProps(nextProps){
        if (this.props.item.itemID != nextProps.item.itemID){
            //Necessary to populate form when existing item is loaded directly.
            this.props.getItemByID();
        }
    }


    render(){

        let {item} = this.props;
        return(
            <UpdateItemForm
            itemTitle={this.state.itemTitle}
            errors={this.state.errors}
            />
        );
    }
}

UpdateItem.propTypes = {
    item: PropTypes.array.isRequired
};

function mapStateToProps(state, ownProps){
    let item = {
        itemTitle: ''
    };


    return {
        item: state.itemReducer
    };
}

function mapDispatchToProps (dispatch, ownProps) {
    return {
        getItemByID:()=>dispatch(loadItemByID(ownProps.params.id)),
        onSubmit: (values) => dispatch(updateItem(values))
    }
}

export default connect(mapStateToProps,mapDispatchToProps)(UpdateItem);

在 render() 方法中，可以从 redux 中获取props，即项目，但不能在构造函数中获取。

以及用于查看 redux 实现是否正确的操作代码，

    export function loadItemByID(ID){
        return function(dispatch){
            return itemAPI.getItemByID(ID).then(item => {
                dispatch(loadItemByIDSuccess(item));
            }).catch(error => {
                throw(error);
            });
        };
    }

export function loadItemByIDSuccess(item){
    return {type: types.LOAD_ITEM_BY_ID_SUCCESS, item}
}

最后我的减速机看起来如下，

export default function itemReducer(state = initialState.item, action) {
    switch (action.type) {
        case types.LOAD_ITEM_BY_ID_SUCCESS:
            return Object.assign([], state = action.item, {
                item: action.item
            });
        default:
            return state;
    }
}

我用谷歌搜索没有运气得到答案，我不知道我在哪里犯了错误。如果有人为我指出这将是一个很大的帮助。提前致谢。