元素的状态是第一次未定义？

Question

我正在尝试在应用程序中创建切换开关，而我是redux的新手。经过非常努力并在网上寻找任何地方后，发布此问题。请提供帮助。我的减速机如下：

const initialState = {
    preview: "off",
    cards:
};

const cardReducer = (state = initialState, action) => {
        switch (action.type) {

            case "FIND_ALL_CARDS":
                return {
                    cards: action.cards
                };
            case "DELETE_CARD":
                return {
                    cards: state.cards.filter(card => card.id !== action.cardId)
                };
            case "CREATE_CARD":
                return {
                    cards: [...state.cards, action.card]
                };
            case "PREVIEW":
                console.log(state); // I get cards[] in the state but no preview.
                let swtch = "on";
                if (state.preview === "on") {
                    swtch = "off";
                }
                console.log(state.preview); // Undefined 

                return {
                    cards: state.cards,
                        preview: swtch
                };
            default:
                return state;
        }

我已经正确连接了组件

const stateToPropertyMapper = state => ({
  cards: state.cards.cards,
  preview: state.cards.cards
});

我第一次切换开关时未定义，但是第一次切换后，我得到了状态。请帮我

Answer 1

基本上是从reducer返回状态的COPY，而当返回{cards：[]}时，您将丢失该状态的所有其他props。在FIND_ALL，DELETE和Create CARD中对状态进行突变时，您还需要返回状态的其余部分。像这样编辑

case "FIND_ALL_CARDS":
    return {
        ...state
        cards: action.cards
     };

在所有动作中都这样做，当您改变单个状态时，不要忘记传递先前的状态道具