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javascript - 异步/等待函数永远不会用 Dispatch 解决

问题描述

在我下面的函数中，addToFlatList只调用一次，即使我知道我的数据库中有几个项目要添加。好像拳头addToFlatList永远解决不了？我究竟做错了什么？

photosSnapshot.forEach(async function(childSnapshot) {
    await addToFlatList(childSnapshot.key, childSnapshot.val())(dispatch);
});

addToFlatList 函数：

const addToFlatList = (photoId, photoObj) => async(dispatch) => { 
    database.ref('users').child(photoObj.author).once('value').then((userSnapshot) => {
        var userInfo = userSnapshot.val();
        dispatch({type: "GOT_USER", payload: userInfo});
    }).catch(error => {
        dispatch({type: "GOT_ERROR"});
    });
}

更新：

试图像这样返回调度。addToFlatList仍然只调用一次。

const addToFlatList = async(photoId, photoObj) => {
    return (dispatch) => { 
        database.ref('users').child(photoObj.author).once('value').then((userSnapshot) => {
            var userInfo = userSnapshot.val();
            dispatch({type: "GOT_USER", payload: userInfo});
        }).catch(error => {
            dispatch({type: "GOT_ERROR"});
        });
    }
}

也试过这个：

const addToFlatList = async(photoId, photoObj) => {
    database.ref('users').child(photoObj.author).once('value').then((userSnapshot) => {
        return (dispatch) => { 
          // never hit this point
          var userInfo = userSnapshot.val();
          dispatch({type: "GOT_USER", payload: userInfo});
        }
    }).catch(error => {
        dispatch({type: "GOT_ERROR"});
    });
}

标签： javascriptecmascript-6async-awaites6-promisedispatch

解决方案

您必须返回承诺：

const addToFlatList = (photoId, photoObj) => (dispatch) => { 
    return database.ref('users').child(photoObj.author).once('value').then((userSnapshot) => {
//  ^^^^^^
        var userInfo = userSnapshot.val();
        return dispatch({type: "GOT_USER", payload: userInfo});
    }).catch(error => {
        return dispatch({type: "GOT_ERROR"});
    });
};

或者，您必须等待承诺，以免您的async功能过早结束：

const addToFlatList = (photoId, photoObj) => async (dispatch) => { 
    try {
        const userSnapshot = await database.ref('users').child(photoObj.author).once('value');
//                           ^^^^^
        var userInfo = userSnapshot.val();
        return dispatch({type: "GOT_USER", payload: userInfo});
    } catch(error) {
        return dispatch({type: "GOT_ERROR"});
    }
}

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