異步/等待功能永遠不會用 Dispatch 解決

Question

在我下面的函數中， addToFlatList只被調用一次，即使我知道我的數據庫中有幾個項目要添加。 似乎拳頭addToFlatList從未解決過？ 我究竟做錯了什么？

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

addToFlatList 函數：

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：

試圖像這樣返回調度。 addToFlatList仍然只調用一次。

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"});
        });
    }
}

也試過這個：

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"});
    });
}

Answer 1

您必須返回承諾：

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，您必須等待承諾，以便您的async功能不會過早結束：

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"});
    }
}