状态不会在 React Native 上更新

Question

我正在做我的 react native 应用的通知页面。它具有无限滚动和“拉动刷新”选项。进入它工作的页面，它也可以拉动刷新。 当我向下滚动时会出现问题，因为它似乎调用服务器来获取新通知，但它没有连接到数组。

import React, { useState, useEffect, useCallback, Component } from "react";
import {
  View,
  Text,
  FlatList,
  Button,
  Platform,
  ActivityIndicator,
  StyleSheet,
  ScrollView,
  RefreshControl,
  SafeAreaView,
} from "react-native";
import { useSelector, useDispatch } from "react-redux";
import i18n from "i18n-js";
import Colors from "../../constants/Colors";
import { getNotificationList } from "../../utils/NotificationsUtils";
import Card from "../../components/UI/Card";

const NotificationsScreen = (props) => {
  const [refreshing, setRefreshing] = useState(false);
  const [isLoading, setIsLoading] = useState(false);
  const [page, setPage] = useState(0);
  const [notifications, setNotifications] = useState([]);
  const [error, setError] = useState();

  const dispatch = useDispatch();

  const onRefresh = useCallback(async () => {
    setRefreshing(true);
    setNotifications([]);
    setPage(0);

    console.log("-- Refreshing --");

    getNotifications().then(() => {
      setRefreshing(false);
    });
  }, [dispatch, setRefreshing]);

  const fetchMoreNotifications = useCallback(async () => {
    const newPage = page + 7;
    setPage(newPage);
    console.log(
      "FETCH MORE from page " + newPage + " on array of " + notifications.length
    );

    getNotifications().then(() => {
      setIsLoading(false);
    });
  }, [dispatch, getNotifications]);

  const getNotifications = useCallback(async () => {
    setError(null);
    setIsLoading(true);
    try {
      console.log("Get from page " + page);
      // let fromRecord = (page - 1) * 7;
      const retrievedNotifications = await getNotificationList(
        page,
        7,
        true,
        false
      );
      console.log(
        "Setting " +
          retrievedNotifications.response.notifications.length +
          " new notifications on an already existing array of " +
          notifications.length +
          " elements"
      );

      let updatedNews = notifications.concat(
        retrievedNotifications &&
          retrievedNotifications.response &&
          retrievedNotifications.response.notifications
      );
      setNotifications(updatedNews);
    } catch (err) {
      setError(err.message);
    }
    setIsLoading(false);
  }, [dispatch, setIsLoading, setNotifications, setError]);

  useEffect(() => {
    setIsLoading(true);
    getNotifications(page).then(() => {
      setIsLoading(false);
    });
  }, [dispatch, getNotifications]);

  return (
    <View>
      {error ? (
        <View style={styles.centered}>
          <Text>Error</Text>
        </View>
      ) : refreshing ? (
        <View style={styles.centered}>
          <ActivityIndicator size="large" color={Colors.primary} />
        </View>
      ) : !notifications || !notifications.length ? (
        <View style={styles.centered}>
          <Text>No data found</Text>
        </View>
      ) : (
        <FlatList
          refreshControl={
            <RefreshControl refreshing={refreshing} onRefresh={onRefresh} />
          }
          data={notifications}
          keyExtractor={(notification) => notification.notificationQueueId}
          onEndReached={fetchMoreNotifications}
          onEndReachedThreshold={0.5}
          initialNumToRender={4}
          renderItem={(itemData) => (
            <View
              style={{
                marginTop: 10,
                height: 150,
                width: "100%",
              }}
            >
              <Card style={{ height: 150, backgroundColor: "white" }}>
                <Text style={{ fontSize: 16, color: Colors.black }}>
                  {itemData.item.text}
                </Text>
              </Card>
            </View>
          )}
        />
      )}
    </View>
  );
};

const styles = StyleSheet.create({
  centered: {
    flex: 1,
    justifyContent: "center",
    alignItems: "center",
  },
});

export default NotificationsScreen;

如果我滚动到结束它会触发“fetchMoreNotifications”功能，我会在控制台中得到这个：

FETCH MORE from page 7 on an array of 0
Get from page 0
Setting 7 new notifications on an already existing array of 0 elements
FETCH MORE from page 7 on an array of 0
Get from page 0
Setting 7 new notifications on an already existing array of 0 elements
FETCH MORE from page 7 on an array of 0
Get from page 0
Setting 7 new notifications on an already existing array of 0 elements
...and so on

如您所见，即使我之前保存了通知，它也会显示“现有 0 个元素的数组”。可能useCallback的依赖有问题？

Answer 1

问题：

有两个主要问题，一个是page，第二个是notifications，由于useCallback 和dependencies，useCallback 函数将始终指向不依赖的旧值，直到其中一个依赖更新。

---

1)page问题的解决方案：

将newPage 作为参数传递给getNotifications，由于setPage 的异步行为，它不会直接更新

第二次，要获取页面的更新值，您可以将page 作为依赖项传递。

2）notification问题的解决方案：

使用setState(prevState => newState)直接从其上一个状态值更新通知。

---

解决方案：

  const fetchMoreNotifications = useCallback(async () => {
    const newPage = page + 7;
    setPage(newPage);
    console.log(
      "FETCH MORE from page " + newPage + " on array of " + notifications.length
    );
    getNotifications(newPage).then(() => { // <---- Pass as param
      setIsLoading(false);
    });
  }, [page]); // <---- Add page as dependency 

  const getNotifications = useCallback(
    async page => { // <---- Get page as a param
      setError(null);
      setIsLoading(true);
      try {
        console.log("Get from page " + page);
        // let fromRecord = (page - 1) * 7;
      const retrievedNotifications = await getNotificationList(
        page,
        7,
        true,
        false
      );

      setNotifications(prevNotification => prevNotification.concat(
        retrievedNotifications &&
          retrievedNotifications.response &&
          retrievedNotifications.response.notifications
      )); // <---- Setting up state directly from previous value, instead of getting it from clone version of use callback
      } catch (err) {
        console.log(err);
        setError(err.message);
      }
      setIsLoading(false);
    },
    [setIsLoading, setNotifications, setError]
  );

工作演示：

检查控制台日志以获取更新的页面值，通知将呈现在 Html 上

注意：删除了一些代码只是为了提高代码的可读性和 调试问题

Answer 2

问题其实很简单。 getNotifications 函数是使用useCallback 创建的，并且没有使用notifications 作为依赖项。现在当通知更新时，由于关闭，getNotications 函数仍然引用旧的通知值。

另请注意，您在设置页面状态后立即在 fetchMoreNotifications 上调用 getNotifications，但页面状态也受到关闭的约束，并且不会在相同的重新渲染中更新

这里的解决方案是使用函数的方式setNotifications，使用useEffect在页面变化时触发4r getNotification

const fetchMoreNotifications = useCallback(async () => {
    const newPage = page + 7;
    setPage(newPage);
  }, [dispatch, getNotifications]);

  useEffect(() => {
    setIsLoading(true);
    getNotifications(page).then(() => {
      setIsLoading(false);
    });
  }, [dispatch, page, getNotifications]);

const getNotifications = useCallback(async () => {
    setError(null);
    setIsLoading(true);
    try {
      console.log("Get from page " + page);
      // let fromRecord = (page - 1) * 7;
      const retrievedNotifications = await getNotificationList(
        page,
        7,
        true,
        false
      );

      setNotifications(prevNotification => prevNotification.concat(
        retrievedNotifications &&
          retrievedNotifications.response &&
          retrievedNotifications.response.notifications
      ));
    } catch (err) {
      setError(err.message);
    }
    setIsLoading(false);
  }, [dispatch, setIsLoading, setNotifications, setError]);