在循环内渲染和 return

Question

在我的代码中，我调用了一个函数showUsers。此函数在样式框中显示所有用户。现在，传递给它的 numberOfUsers 是 1，console.logs 也证实了这一点。

所以循环应该只运行一次。但是，所有这些 console.logs 都重复了两次。输出值保持不变。为什么会这样？

         console.log('Number of Users in Loop: ', numberOfUsers);
         console.log('Whats the Id', userId);
         console.log('UserName', userName);
         console.log('i: ', i);

代码：

export const Page: React.FunctionComponent<PageProps> = ({
  toggleShowPage,
  showAddFriendEmailPage,
}) => {
  const initialValues: FormValues = {
    email: '',
  };

  const [errorMessage, setErrorMessage] = useState('');
  const [userData, setUserData] = useState<UsersLazyQueryHookResult>('');
  const [numberOfUsers, setNumberOfUsers] = useState('');
  const validationSchema = emailValidationSchema;

  useEffect(() => {
    setUserData(userData);
    setNumberOfUsers(numberOfUsers);
  }, [userData, numberOfUsers]);


  const showAlert = () => {
    Alert.alert('Friend Added');
  };

  useEffect(() => {
    if (showAddFriendEmailPage) return;
    initialValues.email = '';
  }, [showAddFriendEmailPage]);

  const _onLoadUserError = React.useCallback((error: ApolloError) => {
    setErrorMessage(error.message);
    Alert.alert('Unable to Add Friend');
  }, []);

  const [
    createUserRelationMutation,
    {
      data: addingFriendData,
      loading: addingFriendLoading,
      error: addingFriendError,
      called: isMutationCalled,
    },
  ] = useCreateUserRelationMutation({
    onCompleted: (data: CreateUserRelationMutationResult) => {
      showAlert();
    },
  });

   const showUsers = React.useCallback(
     (data: UsersLazyQueryHookResult, numberOfUsers: Number) => {
       for (var i = 0; i < numberOfUsers; i++) {
        console.log('Number of Users in Loop: ', numberOfUsers);
         const userId = data.users.nodes[i].id;
         const userName = ((data.users.nodes[i].firstName).concat(' ')).concat(data.users.nodes[i].lastName);
         console.log('Whats the Id', userId);
         console.log('UserName', userName);
         console.log('Loop');
         return(
           <View style={styles.friends}>
             <View style={styles.item}>
              <Text>{userName}</Text>
             </View>
           </View>
         )
       }      
     },
     [createUserRelationMutation],
   );

  const addFriend = React.useCallback(
    (id: Number) => {
      console.log('Whats the Id', id);
      createUserRelationMutation({
        variables: {
          input: { relatedUserId: id, type: RelationType.Friend, userId: 7 },
        },
      });
    },
    [createUserRelationMutation],
  );

  const getFriendId = React.useCallback(
    (data: UsersLazyQueryHookResult) => {
      if (data) {
        if (data.users.nodes.length == 0) {
          setErrorMessage('User Not Found');
          Alert.alert('User Not Found');
        } else {
          setUserData(data);
          setNumberOfUsers(data.users.nodes.length);
          //showUsers(data, Number(numberOfUsers));
          addFriend(Number(data.users.nodes[0].id));
        }
      }
    },
    [addFriend],
  );

  const [loadUsers] = useUsersLazyQuery({
    onCompleted: getFriendId,
    onError: _onLoadUserError,
  });

  const handleSubmitForm = React.useCallback(
    (values: FormValues, helpers: FormikHelpers<FormValues>) => {
      console.log('Submitted');
      loadUsers({
        variables: {
          where: { email: values.email },
        },
      });
      values.email = '';
    },
    [loadUsers],
  );

  return (
    <Modal
      visible={showAddFriendEmailPage}
      animationType="slide"
      transparent={true}>
      <SafeAreaView>
        <View style={styles.container}>
          <View style={styles.searchTopContainer}>
              <Formik
                initialValues={initialValues}
                onSubmit={handleSubmitForm}
                validationSchema={validationSchema}>
                {({ handleChange, handleBlur, handleSubmit, values }) => (
                  <View style={styles.searchFieldContainer}>
                    <View style={styles.form}>
                      <FieldInput
                        handleChange={handleChange}
                        handleBlur={handleBlur}
                        value={values.email}
                        fieldType="email"
                      />
                      <ErrorMessage
                        name="email"
                        render={msg => (
                          <Text style={styles.errorText}>{msg}</Text>
                        )}
                      />
                    </View>
                    <View style={styles.buttonContainer}>
                      <Button
                        rounded
                        style={styles.button}
                        onPress={handleSubmit}>
                        <Text style={styles.text}>Add Friend </Text>
                      </Button>
                    </View>
                  </View>
                )}
              </Formik>
            </View>
            {showUsers(userData, Number(numberOfUsers))}
          </View>
        </View>
      </SafeAreaView>
    </Modal>
  );
};

更新代码：

export const Page: React.FunctionComponent<PageProps> = ({
  toggleShowPage,
  showAddFriendEmailPage,
}) => {
  const initialValues: FormValues = {
    email: '',
  };

  const [errorMessage, setErrorMessage] = useState('');
  const [isSubmitted, setIsSubmitted] = useState(false);
  const [userData, setUserData] = useState<UsersLazyQueryHookResult>('');
  const [numberOfUsers, setNumberOfUsers] = useState('');

  const validationSchema = emailValidationSchema;

   const showUsers = React.useCallback(
     (data: UsersLazyQueryHookResult, numberOfUsers: Number) => {
      if (data){
       for (var i = 0; i < numberOfUsers; i++) {
         console.log('Number of Users in Loop: ', numberOfUsers);
         const userId = data.users.nodes[i].id;
         const userName = data.users.nodes[i].firstName
           .concat(' ')
           .concat(data.users.nodes[i].lastName);
         return (
           <View style={styles.friends}>
             <View style={styles.item}>
               <Text style={styles.userName}>{userName}</Text>
               <View style={styles.addButtonContainer}>
                 <Button
                   onPress={() => {
                     addFriend(Number(data.users.nodes[i].id));
                     setIsSubmitted(false);
                   }}>
                 </Button>
               </View>
             </View>
           </View>
         );
       }}
     },
     [createUserRelationMutation, userData, numberOfUsers],
   );

  const addFriend = React.useCallback(
    (id: Number) => {
      console.log('Whats the Id', id);
      createUserRelationMutation({
        variables: {
          input: { relatedUserId: id, type: RelationType.Friend, userId: 7 },
        },
      });
    },
    [createUserRelationMutation],
  );

  const getFriendId = React.useCallback(
    (data: UsersLazyQueryHookResult) => {
      if (data) {
        if (data.users.nodes.length == 0) {
          setErrorMessage('User Not Found');
          Alert.alert('User Not Found');
        } else {
          setUserData(data);
          setNumberOfUsers(data.users.nodes.length);
        }
      }
    },
    [addFriend],
  );



  const [loadUsers] = useUsersLazyQuery({
    onCompleted: getFriendId,
    onError: _onLoadUserError,
  });

  const handleSubmitForm = React.useCallback(

    (values: FormValues, helpers: FormikHelpers<FormValues>) => {
      setIsSubmitted(true);
      console.log('Submitted');
      loadUsers({
        variables: {
          where: { email: values.email },
        },
      });
      values.email = '';
    },
    [loadUsers],
  );


  if (!addingFriendLoading && isMutationCalled) {
    if (addingFriendError) {
      setErrorMessage(addingFriendError.message);
      Alert.alert('Unable to Add Friend');
    }
  }

  return (
    <Modal
      visible={showAddFriendEmailPage}
      animationType="slide"
      transparent={true}>
      <SafeAreaView>
        <View style={styles.container}>
          <View style={styles.searchTopContainer}>
            <View style={styles.searchTopTextContainer}>
              <Text
                style={styles.searchCancelDoneText}
                onPress={() => {
                 toggleShowPage()
                  setIsSubmitted(false);
                  setUserData(null);
              }}
              >
                Cancel
              </Text>
              <Text style={styles.searchTopMiddleText}>
                Add Friend by Email
              </Text>
              <Text style={styles.searchCancelDoneText}>Done</Text>
            </View>
            <View>
              <Formik
                initialValues={initialValues}
                onSubmit={handleSubmitForm}
                validationSchema={validationSchema}>
                {({ handleChange, handleBlur, handleSubmit, values }) => (
                  <View style={styles.searchFieldContainer}>
                    <View style={styles.form}>
                      <FieldInput
                        handleChange={handleChange}
                        handleBlur={handleBlur}
                        value={values.email}
                        fieldType="email"
                      />
                      <ErrorMessage
                        name="email"
                        render={msg => (
                          <Text style={styles.errorText}>{msg}</Text>
                        )}
                      />
                    </View>
                    <View style={styles.buttonContainer}>
                      <Button
                        rounded
                        style={styles.button}
                        onPress={handleSubmit}>
                        <Text style={styles.text}>Search </Text>
                      </Button>
                    </View>
                  </View>
                )}
              </Formik>
            </View>
            {isSubmitted && showUsers(userData, Number(numberOfUsers))}    
          </View>
        </View>
      </SafeAreaView>
    </Modal>
  );
};

Answer 1

您还从 getFriendId 调用 setUserData，因此它将更新 userData

const getFriendId = React.useCallback(
    ....
    setUserData(data);

因此它将调用 render 和 useEffect

useEffect(() => {
    setUserData(userData);
    setNumberOfUsers(numberOfUsers);
}, [userData, numberOfUsers]);

你可以注释掉useEffect并勾选console.log

在循环内渲染和 return

Render and return inside a loop

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render

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