React Native 的基本级联形式

我想分享我处理级联表单字段的 3 种方法。

1. 第一种方法是通用的，使用状态变量。
2. 第二种是使用普通变量和一个布尔状态变量来触发状态效果（刷新页面）。
3. 第三种是带有普通变量的动态表单字段。

在这篇文章中，我们看到第一种方法，即基于国家、州、城市数据处理级联表单字段的常用方法。

套餐

react-native-element-dropdown
react-native-paper

我们正在使用react-native-element-dropdown作为下拉字段

基页

import React, { useState, useEffect } from "react";
import { View, Text, StyleSheet, TouchableOpacity } from "react-native";
import { Dropdown } from "react-native-element-dropdown";

export default function App() {
  return (
    <view>



<p>ZDropDown is a custom component. </p>

<h3>
  
  
  Sample data
</h3>



<pre class="brush:php;toolbar:false">const listCountry = [
  { countryId: "1", name: "india" },
  { countryId: "2", name: "uk" },
  { countryId: "3", name: "canada" },
  { countryId: "4", name: "us" },
];

const listSate = [
  { stateId: "1", countryId: "1", name: "state1_india" },
  { stateId: "4", countryId: "2", name: "state1_uk" },
  { stateId: "7", countryId: "3", name: "state1_canada" },
  { stateId: "10", countryId: "4", name: "state1_us" },
];

const listCity = [
  {
    cityId: "1",
    stateId: "1",
    countryId: "1",
    name: "city1_state1_country1",
  },
   {
    cityId: "6",
    stateId: "2",
    countryId: "1",
    name: "city6_state2_country1",
  },
  {
    cityId: "7",
    stateId: "3",
    countryId: "1",
    name: "city7_state3_country1",
  }, 
  {
    cityId: "23",
    stateId: "8",
    countryId: "3",
    name: "city23_state8_country3",
  }, 
  {
    cityId: "30",
    stateId: "10",
    countryId: "4",
    name: "city30_state10_country4",
  }, 
  {
    cityId: "35",
    stateId: "12",
    countryId: "4",
    name: "city35_state12_country4",
  },
  {
    cityId: "36",
    stateId: "12",
    countryId: "4",
    name: "city36_state12_country4",
  },
];

表单变量

我们使用 4 个状态变量，其中 3 个用于表单字段，其余一个用于触发焦点效果。

export default function App() {
  const [country, setCountry] = useState({
    data: [],
    selectedCountry: {},
    value: null,
  });
  const [state, setState] = useState({
    data: [],
    selectedState: {},
    value: null,
  });
  const [city, setCity] = useState({ data: [], selectedCity: {}, value: null });

  const [ddfocus, setDdfocus] = useState({
    country: false,
    state: false,
    city: false,
  });

  return (
    <view>



<p>Focus and Blur events get triggered more than the onChange event so for focus changes, here a separate state variable is used to not to mess up with drop down data changes. </p>

<h3>
  
  
  Load Country
</h3>

<p>Load country dropdown from the sample data. (you can use api call)<br>
</p>

<pre class="brush:php;toolbar:false">export default function App() {
. . .
  const loadCountry = () => {
    // load data from api call
    setCountry({ data: [...listCountry], selectedCountry: {}, value: null });
  };

  useEffect(() => {
    loadCountry();
  }, []);

return (
. . .
)

对焦/模糊

当选择一个下拉菜单时，该字段必须被聚焦，而其余字段应该被模糊。我们正在使用一个函数来处理这个问题。

  const focusField = (fld = '') => {
    const obj = { country: false, state: false, city: false };
    if (fld) obj[fld] = true;
    setDdfocus(obj);
  };

        <text>Country</text>
        <zdropdown . isfocus="{ddfocus.country}" onfocus="{()"> focusField('country')}
          onBlur={() => focusField('')}
          onChange={null}
        />
        <text>State</text>
        <zdropdown . isfocus="{ddfocus.state}" onfocus="{()"> focusField('state')}
          onBlur={() => focusField('')}
          onChange={null}
        />
        <text>City</text>
        <zdropdown . isfocus="{ddfocus.city}" onfocus="{()"> focusField('city')}
          onBlur={() => focusField('')}
          onChange={null}       
        />
</zdropdown></zdropdown></zdropdown>

我们现在已经完成了一半。

加载状态状态

在选择的国家/地区，我们需要根据所选国家/地区加载相应的州 STATES。

更新国家/地区字段，根据国家/地区选择加载州并关注国家/地区。

  <text>Country</text>
  <zdropdown . onchange="{(item)"> {
      setCountry({
        ...country,
        selectedCountry: item,
        value: item.countryId,
      });
      loadState(item.countryId);
      focusField("");
    }}
  />
</zdropdown>

当国家/地区发生变化时，州和城市都会发生变化。因此，在设置新值之前，我们需要清除现有数据。

const loadState = async (cntId) => {
  // load data from api call
  setState({ data: [], selectedState: {}, value: null });
  setCity({ data: [], selectedCity: {}, value: null });
  const arr = listSate.filter((ele) => ele.countryId === cntId);
  setState({ ...state, data: [...arr] });
  console.log("respective states ", arr);
};

负载城市

并根据选择加载城市字段。

    <text>State</text>
    <zdropdown . onchange="{(item)"> {
        setState({ ...state, selectedState: item, value: item.stateId });
        loadCity(item.stateId);
        focusField("");
      }}
     />
</zdropdown>

const loadCity = async (stId) => {
  // load data from api call
  setCity({ data: [], selectedCity: {}, value: null });
  const arr = listCity.filter((ele) => ele.stateId === stId);
  setCity({ ...city, data: [...arr] });
};

一切就绪，表单字段现在可以正常工作了。

如果我们再处理 2 个附加功能，我们就完成了。一种是休息页面，另一种是验证表单并显示警告。

重置页面

表单变量和焦点变量应该被清除。

. . .
const resetForm = () => {
  focusField("");
  setCountry({ data: [...listCountry], selectedCountry: {}, value: null });
  setState({ data: [], selectedState: {}, value: null });
  setCity({ data: [], selectedCity: {}, value: null });
};
. . .

  <touchableopacity onpress="{()"> resetForm()}>



<h3>
  
  
  Warning
</h3>

<p>We have to show a warning msg if the parent field value is null. For that we are using SnackBar component from paper.<br>
</p>

<pre class="brush:php;toolbar:false">import { Snackbar } from "react-native-paper";

export default function App() {
  . . .
  const [visible, setVisible] = useState(false);
  const [snackMsg, setSnackMsg] = useState("");

  const onToggleSnackBar = () => setVisible(!visible);
  const onDismissSnackBar = () => setVisible(false);
  . . .

  return (
    <view>



<p>Since State and City fields have parent field, they have to be validated.<br>
</p>

<pre class="brush:php;toolbar:false">        <text>State</text>
        <zdropdown onfocus="{()"> {
            focusField('state');
            if (!country.value) {
              setSnackMsg('Select country');
              onToggleSnackBar();
              focusField('country');
            }
          }}
          . . .
        />
        <text>City</text>
        <zdropdown onfocus="{()"> {
            focusField('city');
            if (!country.value) {
              setSnackMsg('Select country');
              onToggleSnackBar();
              focusField('country');
            } else if (!state.value) {
              setSnackMsg('Select state');
              onToggleSnackBar();
              focusField('state');
            }
          }}
          . . .
        />
</zdropdown></zdropdown>

是的，就是这样！我们完成了。谢谢。

完整代码参考这里

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