提取api时未调用useEffect且未更新状态

问题描述

我正在使用useEffect挂钩从天气api获取数据，并正确声明依赖项.我的状态仍然没有更新，因此我的渲染函数出现错误.我已经做了很多尝试，从摆脱依赖关系数组到在依赖关系数组中声明倍数.我不知道我的功能出了什么问题.API的JSON响应采用以下格式:

I'm fetching data from a weather api using useEffect hook and declaring the dependency correctly as well. My state is still not being updated and I get errors in my render function because of that. I've pretty much tried everything from getting rid of the dependency array to declaring multiples in the dependency array. I don't know what's wrong with my function. The API's JSON response is in this format:

{
 location: {
 name: "Paris",
 region: "Ile-de-France",
 },
 current: {
  last_updated_epoch: 1564279222,
  last_updated: "2019-07-28 04:00",
  temp_c: 16,
  temp_f: 60.8,
  is_day: 0,
  condition: {
    text: "Clear",
    icon: "//cdn.apixu.com/weather/64x64/night/113.png",
    code: 1000
  },
  wind_mph: 6.9,
  wind_kph: 11.2
 }
}

这就是我的代码:

const Weather = ({ capital }) => {
  const [weather, setWeather] = useState(null);

  useEffect(() => {
    console.log("useEffect called");
    const getWeather = async () => {
      try {
        const res = await axios.get(
          `http://api.apixu.com/v1/current.json?key=53d601eb03d1412c9c004840192807&q=${capital}`
        );
        setWeather(res.data);
      } catch (e) {
        console.log(e);
      }
    };
    getWeather();
  }, [capital]);
  console.log(weather);

  return (
    <Card style={{ width: "18rem", marginTop: "25px" }}>
      <Card.Img variant="top" src={weather.current.condition.icon} />

      <Card.Header style={{ textAlign: "center", fontSize: "25px" }}>
        Weather in {capital}
      </Card.Header>
    </Card>
  )
}

我希望能够显示该图标的图像，但是我在渲染函数中收到此错误消息:

I expect to get to be shown image of the icon but I get this error message in my render function:

TypeError: Cannot read property 'current' of null
Weather
src/components/Weather.js:26
  23 | 
  24 | return (
  25 |   <Card style={{ width: "18rem", marginTop: "25px" }}>
  26 |     <Card.Img variant="top" src={weather.current.condition.icon} />
     | ^  27 | 
  28 |     <Card.Header style={{ textAlign: "center", fontSize: "25px" }}>
  29 |       Weather in {capital}

和我的 console.log(天气)返回null，即使在useEffect()和 console.log(称为useEffect被调用)之后被调用，其原始状态也不会返回null记录所有日志，这意味着未调用useEffect.

and my console.log(weather) return null, the original state even though its being called after useEffect() and console.log(useEffect called) does not log at all which mean useEffect is not being called.

推荐答案

错误消息将其释放，无法读取null的'current'属性，这是 current唯一的位置被称为 Card.Img 的 src 中的 weather.current 中，因此我们推断出 weather 在渲染过程中为 null .

The error message gives it away, Cannot read property 'current' of null, the only place where current is called is in weather.current in the src of Card.Img, so we deduce that weather was null during the render.

发生这种情况的原因是因为api调用是异步的，它不会立即填充状态，因此渲染首先发生，并尝试从初始天气状态读取 .current .空.

The reason this happens is because the api call is asynchronus, it doesn't populate the state immediately, so the render happens first and tries to read .current from the initial weather state null.

解决方案:在您的 render 方法中，请确保当 weather 为 null 时，不要读取 weather.current

Solution: in your render method, make sure not to read weather.current while weather is null.

例如，您可以使用 {weather&&< Card> ...</Card} 隐藏整个卡片，直到数据加载完毕并显示加载指示符为止，或者您可以使用 src = {weather&&weather.current.condition.icon} 作为一种快速的解决方法.

You can for example use {weather && <Card>...</Card} to hide the whole card until the data is loaded and show a loading indicator, or you can use src={weather && weather.current.condition.icon} as a quick workaround.

const Weather = ({capital}) => {
  const [weather, setWeather] = useState(null);

  useEffect(() => {
    console.log("useEffect called");
    const getWeather = async () => {
      try {
        const res = await axios.get(
          `http://api.apixu.com/v1/current.json?key=53d601eb03d1412c9c004840192807&q=${capital}`,
        );
        setWeather(res.data);
      } catch (e) {
        console.log(e);
      }
    };
    getWeather();
  }, [capital]);
  console.log(weather);

  return (
    <Card style={{width: "18rem", marginTop: "25px"}}>
      <Card.Img variant="top" src={weather && weather.current.condition.icon} />

      <Card.Header style={{textAlign: "center", fontSize: "25px"}}>
        Weather in {capital}
      </Card.Header>
    </Card>
  );
};

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