fetchData

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In this tutorial, I want to show you how to fetch data in React with Hooks by using the state and effect hooks. We will use the widely known Hacker News API to fetch popular articles from the tech world. You will also implement your custom hook for the data fetching that can be reused anywhere in your application or published on npm as standalone node package.
在本教程中，我想向您展示如何使用状态和效果挂钩在React with Hooks中获取数据。我们将使用广为人知的Hacker News API从科技界获取热门文章。您还将实现用于数据获取的自定义钩子，该钩子可在应用程序中的任何位置重用或作为独立的节点程序包发布在npm上。

If you don't know anything about this new React feature, checkout this introduction to React Hooks. If you want to checkout the finished project for the showcased examples that show how to fetch data in React with Hooks, checkout this GitHub repository.
如果您对这个新的React功能一无所知，请查看此React Hooks简介。如果您想签出完成的项目以获取展示如何使用Hooks在React中获取数据的展示示例，请签出该GitHub存储库。

If you just want to have a ready to go React Hook for data fetching: npm install use-data-api and follow the documentation. Don't forget to star it if you use it 😃

Note: In the future, React Hooks are not be intended for data fetching in React. Instead, a feature called Suspense will be in charge for it. The following walkthrough is nonetheless a great way to learn more about state and effect hooks in React.

DATA FETCHING WITH REACT HOOKS

If you are not familiar with data fetching in React, checkout my extensive data fetching in React article. It walks you through data fetching with React class components, how it can be made reusable with Render Prop Components and Higher-Order Components, and how it deals with error handling and loading spinners. In this article, I want to show you all of it with React Hooks in function components.

import React, { useState } from 'react';
function App() {
  const [data, setData] = useState({ hits: [] });
  return (
    <ul>
      {data.hits.map(item => (
        <li key={item.objectID}>
          <a href={item.url}>{item.title}</a>
        </li>
      ))}
    </ul>
  );
}

export default App;

The App component shows a list of items (hits = Hacker News articles). The state and state update function come from the state hook called useState that is responsible to manage the local state for the data that we are going to fetch for the App component. The initial state is an empty list of hits in an object that represents the data. No one is setting any state for this data yet.

We are going to use axios to fetch data, but it is up to you to use another data fetching library or the native fetch API of the browser. If you haven't installed axios yet, you can do so by on the command line with npm install axios. Then implement your effect hook for the data fetching:

import React, { useState, useEffect } from 'react';
import axios from 'axios';
function App() {
  const [data, setData] = useState({ hits: [] });
  useEffect(async () => {
    const result = await axios(
      'https://hn.algolia.com/api/v1/search?query=redux',
    );
    setData(result.data);
  });
  return (
    <ul>
      {data.hits.map(item => (
        <li key={item.objectID}>
          <a href={item.url}>{item.title}</a>
        </li>
      ))}
    </ul>
  );
}
export default App;

The effect hook called useEffect is used to fetch the data with axios from the API and to set the data in the local state of the component with the state hook's update function. The promise resolving happens with async/await.

However, when you run your application, you should stumble into a nasty loop. The effect hook runs when the component mounts but also when the component updates. Because we are setting the state after every data fetch, the component updates and the effect runs again. It fetches the data again and again. That's a bug and needs to be avoided. We only want to fetch data when the component mounts. That's why you can provide an empty array as second argument to the effect hook to avoid activating it on component updates but only for the mounting of the component.

import React, { useState, useEffect } from 'react';
import axios from 'axios';
function App() {
  const [data, setData] = useState({ hits: [] });
  useEffect(async () => {
    const result = await axios(
      'https://hn.algolia.com/api/v1/search?query=redux',
    );
    setData(result.data);
  }, []);
  return (
    <ul>
      {data.hits.map(item => (
        <li key={item.objectID}>
          <a href={item.url}>{item.title}</a>
        </li>
      ))}
    </ul>
  );
}
export default App;

The second argument can be used to define all the variables (allocated in this array) on which the hook depends. If one of the variables changes, the hook runs again. If the array with the variables is empty, the hook doesn't run when updating the component at all, because it doesn't have to watch any variables.

There is one last catch. In the code, we are using async/await to fetch data from a third-party API. According to the documentation every function annotated with async returns an implicit promise: "The async function declaration defines an asynchronous function, which returns an AsyncFunction object. An asynchronous function is a function which operates asynchronously via the event loop, using an implicit Promise to return its result. ". However, an effect hook should return nothing or a clean up function. That's why you may see the following warning in your developer console log:

07:41:22.910 index.js:1452 Warning: useEffect function must return a cleanup function or nothing. Promises and useEffect(async () => ...) are not supported, but you can call an async function inside an effect.. That's why using async directly in the useEffect function isn't allowed. Let's implement a workaround for it, by using the async function inside the effect.

import React, { useState, useEffect } from 'react';
import axios from 'axios';
function App() {
  const [data, setData] = useState({ hits: [] });
  useEffect(() => {
    const fetchData = async () => {
      const result = await axios(
        'https://hn.algolia.com/api/v1/search?query=redux',
      );
      setData(result.data);
    };
    fetchData();
  }, []);
  return (
    <ul>
      {data.hits.map(item => (
        <li key={item.objectID}>
          <a href={item.url}>{item.title}</a>
        </li>
      ))}
    </ul>
  );
}
export default App;

That's data fetching with React hooks in a nutshell. But continue reading if you are interested about error handling, loading indicators, how to trigger the data fetching from a form, and how to implement a reusable data fetching hook.

HOW TO TRIGGER A HOOK PROGRAMMATICALLY / MANUALLY

Great, we are fetching data once the component mounts. But what about using an input field to tell the API in which topic we are interested in? "Redux" is taken as default query. But what about topics about "React"? Let's implement an input element to enable someone to fetch other stories than "Redux" stories. Therefore, introduce a new state for the input element.

import React, { Fragment, useState, useEffect } from 'react';
import axios from 'axios';
function App() {
  const [data, setData] = useState({ hits: [] });
  const [query, setQuery] = useState('redux');
  useEffect(() => {
    const fetchData = async () => {
      const result = await axios(
        'https://hn.algolia.com/api/v1/search?query=redux',
      );
      setData(result.data);
    };
    fetchData();
  }, []);
  return (
    <Fragment>
      <input
        type="text"
        value={query}
        onChange={event => setQuery(event.target.value)}
      />
      <ul>
        {data.hits.map(item => (
          <li key={item.objectID}>
            <a href={item.url}>{item.title}</a>
          </li>
        ))}
      </ul>
    </Fragment>
  );
}
export default App;

At the moment, both states are independent from each other, but now you want to couple them to only fetch articles that are specified by the query in the input field. With the following change, the component should fetch all articles by query term once it mounted.
目前，这两种状态彼此独立，但是现在您希望将它们耦合以仅获取输入字段中查询指定的文章。进行以下更改后，该组件应在安装后按查询字词获取所有文章。

function App() {
  const [data, setData] = useState({ hits: [] });
  const [query, setQuery] = useState('redux');
  useEffect(() => {
    const fetchData = async () => {
      const result = await axios(
        `http://hn.algolia.com/api/v1/search?query=${query}`,
      );
      setData(result.data);
    };
    fetchData();
  }, []);
  return (
    ...
  );
}
export default App;

One piece is missing: When you try to type something into the input field, there is no other data fetching after the mounting triggered from the effect. That's because you have provided the empty array as second argument to the effect. The effect depends on no variables, so it is only triggered when the component mounts. However, now the effect should depend on the query. Once the query changes, the data request should fire again.

function App() {
  const [data, setData] = useState({ hits: [] });
  const [query, setQuery] = useState('redux');
  useEffect(() => {
    const fetchData = async () => {
      const result = await axios(
        `http://hn.algolia.com/api/v1/search?query=${query}`,
      );
      setData(result.data);
    };
    fetchData();
  }, [query]);
  return (
    ...
  );
}
export default App;

The refetching of the data should work once you change the value in the input field. But that opens up another problem: On every character you type into the input field, the effect is triggered and executes another data fetching request. How about providing a button that triggers the request and therefore the hook manually?

function App() {
  const [data, setData] = useState({ hits: [] });
  const [query, setQuery] = useState('redux');
  const [search, setSearch] = useState('');
  useEffect(() => {
    const fetchData = async () => {
      const result = await axios(
        `http://hn.algolia.com/api/v1/search?query=${query}`,
      );
      setData(result.data);
    };
    fetchData();
  }, [query]);
  return (
    <Fragment>
      <input
        type="text"
        value={query}
        onChange={event => setQuery(event.target.value)}
      />
      <button type="button" onClick={() => setSearch(query)}>
        Search
      </button>
      <ul>
        {data.hits.map(item => (
          <li key={item.objectID}>
            <a href={item.url}>{item.title}</a>
          </li>
        ))}
      </ul>
    </Fragment>
  );
}

Now, make the effect dependant on the search state rather than the fluctuant query state that changes with every key stroke in the input field. Once the user clicks the button, the new search state is set and should trigger the effect hook kinda manually.

function App() {
  const [data, setData] = useState({ hits: [] });
  const [query, setQuery] = useState('redux');
  const [search, setSearch] = useState('redux');
  useEffect(() => {
    const fetchData = async () => {
      const result = await axios(
        `http://hn.algolia.com/api/v1/search?query=${search}`,
      );
      setData(result.data);
    };
    fetchData();
  }, [search]);
  return (
    ...
  );
}
export default App;

Also the initial state of the search state is set to the same state as the query state, because the component fetches data also on mount and therefore the result should mirror the value in the input field. However, having a similar query and search state is kinda confusing. Why not set the actual URL as state instead of the search state?

function App() {
  const [data, setData] = useState({ hits: [] });
  const [query, setQuery] = useState('redux');
  const [url, setUrl] = useState(
    'https://hn.algolia.com/api/v1/search?query=redux',
  );
  useEffect(() => {
    const fetchData = async () => {
      const result = await axios(url);
      setData(result.data);
    };
    fetchData();
  }, [url]);
  return (
    <Fragment>
      <input
        type="text"
        value={query}
        onChange={event => setQuery(event.target.value)}
      />
      <button
        type="button"
        onClick={() =>
          setUrl(`http://hn.algolia.com/api/v1/search?query=${query}`)
        }
      >
        Search
      </button>
      <ul>
        {data.hits.map(item => (
          <li key={item.objectID}>
            <a href={item.url}>{item.title}</a>
          </li>
        ))}
      </ul>
    </Fragment>
  );
}

That's if for the implicit programmatic data fetching with the effect hook. You can decide on which state the effect depends. Once you set this state on a click or in another side-effect, this effect will run again. In this case, if the URL state changes, the effect runs again to fetch stories from the API.
如果是使用效果钩子进行隐式编程数据获取的话。您可以决定效果取决于哪个状态。在单击或其他副作用上设置此状态后，该效果将再次运行。在这种情况下，如果URL状态发生更改，则效果将再次运行以从API提取故事。

LOADING INDICATOR WITH REACT HOOKS

Let's introduce a loading indicator to the data fetching. It's just another state that is manage by a state hook. The loading flag is used to render a loading indicator in the App component.
让我们为数据获取引入一个加载指示器。这只是由状态挂钩管理的另一个状态。加载标志用于在App组件中呈现加载指示器。

import React, { Fragment, useState, useEffect } from 'react';
import axios from 'axios';
function App() {
  const [data, setData] = useState({ hits: [] });
  const [query, setQuery] = useState('redux');
  const [url, setUrl] = useState(
    'https://hn.algolia.com/api/v1/search?query=redux',
  );
  const [isLoading, setIsLoading] = useState(false);
  useEffect(() => {
    const fetchData = async () => {
      setIsLoading(true);
      const result = await axios(url);
      setData(result.data);
      setIsLoading(false);
    };
    fetchData();
  }, [url]);
  return (
    <Fragment>
      <input
        type="text"
        value={query}
        onChange={event => setQuery(event.target.value)}
      />
      <button
        type="button"
        onClick={() =>
          setUrl(`http://hn.algolia.com/api/v1/search?query=${query}`)
        }
      >
        Search
      </button>
      {isLoading ? (
        <div>Loading ...</div>
      ) : (
        <ul>
          {data.hits.map(item => (
            <li key={item.objectID}>
              <a href={item.url}>{item.title}</a>
            </li>
          ))}
        </ul>
      )}
    </Fragment>
  );
}
export default App;

Once the effect is called for data fetching, which happens when the component mounts or the URL state changes, the loading state is set to true. Once the request resolves, the loading state is set to false again.
一旦调用了效果以进行数据获取（在组件安装或URL状态更改时发生），则加载状态将设置为true。请求解决后，加载状态将再次设置为false

ERROR HANDLING WITH REACT HOOKS

What about error handling for data fetching with a React hook? The error is just another state initialized with a state hook. Once there is an error state, the App component can render feedback for the user. When using async/await, it is common to use try/catch blocks for error handling. You can do it within the effect:
使用React挂钩获取数据的错误处理怎么样？该错误只是使用状态挂钩初始化的另一个状态。一旦出现错误状态，App组件即可为用户提供反馈。使用异步/等待时，通常使用try /catch块进行错误处理。您可以在效果范围内做到这一点

import React, { Fragment, useState, useEffect } from 'react';
import axios from 'axios';
function App() {
  const [data, setData] = useState({ hits: [] });
  const [query, setQuery] = useState('redux');
  const [url, setUrl] = useState(
    'https://hn.algolia.com/api/v1/search?query=redux',
  );
  const [isLoading, setIsLoading] = useState(false);
  const [isError, setIsError] = useState(false);
  useEffect(() => {
    const fetchData = async () => {
      setIsError(false);
      setIsLoading(true);
      try {
        const result = await axios(url);
        setData(result.data);
      } catch (error) {
        setIsError(true);
      }
      setIsLoading(false);
    };
    fetchData();
  }, [url]);
  return (
    <Fragment>
      <input
        type="text"
        value={query}
        onChange={event => setQuery(event.target.value)}
      />
      <button
        type="button"
        onClick={() =>
          setUrl(`http://hn.algolia.com/api/v1/search?query=${query}`)
        }
      >
        Search
      </button>
      {isError && <div>Something went wrong ...</div>}
      {isLoading ? (
        <div>Loading ...</div>
      ) : (
        <ul>
          {data.hits.map(item => (
            <li key={item.objectID}>
              <a href={item.url}>{item.title}</a>
            </li>
          ))}
        </ul>
      )}
    </Fragment>
  );
}
export default App;

The error state is reset every time the hook runs again. That's useful because after a failed request the user may want to try it again which should reset the error. In order to enforce an error yourself, you can alter the URL into something invalid. Then check whether the error message shows up.
每次挂钩再次运行时，都会重置错误状态。这很有用，因为在失败的请求之后，用户可能想要再次尝试，这将重置错误。为了自己执行错误，您可以将URL更改为无效的内容。然后检查是否显示错误消息。

FETCHING DATA WITH FORMS AND REACT

What about a proper form to fetch data? So far, we have only a combination of input field and button. Once you introduce more input elements, you may want to wrap them with a form element. In addition, a form makes it possible to trigger the button with "Enter" on the keyboard too.
适当的形式来获取数据呢？到目前为止，我们只有输入字段和按钮的组合。引入更多输入元素后，您可能需要用form元素包装它们。此外，还可以通过表格使用键盘上的“ Enter”来触发按钮。

function App() {
  ...
  return (
    <Fragment>
      <form
        onSubmit={() =>
          setUrl(`http://hn.algolia.com/api/v1/search?query=${query}`)
        }
      >
        <input
          type="text"
          value={query}
          onChange={event => setQuery(event.target.value)}
        />
        <button type="submit">Search</button>
      </form>
      {isError && <div>Something went wrong ...</div>}
      ...
    </Fragment>
  );
}

But now the browser reloads when clicking the submit button, because that's the native behavior of the browser when submitting a form. In order to prevent the default behavior, we can invoke a function on the React event. That's how you do it in React class components too.
但是现在，单击“提交”按钮时浏览器会重新加载，因为这是浏览器提交表单时的本机行为。为了防止默认行为，我们可以在React事件上调用一个函数。这也是您在React类组件中执行此操作的方式。

function App() {
  ...
  return (
    <Fragment>
      <form onSubmit={event => {
        setUrl(`http://hn.algolia.com/api/v1/search?query=${query}`);
        event.preventDefault();
      }}>
        <input
          type="text"
          value={query}
          onChange={event => setQuery(event.target.value)}
        />
        <button type="submit">Search</button>
      </form>
      {isError && <div>Something went wrong ...</div>}
      ...
    </Fragment>
  );
}

Now the browser shouldn't reload anymore when you click the submit button. It works as before, but this time with a form instead of the naive input field and button combination. You can press the "Enter" key on your keyboard too.
现在，单击“提交”按钮后，浏览器不再需要重新加载。它像以前一样工作，但是这次使用的是表单而不是朴素的输入字段和按钮组合。您也可以按键盘上的“ Enter”键

CUSTOM DATA FETCHING HOOK

In order to extract a custom hook for data fetching, move everything that belongs to the data fetching, except for the query state that belongs to the input field, but including the loading indicator and error handling, to its own function. Also make sure you return all the necessary variables from the function that are used in the App component.
为了提取用于数据获取的自定义钩子，请将属于数据获取的所有内容（属于输入字段的查询状态（但包括加载指示符和错误处理））移至其自身的功能。另外，请确保您从App组件中使用的函数返回所有必需的变量

const useHackerNewsApi = () => {
  const [data, setData] = useState({ hits: [] });
  const [url, setUrl] = useState(
    'https://hn.algolia.com/api/v1/search?query=redux',
  );
  const [isLoading, setIsLoading] = useState(false);
  const [isError, setIsError] = useState(false);
  useEffect(() => {
    const fetchData = async () => {
      setIsError(false);
      setIsLoading(true);
      try {
        const result = await axios(url);
        setData(result.data);
      } catch (error) {
        setIsError(true);
      }
      setIsLoading(false);
    };
    fetchData();
  }, [url]);
  return [{ data, isLoading, isError }, setUrl];
}

Now, your new hook can be used in the App component again:

function App() {
  const [query, setQuery] = useState('redux');
  const [{ data, isLoading, isError }, doFetch] = useHackerNewsApi();
  return (
    <Fragment>
      <form onSubmit={event => {
        doFetch(`http://hn.algolia.com/api/v1/search?query=${query}`);
        event.preventDefault();
      }}>
        <input
          type="text"
          value={query}
          onChange={event => setQuery(event.target.value)}
        />
        <button type="submit">Search</button>
      </form>
      ...
    </Fragment>
  );
}

The initial state can be made generic too. Pass it simply to the new custom hook:
初始状态也可以设为通用。将其简单地传递到新的自定义钩子

import React, { Fragment, useState, useEffect } from 'react';
import axios from 'axios';
const useDataApi = (initialUrl, initialData) => {
  const [data, setData] = useState(initialData);
  const [url, setUrl] = useState(initialUrl);
  const [isLoading, setIsLoading] = useState(false);
  const [isError, setIsError] = useState(false);
  useEffect(() => {
    const fetchData = async () => {
      setIsError(false);
      setIsLoading(true);
      try {
        const result = await axios(url);
        setData(result.data);
      } catch (error) {
        setIsError(true);
      }
      setIsLoading(false);
    };
    fetchData();
  }, [url]);
  return [{ data, isLoading, isError }, setUrl];
};
function App() {
  const [query, setQuery] = useState('redux');
  const [{ data, isLoading, isError }, doFetch] = useDataApi(
    'https://hn.algolia.com/api/v1/search?query=redux',
    { hits: [] },
  );
  return (
    <Fragment>
      <form
        onSubmit={event => {
          doFetch(
            `http://hn.algolia.com/api/v1/search?query=${query}`,
          );
          event.preventDefault();
        }}
      >
        <input
          type="text"
          value={query}
          onChange={event => setQuery(event.target.value)}
        />
        <button type="submit">Search</button>
      </form>
      {isError && <div>Something went wrong ...</div>}
      {isLoading ? (
        <div>Loading ...</div>
      ) : (
        <ul>
          {data.hits.map(item => (
            <li key={item.objectID}>
              <a href={item.url}>{item.title}</a>
            </li>
          ))}
        </ul>
      )}
    </Fragment>
  );
}
export default App;

That's it for the data fetching with a custom hook. The hook itself doesn't know anything about the API. It receives all parameters from the outside and only manages necessary states such as the data, loading and error state. It executes the request and returns the data to the component using it as custom data fetching hook.
使用自定义钩子获取数据即已完成。挂钩本身对API一无所知。它从外部接收所有参数，并且仅管理必要的状态，例如数据，加载和错误状态。它执行请求并将数据用作自定义数据获取挂钩，将数据返回给组件

REDUCER HOOK FOR DATA FETCHING

So far, we have used various state hooks to manage our data fetching state for the data, loading and error state. However, somehow all these states, managed with their own state hook, belong together because they care about the same cause. As you can see, they are all used within the data fetching function. A good indicator that they belong together is that they are used one after another (e.g. setIsError, setIsLoading). Let's combine all three of them with a Reducer Hook instead.
到目前为止，我们已经使用各种状态挂钩来管理数据的获取状态，加载和错误状态。但是，以某种方式用自己的状态钩子管理的所有这些状态属于同一类，因为它们关心相同的原因。如您所见，它们都在数据获取功能中使用。它们在一起的一个很好的指标是它们被一个接一个地使用（例如setIsError，setIsLoading）。让我们将它们全部三个与一个Reducer Hook结合使用

A Reducer Hook returns us a state object and a function to alter the state object. The function -- called dispatch function -- takes an action which has a type and an optional payload. All this information is used in the actual reducer function to distill a new state from the previous state, the action's optional payload and type. Let's see how this works in code:
Reducer Hook向我们返回一个状态对象和一个更改状态对象的函数。该函数-称为调度功能-采取具有类型和可选有效载荷的动作。所有这些信息都在实际的reducer功能中使用，以从先前状态，操作的可选有效负载和类型中提取新状态。让我们看看这在代码中是如何工作的

import React, {
  Fragment,
  useState,
  useEffect,
  useReducer,
} from 'react';
import axios from 'axios';
const dataFetchReducer = (state, action) => {
  ...
};
const useDataApi = (initialUrl, initialData) => {
  const [url, setUrl] = useState(initialUrl);
  const [state, dispatch] = useReducer(dataFetchReducer, {
    isLoading: false,
    isError: false,
    data: initialData,
  });
  ...
};

The Reducer Hook takes the reducer function and an initial state object as parameters. In our case, the arguments of the initial states for the data, loading and error state didn't change, but they have been aggregated to one state object managed by one reducer hook instead of single state hooks.
减速器挂钩将减速器功能和初始状态对象作为参数。在我们的例子中，数据，加载状态和错误状态的初始状态的参数没有改变，但是它们已经聚合到一个由减少器钩子而不是单个状态钩子管理的状态对象中

const dataFetchReducer = (state, action) => {
  ...
};
const useDataApi = (initialUrl, initialData) => {
  const [url, setUrl] = useState(initialUrl);
  const [state, dispatch] = useReducer(dataFetchReducer, {
    isLoading: false,
    isError: false,
    data: initialData,
  });
  useEffect(() => {
    const fetchData = async () => {
      dispatch({ type: 'FETCH_INIT' });
      try {
        const result = await axios(url);
        dispatch({ type: 'FETCH_SUCCESS', payload: result.data });
      } catch (error) {
        dispatch({ type: 'FETCH_FAILURE' });
      }
    };
    fetchData();
  }, [url]);
  ...
};

Now, when fetching data, the dispatch function can be used to send information to the reducer function. The object being send with the dispatch function has a mandatory type property and an optional payload property. The type tells the reducer function which state transition needs to be applied and the payload can additionally be used by the reducer to distill the new state. After all, we only have three state transitions: initializing the fetching process, notifying about a successful data fetching result, and notifying about an errornous data fetching result.
现在，在获取数据时，可以使用分派功能将信息发送给reducer功能。使用分派功能发送的对象具有强制类型属性和可选有效负载属性。该类型告诉化简器功能需要应用哪个状态转换，并且化简器可以额外使用有效负载来提取新状态。毕竟，我们只有三个状态转换：初始化获取过程，通知成功的数据获取结果，以及通知错误的数据获取结果

In the end of the custom hook, the state is returned as before, but because we have a state object and not the standalone states anymore. This way, the one who calls the useDataApi custom hook still gets access to data, isLoading and isError:
在自定义钩子的末尾，状态将像以前一样返回，但是因为我们有一个状态对象，而不再是独立状态。这样，调用useDataApi自定义钩子的人仍然可以访问数据，isLoading和isError

const useDataApi = (initialUrl, initialData) => {
  const [url, setUrl] = useState(initialUrl);
  const [state, dispatch] = useReducer(dataFetchReducer, {
    isLoading: false,
    isError: false,
    data: initialData,
  });
  ...
  return [state, setUrl];
};

Last but not least, the implementation of the reducer function is missing. It needs to act on three different state transitions called FETCH_INIT, FETCH_SUCCESS and FETCH_FAILURE. Each state transition needs to return a new state object. Let's see how this can be implemented with a switch case statement:
最后但并非最不重要的一点是，缺少reducer功能的实现。它需要对三种不同的状态过渡（称为FETCH_INIT，FETCH_SUCCESS和FETCH_FAILURE）采取行动。每个状态转换都需要返回一个新的状态对象。让我们看看如何用switch case语句实现这一点

const dataFetchReducer = (state, action) => {
  switch (action.type) {
    case 'FETCH_INIT':
      return { ...state };
    case 'FETCH_SUCCESS':
      return { ...state };
    case 'FETCH_FAILURE':
      return { ...state };
    default:
      throw new Error();
  }
};

A reducer function has access to the current state and the incoming action via its arguments. So far, in out switch case statement each state transition only returns the previous state. A destructuring statement is used to keep the state object immutable -- meaning the state is never directly mutated -- to enforce best practices. Now let's override a few of the current's state returned properties to alter the state with each state transition:
educer函数可以通过其参数访问当前状态和传入动作。到目前为止，在out switch case语句中，每个状态转换仅返回先前的状态。销毁语句用于使状态对象保持不变-意味着状态永远不会直接发生变化-可以实施最佳做法。现在，让我们重写一些当前状态返回的属性，以在每次状态转换时更改状态

const dataFetchReducer = (state, action) => {
  switch (action.type) {
    case 'FETCH_INIT':
      return {
        ...state,
        isLoading: true,
        isError: false
      };
    case 'FETCH_SUCCESS':
      return {
        ...state,
        isLoading: false,
        isError: false,
        data: action.payload,
      };
    case 'FETCH_FAILURE':
      return {
        ...state,
        isLoading: false,
        isError: true,
      };
    default:
      throw new Error();
  }
};

Now every state transition, decided by the action's type, returns a new state based on the previous state and the optional payload. For instance, in the case of a successful request, the payload is used to set the data of the new state object.
现在，由操作类型决定的每个状态转换都会根据先前的状态和可选的有效负载返回一个新的状态。例如，在成功请求的情况下，有效负载用于设置新状态对象的数据

In conclusion, the Reducer Hook makes sure that this portion of the state management is encapsulated with its own logic. By providing action types and optional payloads, you will always end up with a predicatbale state change. In addition, you will never run into invalid states. For instance, previously it would have been possible to accidently set the isLoading and isError states to true. What should be displayed in the UI for this case? Now, each state transition defined by the reducer function leads to a valid state object.
总之，Reducer Hook确保状态管理的这一部分使用其自己的逻辑进行封装。通过提供动作类型和可选的有效负载，您将始终以谓词状态更改为最终结果。此外，您将永远不会陷入无效状态。例如，以前可能会意外地将isLoading和isError状态设置为true。在这种情况下，UI中应显示什么？现在，由reducer函数定义的每个状态转换都导致一个有效的状态对象。

ABORT DATA FETCHING IN EFFECT HOOK

It's a common problem in React that component state is set even though the component got already unmounted (e.g. due to navigating away with React Router). I have written about this issue previously over here which describes how to prevent setting state for unmounted components in various scenarios. Let's see how we can prevent to set state in our custom hook for the data fetching:
在React中，一个常见的问题是即使组件已经被卸载（例如由于使用React Router导航），也会设置组件状态。在此之前，我已经写过有关此问题的文章，它描述了如何在各种情况下防止未安装组件的设置状态。让我们看看如何防止在自定义钩子中为数据获取设置状态

const useDataApi = (initialUrl, initialData) => {
  const [url, setUrl] = useState(initialUrl);
  const [state, dispatch] = useReducer(dataFetchReducer, {
    isLoading: false,
    isError: false,
    data: initialData,
  });
  useEffect(() => {
    let didCancel = false;
    const fetchData = async () => {
      dispatch({ type: 'FETCH_INIT' });
      try {
        const result = await axios(url);
        if (!didCancel) {
          dispatch({ type: 'FETCH_SUCCESS', payload: result.data });
        }
      } catch (error) {
        if (!didCancel) {
          dispatch({ type: 'FETCH_FAILURE' });
        }
      }
    };
    fetchData();
    return () => {
      didCancel = true;
    };
  }, [url]);
  return [state, setUrl];
};

Every Effect Hook comes with a clean up function which runs when a component unmounts. The clean up function is the one function returned from the hook. In our case, we use a boolean flag called didCancel to let our data fetching logic know about the state (mounted/unmounted) of the component. If the component did unmount, the flag should be set to true which results in preventing to set the component state after the data fetching has been asynchronously resolved eventually.
每个效果挂钩都带有清理功能，该功能在卸载组件时运行。清理函数是从挂钩返回的一个函数。在我们的例子中，我们使用一个名为didCancel的布尔标志来让我们的数据获取逻辑知道组件的状态（已安装/未安装）。如果确实卸载了组件，则应将标志设置为true，这将导致在最终异步地解析了数据提取之后阻止设置组件状态

Note: Actually not the data fetching is aborted -- which could be achieved with Axios Cancellation -- but the state transition is not performed anymore for the unmounted component. Since Axios Cancellation has not the best API in my eyes, this boolean flag to prevent setting state does the job as well.
注意：实际上，数据获取不会中止-这可以通过Axios Cancellation来实现-但已卸载的组件不再执行状态转换。由于Axios Cancellation在我眼中并不是最好的API，因此该防止设置状态的布尔值标记也可以完成这项工作

You have learned how the React hooks for state and effects can be used in React for data fetching. If you are curious about data fetching in class components (and function components) with render props and higher-order components, checkout out my other article from the beginning. Otherwise, I hope this article was useful to you for learning about React Hooks and how to use them in a real world scenario.
您已经了解了状态和效果的React钩子如何在React中用于数据获取。如果您对使用渲染道具和高阶组件的类组件（和函数组件）中的数据获取感到好奇，请从头开始阅读我的其他文章。否则，我希望本文对您了解React Hooks以及如何在实际场景中使用它们很有帮助