javascript - React Hooks setState 函数未设置状态

Question

我正在尝试构建一个聊天应用程序，并且我有一个用于选择要添加到聊天中的用户的功能。它位于以下组件中(为了简洁起见，我省略了导入语句和其中一个函数):

const AddUserBox = (props) => {

  // context:
  const { friends, getFriends, user, whoToMsg } = useContext(AppContext);

  // state:
  const [friendsDisplay, setFriendsDisplay] = useState();
  const [selected, setSelected] = useState();

  const getFriendsDisplay = async function() {
    if (!friends) await getFriends(user.username);

    if (friends) {
      let currentFriend;
      let result;
      let friendsInfo = [];

      friends.sort();

      for (let i = 0; i < friends.length; i++) {

        if (friends[i].user_1 === user.username) {
          currentFriend = friends[i].user_2;
        } else {
          currentFriend = friends[i].user_1;
        }

        if (currentFriend === whoToMsg) {
          return;
        } else if (typeof whoToMsg === Array) {
          if (whoToMsg.includes(currentFriend)) return;
        }

        await fetch('/proxy/get-user-update', {
          method: 'POST',
          headers: {
            'Accept': 'application/json, text/plain, */*',
            'Content-Type':'application/json'
          },
          body: JSON.stringify({username: currentFriend})
        })
        .then(results => results.json())
        .then(data => result = data)
        .catch(err => console.error(err));

        friendsInfo.push(result);
      }

      setFriendsDisplay(friends.map((friend, i) => {
        return <div 
          className="add-user-friends-button" 
          key={i}
          i={i}
          onClick={(e) => {selectUser(e, friend)}}
        >
          {friendsInfo[i].name}
        </div>
      }));
    }
  }

  const selectUser = function(e, friend) {
    let newArr = [];
    let currentFriend;

    if (friend.user_1 === user.username) {
      currentFriend = friend.user_2;
    } else {
      currentFriend = friend.user_1;
    }

    if (selected) {
      newArr = [...selected];
      if (selected.includes(currentFriend)) {
        e.target.style.backgroundColor = "";
        newArr.splice(newArr.indexOf(currentFriend), 1);
        setSelected(newArr);
        return;
      }
    }

    newArr.push(currentFriend);
    newArr.sort();

    e.target.style.backgroundColor = "#E6FCFF";
    setSelected(newArr);
    console.log(selected);
  }

  return ( 
    <div className="add-user">
      <span>Add users to chat:</span>
      <div className="add-user-friends">
        {friendsDisplay}
      </div>
      <button onClick={() => props.close()}>Close</button>
      <button onClick={() => addUsers()}>Add users</button>
    </div>
   );
} 

这个想法是，您从列表中选择一个用户，并将其添加到选定用户的数组中，这样当他们单击“添加用户”按钮时，用户就会添加到聊天中。出于某种原因setSelected(newArr ) 不会将状态设置为新数组。谁能帮助我理解/解决这个问题？

Answer 1

由于 setState 是一个异步方法，因此当您立即调用它时，并不总是能获得预期的值。您可以将回调方法传递给 setState。如果将 console.log 语句移至此处，您应该会看到预期的结果。

这里有一些信息: https://dev.to/dance2die/accessing-react-state-right-after-setting-it-2kc8