2

我有一个组件，我需要一个接一个地执行几个查询。我使用了 Promise，但似乎它不起作用/我无法这样做。这是我的组件的一部分：

export default function AuthWrapper() {
    useEffect(() => {
        // creating app_tokens table if not exist
        console.log('before first query');
        new Promise((resolve) => {
            db.transaction(function (txn) {
                txn.executeSql(
                    "SELECT name FROM sqlite_master WHERE type='table' AND name='app_tokens'",
                    [],
                    function (tx, res) {
                        console.log('first query executed');
                        resolve(res);
                    },
                    function(error) {
                        console.log(error);
                    }
                );
            });
        });

        console.log('before second query');
        // creating users table
        new Promise((resolve) => {
            db.transaction(function (txn) {
                txn.executeSql(
                    "SELECT name FROM sqlite_master WHERE type='table' AND name='users'",
                    [],
                    function (tx, res) {
                        console.log('second query executed');
                        resolve(res);
                    },
                    function(error) {
                        console.log(error);
                    }
                );
            });
        });

        console.log('before third query');
        // creating institute_details table
        new Promise((resolve) => {
            db.transaction(function (txn) {
                txn.executeSql(
                    "SELECT name FROM sqlite_master WHERE type='table' AND name='institute_details'",
                    [],
                    function (tx, res) {
                        console.log('third query executed');
                        resolve(res);
                    },
                    function(error) {
                        console.log(error);
                    }
                );
            });
        });
    }, []);
}

当我运行此代码日志时显示如下：

before first query
before second query
before third query

first query executed
second query executed
third query executed

但它应该是这样的：

before first query
first query executed

before second query
second query executed

before third query
third query executed

谁能帮我解决我所缺少的！

仅供参考：我正在使用

"react": "16.11.0",
"react-native": "0.62.2",
"react-native-sqlite-storage": "^5.0.0",
4

1 回答 1

2

你问错了，你想通过查看你想要的输出来按照我的同步。

对于那个 async - await 就在那里。

我不知道您为什么使用 promise，但是，是的，使用您现在拥有的代码，您可以做这样的事情，

const myFirstFunction = () => {
  return new Promise((resolve) => {
    db.transaction(function (txn) {
      txn.executeSql(
        "SELECT name FROM sqlite_master WHERE type='table' AND name='app_tokens'",[],
        function (tx, res) {
          console.log('first query executed');
          resolve(res);
        },
        function (error) {
          console.log(error);
        },
      );
    });
  });
};

const mainFunction = async () => {
  console.log('before first query');
  await myFirstFunction();
  // ... similarlly create and call other functions here
  // ... creating different methods will be easy to maintain
};

useEffect(() => {
  mainFunction();
}, []);
于 2020-12-01T10:55:48.497 回答