Question

无法在 react-native 中异步执行 sqlite db 查询

2020-12-01

1294

react-native sqlite react-native-sqlite-storage

我有一个组件，需要依次执行几个查询。我使用了承诺，但似乎不起作用/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);
                    }
                );
            });
        });
    }, []);
}

当我运行此代码时，日志显示如下：

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

有人能帮我解决我遗漏的内容吗！

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",

Answer 1

你问错了，按照我的说法，你想要同步，通过查看你想要的输出。

对于那个异步 - await 就在那里。

我不知道你为什么使用承诺，但是，是的，使用你现在拥有的代码，你可以做这样的事情，

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();
}, []);