sqlite3-异步调用的承诺 [英] sqlite3 - promise for asynchronous calls

问题描述

我想选择一个sqlite3数据库的异步数据.但是由于 db.each 是一个异步函数，所以我下面的 select 函数无法正常工作.如何添加一个Promise以等待结果?

I want to select asynchronous some data of a sqlite3 database. But since db.each is a asynchron function my following select function doesn't work properly. How to add a Promise to wait for the result?

const sqlite3 = require('sqlite3').verbose();

export default function select(database, table) {
  return new Promise((resolve, reject) => {
    const db = new sqlite3.Database(database);
    const queries = [];
    db.each(`SELECT rowid as key, * FROM ${table}`, (err, row) => {
      if (err) {
        reject(err);
      }
      console.log(`Push row ${row.key} from database.`);
      queries.push(row);
    });
    console.log(queries);
    console.log(JSON.stringify(queries));
  });
}

推荐答案

db.each()由于其回调的性质而需要稍微麻烦的，非标准的promisification，通过该回调，它可以:

db.each() requires a slightly cumbersome, non-standard promisification due to the nature of its callbacks, through which it :

• 一次传送一行到第一个回调，
• 信号完成到第二个回调.

将其与 db.all()中显示的标准模式进行比较，该模式将使用签名为(err，rows)的单个回调.

Compare that with the standard pattern exhibited in db.all(), which takes a single callback with the signature (err, rows).

与 db.each()匹配，您所写的内容就目前而言是正确的，但在完成 db.each()后仍无法解决Promise>.

Sticking with db.each(), what you have written is correct as far as it goes but stops short of resolving the Promise on completion of db.each().

幸运的是，尽管很麻烦，但解决方案还是很简单的.可以从第二个回调中调用 resolve(queries).

Fortunately the solution, despite being cumbersome, is fairly simple. resolve(queries) can be called from a second, callback.

export default function select(database, table) {
    return new Promise((resolve, reject) => {
        const db = new sqlite3.Database(database);
        const queries = [];
        db.each(`SELECT rowid as key, * FROM ${table}`, (err, row) => {
            if (err) {
                reject(err); // optional: you might choose to swallow errors.
            } else {
                queries.push(row); // accumulate the data
            }
        }, (err, n) => {
            if (err) {
                reject(err); // optional: again, you might choose to swallow this error.
            } else {
                resolve(queries); // resolve the promise
            }
        });
    });
}

如果预期的行数非常有限"(如SQLite文档中所述)，请改用 db.all().

If the expected number of rows is "very limited" (as it says in the SQLite documentation), then use db.all() instead.

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