我想逐行读取大文件并将数据插入数据库存储中。
我的函数在它创建的流中返回一个 Promise,并在调用事件 stream.on('end')
时解析它,但这不是我真正想要的,因为在 stream.on( 'data')
它在每一行生成 Promise.map()
,我想确定所有插入操作在 resolve()
调用之前完成。在这种情况下我怎样才能生产出正确的链条?
var loadFromFile = (options) => new Promise(function (resolve, reject) {
let stream = fs.createReadStream(options.filePath, {
flags: 'r',
});
stream.on('data', (chunk) => {
/* process data chunk to string Array */
Promise.map(lines, (line) => {
/* process and insert each line here */
})
.catch((err)=>{
reject(err);
});
});
stream.on('end', () => {
if (someBuisnessLogicCheckHere) {
reject('Invalid data was found in file');
}
resolve(); // Here I am not sure, that all inserts for each chunk in Promise.map was completed
});
});
最佳答案
如果您想确保在映射操作中的所有 promise 都已解决之前不会解决,请等待 Promise.map
返回的 promise ;见评论:
var loadFromFile = (options) => new Promise(function(resolve, reject) {
let stream = fs.createReadStream(options.filePath, {
flags: 'r',
});
let promises = []; // Array of promises we'll wait for
stream.on('data', (chunk) => {
promises.push( // Remember this promise
Promise.map(lines, (line) => {
/* process and insert each line here */
})
.catch((err) => {
reject(err);
})
);
});
stream.on('end', () => {
if (someBuisnessLogicCheckHere) {
reject('Invalid data was found in file');
}
Promise.all(promises).then(() => { resolve()}); // Wait for it before resolving
});
});
请注意,我不只是执行 Promise.all(promises).then(resolve);
,因为这会将一系列分辨率传递给 resolve
,这您的原始代码未与调用者共享。
如果数组有可能在充满大多数已解决的 promise 时变得非常大,您可以在它们解决时主动删除它们,只等待最后留下的 promise 。
关于javascript - Promise 合并流,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/37254448/