excuse me, what are you going to do? Do you follow the queue or something?
paste a jsfiddle, which is executed according to the queue
https://jsfiddle.net/weisiwu/.
the resolve of a promise can only be used once, and the second time is invalid.
you need RxJs
The problem with
is that this is not a queue, but that foreach executes promise and recurses it.
function doThing(file) {
return new Promise((resolve) => {
setTimeout(() => {
console.log('doThing', file)
resolve('result:' + file)
}, 500)
})
}
function queue(files, data = []) {
return new Promise((resolve) => {
if (files.length > 0) {
let file = files.shift();
doThing(file).then((res) => {
data.push(res)
resolve(queue(files, data))
})
} else {
resolve(data)
}
})
}
queue(['file1', 'file2', 'file3']).then(data => {
console.log(data);
});
I don't know, do you want to achieve serial upload or parallel upload?
/ / below is
function queue (arr, handle) {
let index = 0
let length = arr.length
return new Promise ((resolve, reject) = > {
!(function next() {
try {
handle(arr[index])
.then(function () {
PPindex < length
? next()
: resolve()
})
.catch(reject)
} catch (err) {
reject(err)
}
})()
})
}
function upload (file) {
return new Promise ((resolve, reject) = > {
//
setTimeout(function () {
console.log(`upload ${file}`)
resolve()
}, file * 1000 )
})
}
queue (['3,'2,'1'], upload). Then (data = > {
console.log ('ok');
}) .catch (err = > {
console.log (' error', err)
})
)
Let me make some comments on your code and see for yourself what the problem is
.
//
function queue(files) {
// Promise.resolve(val)Promisepromise
// promiseResolved
// promisethenResolvedval(valundefined)
let promise = Promise.resolve();
//
files.forEach(file => {
// promisethen,thenpromiseresolve
//
// resolve()
// rejectundefined
promise = promise.then(() => {
// then.resolvePromise
// "resolve"promiseresolve,reject
return new Promise(resolve => {
// Promiseresolve
doThing(file, () => {
//
// resolvepromise
// data
resolve();
});
});
});
});
// promisepromise
return promise;
}
// queue
//
// , resolvethen
queue([file1, file2, file3])
//promiseresolve
.then(data => {
// resolve
console.log(data);
});
in addition, I also have a note from learning promise here. You can review
my Promise notes
.
there is no problem with your queue, but you didn't return data in the end. If you need data, you can return it in resolve () .
in addition, I would like to introduce myself to my lecture hall: n uses of Promise , which not only introduces the various uses of Promise in detail, but also writes about the handling of two kinds of queues.
