如何同步FileReader onload函数？
如何同步FileReader onload函数？
提问于 2019-02-20 21:01:12
我想读一些文本文件，并对它们进行排序。我写了以下函数：
function load_files (event)
{
var file;
var reader;
var lines = [];
for (file of event.target.files)
{
reader = new FileReader();
reader.onload = function (event)
{
let l = this.result.split (/\r?\n/); // lines to add
for (let i = 0; i < l.length; i++)
lines.push(l[i]);
};
reader.readAsText (file);
}
// how to wait for all onload functions here?
replace_log(lines.sort());
}在我可以对行进行排序之前，我必须等待，所有加载都已完成。如何等待异步调用？
回答 2
Stack Overflow用户
回答已采纳
发布于 2019-02-25 22:40:15
多亏了Karim和他的suggestion to use promises，我找到了。
function load_files (event)
{
Promise.all(
(function* () {
for (let file of event.target.files) {
yield new Promise(resolve => {
let reader = new FileReader();
reader.onload = (event) => resolve(event.target.result);
reader.readAsText(file);
})}})())
.then(texts => {
let lines = [].concat.apply([], texts.map(text => text.split (/\r?\n/)));
replace_log (lines.sort());
});
}Stack Overflow用户
发布于 2019-02-20 21:13:00
您甚至可以计算回调的发生次数，并在读取所有文件时触发排序
function load_files (event) {
var file;
var reader;
var lines = [];
let count = 0;
for (file of event.target.files) {
reader = new FileReader();
// this event is triggered only in case of success
reader.onload = function (event) {
let l = this.result.split (/\r?\n/); // lines to add
for (let i = 0; i < l.length; i++)
lines.push(l[i]);
};
// this event is triggered both in case of success or failure when the op. is completed
reader.onloadend = function(event) {
if(++count === event.target.files.length)
replace_log(lines.sort());
}
reader.readAsText (file);
}
} 或者，您也可以使用Promises
function read_file(file) {
return new Promise((resolve, reject) => {
var fr = new FileReader();
fr.onload = () => {
resolve(fr.result);
};
fr.readAsText(file);
});
}
function load_files(event) {
var file;
var reader;
var lines = [];
let count = 0;
let promises = []; // collect all promises
for (file of event.target.files) {
promises.push(read_file(file));
}
Promise.all(promises) // wait for the resolutions
.then(results => {
for (result of results) {
let l = result.split(/\r?\n/); // lines to add
for (let i = 0; i < l.length; i++)
lines.push(l[i]);
}
replace_log(lines.sort());
})
}页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/54786996
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