我有以下HTML代码:
<input type='file' multiple>
这是我的JS代码:
var inputFiles = document.getElementsByTagName("input")[0];
inputFiles.onchange = function(){
var fr = new FileReader();
for(var i = 0; i < inputFiles.files.length; i++){
fr.onload = function(){
console.log(i) // Prints "0, 3, 2, 1" in case of 4 chosen files
}
}
fr.readAsDataURL(inputFiles.files[i]);
}
所以我的问题是,如何使这个循环同步?首先等待文件完成加载,然后转到下一个文件。有人告诉我使用JS承诺。但我无法工作。这是我正在尝试的:
var inputFiles = document.getElementsByTagName("input")[0];
inputFiles.onchange = function(){
for(var i = 0; i < inputFiles.files.length; i++){
var fr = new FileReader();
var test = new Promise(function(resolve, reject){
console.log(i) // Prints 0, 1, 2, 3 just as expected
resolve(fr.readAsDataURL(inputFiles.files[i]));
});
test.then(function(){
fr.onload = function(){
console.log(i); // Prints only 3
}
});
};
}
提前感谢...
我们修改了 midos 答案以使其工作如下:
function readFile(file){
return new Promise((resolve, reject) => {
var fr = new FileReader();
fr.onload = () => {
resolve(fr.result )
};
fr.onerror = reject;
fr.readAsText(file.blob);
});
}
如果你想使用 Promise 按顺序(而不是同步(执行此操作,您可以执行以下操作:
var inputFiles = document.getElementsByTagName("input")[0];
inputFiles.onchange = function(){
var promise = Promise.resolve();
inputFiles.files.map( file => promise.then(()=> pFileReader(file)));
promise.then(() => console.log('all done...'));
}
function pFileReader(file){
return new Promise((resolve, reject) => {
var fr = new FileReader();
fr.onload = resolve; // CHANGE to whatever function you want which would eventually call resolve
fr.onerror = reject;
fr.readAsDataURL(file);
});
}
前言:这个答案最初写于 2015 年,显示了将FileReader
包裹在一个承诺中。这仍然是执行问题所问readAsDataURL
操作的完全有效的方法,但是如果您要使用readAsText
或readAsArrayBuffer
(通常,新代码不应使用较旧的readAsBinaryString
(,则需要使用File
对象的内置基于promise的方法text
或arrayBuffer
代替(如果您想在数据流经时对数据进行内联处理,则可能stream
(, 所有这些都是从Blob
继承而来的。
FileReader
的本质是您无法使其操作同步。
我怀疑你并不真正需要或希望它是同步的,只是你想正确获取生成的 URL。建议使用 promise 的人可能是对的,但不是因为 promise 使过程同步(它们不是(,而是因为它们为处理异步操作(无论是并行还是串联(提供了标准化的语义:
使用promises,您可以从 readAsDataURL
的 promise 包装器开始(我在这里使用 ES2015+,但您可以使用 promise 库将其转换为 ES5(:
function readAsDataURL(file) {
return new Promise((resolve, reject) => {
const fr = new FileReader();
fr.onerror = reject;
fr.onload = () => {
resolve(fr.result);
}
fr.readAsDataURL(file);
});
}
然后,您将使用我在此答案中描述的基于承诺的操作来并行阅读这些操作:
Promise.all(Array.prototype.map.call(inputFiles.files, readAsDataURL))
.then(urls => {
// ...use `urls` (an array) here...
})
.catch(error => {
// ...handle/report error...
});
。或串联:
let p = Promise.resolve();
for (const file of inputFiles.files) {
p = p.then(() => readAsDataURL(file).then(url => {
// ...use `url` here...
}));
}
p.catch(error => {
// ...handle/report error...
});
在 ES2017 async
函数中,您可以使用 await
.它对并行版本没有太大作用:
// Inside an `async` function
try {
const urls = await Promise.all(Array.prototype.map.call(inputFiles.files, readAsDataURL));
} catch (error) {
// ...handle/report error...
}
。但它使系列版本更简单、更清晰:
// Inside an `async` function
try {
for (const file of inputFiles.files) {
const url = await readAsDataURL(file);
// ...use `url` here...
}
} catch (error) {
// ...handle/report error...
}
如果没有承诺,您可以通过跟踪您有多少未完成的操作来做到这一点,以便您知道何时完成:
const inputFiles = document.getElementsByTagName("input")[0];
inputFiles.onchange = () => {
const data = []; // The results
let pending = 0; // How many outstanding operations we have
// Schedule reading all the files (this finishes before the first onload
// callback is allowed to be executed). Note that the use of `let` in the
// `for` loop is important, `var` would not work correctly.
for (let index = 0; index < inputFiles.files.length; ++index) {
const file = inputFiles.files[index];
// Read this file, remember it in `data` using the same index
// as the file entry
const fr = new FileReader();
fr.onload = () => {
data[index] = fr.result;
--pending;
if (pending == 0) {
// All requests are complete, you're done
}
}
fr.readAsDataURL(file);
++pending;
});
};
或者,如果您出于某种原因希望按顺序(但仍异步(读取文件,则可以通过在前一个调用完成后安排下一次调用来实现:
// Note: This assumes there is at least one file, if that
// assumption isn't valid, you'll need to add an up-front check
var inputFiles = document.getElementsByTagName("input")[0];
inputFiles.onchange = () => {
let index = 0;
readNext();
function readNext() {
const file = inputFiles.files[index++];
const fr = new FileReader();
fr.onload = () => {
// use fr.result here
if (index < inputFiles.files.length) {
// More to do, start loading the next one
readNext();
}
}
fr.readAsDataURL(file);
}
};
我通过添加工作示例并引入异步/等待语法来升级 Jens Lincke 答案
function readFile(file) {
return new Promise((resolve, reject) => {
let fr = new FileReader();
fr.onload = x=> resolve(fr.result);
fr.onerrror = reject;
fr.readAsDataURL(file) // or readAsText(file) to get raw content
})}
function readFile(file) {
return new Promise((resolve, reject) => {
let fr = new FileReader();
fr.onload = x=> resolve(fr.result);
fr.readAsDataURL(file) // or readAsText(file) to get raw content
})}
async function load(e) {
for(let [i,f] of [...e.target.files].entries() ){
msg.innerHTML += `<h1>File ${i}: ${f.name}</h1>`;
let p = document.createElement("pre");
p.innerText += await readFile(f);
msg.appendChild(p);
}
}
<input type="file" onchange="load(event)" multiple />
<div id="msg"></div>
Promisized FileReader
/**
* Promisified FileReader
* More info https://developer.mozilla.org/en-US/docs/Web/API/FileReader
* @param {*} file
* @param {*} method: readAsArrayBuffer, readAsBinaryString, readAsDataURL, readAsText
*/
export const readFile = (file = {}, method = 'readAsText') => {
const reader = new FileReader()
return new Promise((resolve, reject) => {
reader[method](file)
reader.onload = () => {
resolve(reader)
}
reader.onerror = (error) => reject(error)
})
}
用法
const file = new File(["foo"], "foo.txt", {
type: "text/plain",
});
// Text
const resp1 = await readFile(file)
console.log(resp1.result)
// DataURL
const resp2 = await readFile(file, 'readAsDataURL')
console.log(resp2.result)
使用承诺可以使它更加优雅,
// opens file dialog waits till user selects file and return dataurl of uploaded file
async function pick() {
var filepicker = document.createElement("input");
filepicker.setAttribute("type","file");
filepicker.click();
return new Promise((resolve,reject) => {
filepicker.addEventListener("change", e => {
var reader = new FileReader();
reader.addEventListener('load', file => resolve(file.target.result));
reader.addEventListener('error', reject);
reader.readAsDataURL(e.target.files[0]);
});
});
}
// Only call this function on a user event
window.onclick = async function() {
var file = await pick();
console.log(file);
}
这是对 Jen 答案的另一个修改(捎带 Mido 的答案(以额外检查文件大小:
function readFileBase64(file, max_size){
max_size_bytes = max_size * 1048576;
return new Promise((resolve, reject) => {
if (file.size > max_size_bytes) {
console.log("file is too big at " + (file.size / 1048576) + "MB");
reject("file exceeds max size of " + max_size + "MB");
}
else {
var fr = new FileReader();
fr.onload = () => {
data = fr.result;
resolve(data)
};
fr.readAsDataURL(file);
}
});
}
我们可以使用回调函数来获取 reader.result
function myDisplay(some) {
document.getElementById('demo').innerHTML = some;
}
function read(file, callback) {
const reader = new FileReader();
reader.onload = () => {
callback(reader.result);
}
reader.readAsText(file);
}
// When you pass a function as an argument, remember not to use parenthesis.
read(this.files[0], myDisplay);