带有 FileReader() 的 Javascript 承诺

Question

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，我怎样才能使这个循环同步？那就是首先等待文件完成加载，然后继续下一个文件。有人告诉我使用 JS Promises。但我无法让它工作。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
            }
        });
    };
}

提前谢谢...

Answer 1

我们修改了midos answer 以使其在以下情况下工作：

function readFile(file){
  return new Promise((resolve, reject) => {
    var fr = new FileReader();  
    fr.onload = () => {
      resolve(fr.result )
    };
    fr.onerror = reject;
    fr.readAsText(file.blob);
  });
}

Answer 2

如果您想使用 Promises 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);
  });
}

Answer 3

FileReader 的本质是不能使其操作同步。

我怀疑您并不真正需要或不希望它是同步的，只是您想正确获取生成的 URL。建议使用 Promise 的人可能是对的，但不是因为 Promise 使流程同步（它们不会），而是因为它们为处理异步操作（无论是并行还是串行）提供了标准化的语义：

使用 Promise，您将从 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);
    });
}

然后您将使用我描述的基于 Promise 的操作 in this answer 来并行读取这些操作：

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。它对并行版本没有多大作用：

// 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明了：

// 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想要按顺序读取文件（但仍然是异步的），您可以通过仅在前一个调用完成时安排下一个调用来做到这一点：

// 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);
    }
};

Answer 4

我通过添加工作示例升级 Jens Lincke answer 并引入 async/await 语法

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>

Answer 5

承诺的文件阅读器

/**
 * 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)

Answer 6

使用 Promise 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);
}

Answer 7

这是对 Jens 答案的另一个修改（借鉴 Mido 的答案），以另外检查文件大小：

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);
            }
        });
    }