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,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__CODE__.但我不能让它发挥作用.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
            }
        });
    };
}

提前致谢...

Answer 1

如果你想使用Promises顺序(不同步),你可以这样做:

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.readAsDataURL(file);
  });
}

Answer 2

我们修改了midos的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.readAsText(file.blob);
  });
}

Answer 3

我通过添加工作示例来升级Jens Lincke 答案并引入 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.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);
  }
}

Answer 4

性质FileReader是你无法使其操作同步.

我怀疑你并不真正需要或希望它是同步的,只是你想要正确获得结果URL.如果是这样,我不认为承诺会真的有帮助.相反,只需跟踪您有多少未完成的操作,以便在完成后知道:

var inputFiles = document.getElementsByTagName("input")[0];
inputFiles.onchange = function(){
    var data = [];      // The results
    var 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)
    Array.prototype.forEach.call(inputFiles.files, function(file, index) {
        // Read this file, remember it in `data` using the same index
        // as the file entry
        var fr = new FileReader();
        fr.onload = function() {
            data[index] = fr.result;
            --pending;
            if (pending == 0) {
                // All requests are complete, you're done
            }
        }
        fr.readAsDataURL(file);
        ++pending;
    });
}

或者,如果您由于某种原因要按顺序(但仍然是异步)读取文件,则可以通过仅在前一个调用完成时调度下一个调用来执行此操作:

// 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 = function(){
    var index = 0;

    readNext();

    function readNext() {
        var file = inputFiles.files[index++];
        var fr = new FileReader();
        fr.onload = function() {
            // use fr.result here
            if (index < inputFiles.files.length) {
                // More to do, start loading the next one
                readNext();
            }
        }
        fr.readAsDataURL(file);
    }
}