Using the File API, which was added to the DOM in HTML5, it's now possible for web content to ask the user to select local files and then read the contents of those files. This selection can be done by either using an HTML <input> element or by drag and drop.
使用在HTML5中被加入DOM的File API，使页面内容请求用户选择本地文件，然后读取这些文件的内容成为可能。选择可以由使用 HTML <input>元素或者通过drag 和 drop实现。
If you want to use the DOM File API from extensions or other browser chrome code, you can; however, note there are some additional features to be aware of. See Using the DOM File API in chrome code for details.
如果你想通过拓展或者其它的chrome源码浏览器（browser chrome code）使用DOM File API，是可行的；然而，注意有一些附加特性需要知道。看 Using the DOM File API in chrome code 了解详情。
访问选择的文件
考虑这段HTML:
<input type="file" id="input">
The File API makes it possible to access a FileList containing File objects representing the files selected by the user.
File API使访问包含 File 对象的 FileList成为可能，FileList代表被用户选择的文件。
If the user selects just one file, it is then only necessary to consider the first file of the list.
如果用户只选择了一个文件，那么只需要考虑FileList中的第一个File对象。
Accessing one selected file using a classical DOM selector:
使用传统的DOM选择器访问一个被选择的文件。
var selectedFile = document.getElementById('input').files[0];
Accessing selected file(s) on a change event
通过change事件访问被选择的文件
It is also possible (but not mandatory) to access the FileList through the change event:
可以通过change事件访问 FileList（但不是强制的）。
<input type="file" id="input" onchange="handleFiles(this.files)">
When the user selects a file, the handleFiles() function gets called with a FileList object containing File objects representing the files selected by the user.
当用户选择一个文件时，handleFiles()方法会被调用，同时传入包含File对象的 FileList 对象。File对象代表被用户选择的文件。
If you want to let the user select multiple files, simply use the multiple attribute on the input element:
如果你想让用户选择多个文件，只需在input元素上使用multiple属性：
<input type="file" id="input" multiple onchange="handleFiles(this.files)">
In this case, the file list passed to the handleFiles() function contains one File object for each file the user selected.
在这个例子中，被传入handleFiles()方法的文件列表包含一组代表用户所选文件的 File 对象。
Dynamically adding a change listener
动态添加一个change监听
You need to use EventTarget.addEventListener() to add the change event listener, like this:
你需要使用EventTarget.addEventListener()去添加change事件监听，像这样：
var inputElement = document.getElementById("input");
inputElement.addEventListener("change", handleFiles, false);
function handleFiles() {
var fileList = this.files; /* now you can work with the file list */
}
Note that in this case, the handleFiles() function itself is the event handler, unlike previous examples where it was called by an event handler which passed it a parameter.
注意在这个例子里，handleFiles() 方法本事是一个事件处理器，不像之前的例子中，它被事件处理器调用然后传递给它一个参数。
Getting information about selected file(s)
获取被选择文件的信息
The FileList object provided by the DOM lists all of the files selected by the user, each specified as a File object. You can determine how many files the user selected by checking the value of the file list's length attribute:
FileList 对象由DOM提供，列出了所有用户选择的文件，每一个代表了一个 File 对象。你可以通过检查文件列表的length属性决定用户可以选则多少文件
var numFiles = files.length;
Individual File objects can be retrieved by simply accessing the list as an array:
非法的File对象可以通过访问文件列表被恢复，像访问数组那样。
for (var i = 0, numFiles = files.length; i < numFiles; i++) {
var file = files[i];
..
}
This loop iterates over all the files in the file list.
这个循环迭代文件列表里的所有文件。
There are three attributes provided by the File object that contain useful information about the file.
这是 File 对象提供的三个属性，包含了关于文件的一些有价值的信息。
name- The file's name as a read-only string. This is just the file name, and does not include any path information.
- 文件名称，只读字符串。只包含文件名称，不包含任何路径信息。
size- The size of the file in bytes as a read-only 64-bit integer.
- 文件大小，使用bytes描述，是一个只读的64位整数.
type- The MIME type of the file as a read-only string or
""if the type couldn't be determined. - 文件的MIME type，只读字符串，当类型不能确定时为
""。
Example: Showing file(s) size
例子：显示文件大小
The following example shows a possible use of the size property:
下面的例子展示了size属性的一种可能用法。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>File(s) size</title>
<script>
function updateSize() {
var nBytes = 0,
oFiles = document.getElementById("uploadInput").files,
nFiles = oFiles.length;
for (var nFileId = 0; nFileId < nFiles; nFileId++) {
nBytes += oFiles[nFileId].size;
}
var sOutput = nBytes + " bytes";
// optional code for multiples approximation
for (var aMultiples = ["KiB", "MiB", "GiB", "TiB", "PiB", "EiB", "ZiB", "YiB"], nMultiple = 0, nApprox = nBytes / 1024; nApprox > 1; nApprox /= 1024, nMultiple++) {
sOutput = nApprox.toFixed(3) + " " + aMultiples[nMultiple] + " (" + nBytes + " bytes)";
}
// end of optional code
document.getElementById("fileNum").innerHTML = nFiles;
document.getElementById("fileSize").innerHTML = sOutput;
}
</script>
</head>
<body onload="updateSize();">
<form name="uploadForm">
<p><input id="uploadInput" type="file" name="myFiles" onchange="updateSize();" multiple> selected files: <span id="fileNum">0</span>; total size: <span id="fileSize">0</span></p>
<p><input type="submit" value="Send file"></p>
</form>
</body>
</html>
Using hidden file input elements using the click() method
通过click()方法使用隐藏的file input元素
Starting in Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1), you can hide the admittedly ugly file <input> element and present your own interface for opening the file picker and displaying which file or files the user has selected. You can do this by styling the input element with display:none and calling the click() method on the <input> element.
从Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1)开始，你可以隐藏丑陋的 file <input> 元素然后使用你自己的接口打开文件选择器，然后显示哪个文件（或多个）被用户选择了。你可以通过给input元素添加display:none的样式然后在 <input> 元素上调用 click()方法。
Consider this HTML:
考虑这段HTML：
<input type="file" id="fileElem" multiple accept="image/*" style="display:none" onchange="handleFiles(this.files)"> <a href="#" id="fileSelect">Select some files</a>
The code that handles the click event can look like this:
处理点击事件的代码可以像这样：
var fileSelect = document.getElementById("fileSelect"),
fileElem = document.getElementById("fileElem");
fileSelect.addEventListener("click", function (e) {
if (fileElem) {
fileElem.click();
}
e.preventDefault(); // prevent navigation to "#"
}, false);
You can style the new button for opening the file picker as you wish.
你可以给这个用来打开文件选择器新按钮添加任何你想要的样式。
Using a label element to trigger a hidden file input element
To allow opening the file picker without using JavaScript (the click() method), a <label> element can be used.
如果想不使JavaScript (click() 方法)打开文件选择器，使用 <label> 元素也可以。
Consider this HTML:
考虑这段HTML
<input type="file" id="fileElem" multiple accept="image/*" style="display:none" onchange="handleFiles(this.files)"> <label for="fileElem">Select some files</label>
There is no need to add JavaScript code to call fileElem.click(). Also in this case you can style the label element as you wish.
没有必要添加JavaScript代码调用fileElem.click().在这个例子里你也可以给label元素添加你想要的样式
Selecting files using drag and drop
You can also let the user drag and drop files into your web application.
你可以在让用户使用拖拽将文件传入你的web应用中。
The first step is to establish a drop zone. Exactly what part of your content will accept drops may vary depending on the design of your application, but making an element receive drop events is easy:
第一步是创建一个drop区域。就是那在个你内容中将要接收拖放的部分，这部分根据你的设计而定，但是使一个元素接收drop事件是很容易的。
var dropbox;
dropbox = document.getElementById("dropbox");
dropbox.addEventListener("dragenter", dragenter, false);
dropbox.addEventListener("dragover", dragover, false);
dropbox.addEventListener("drop", drop, false);
In this example, we're turning the element with the ID dropbox into our drop zone. This is done by adding listeners for the dragenter, dragover, and drop events.
在这个例子中，我将ID为dropbox的元素转换成我们的drop区域。这个操作通过给元素添加dragenter, dragover, 和drop 事件监听完成。
We don't actually need to do anything with the dragenter and dragover events in our case, so these functions are both simple. They just stop propagation of the event and prevent the default action from occurring:
function dragenter(e) {
e.stopPropagation();
e.preventDefault();
}
function dragover(e) {
e.stopPropagation();
e.preventDefault();
}
The real magic happens in the drop() function:
function drop(e) {
e.stopPropagation();
e.preventDefault();
var dt = e.dataTransfer;
var files = dt.files;
handleFiles(files);
}
Here, we retrieve the dataTransfer field from the event, pull the file list out of it, and then pass that to handleFiles(). From this point on, handling the files is the same whether the user used the input element or drag and drop.
Example: Showing thumbnails of user-selected images
Let's say you're developing the next great photo-sharing website and want to use HTML5 to display thumbnail previews of images before the user actually uploads them. You can establish your input element or drop zone as discussed previously and have them call a function such as the handleFiles() function below.
function handleFiles(files) {
for (var i = 0; i < files.length; i++) {
var file = files[i];
var imageType = /^image\//;
if (!imageType.test(file.type)) {
continue;
}
var img = document.createElement("img");
img.classList.add("obj");
img.file = file;
preview.appendChild(img); // Assuming that "preview" is the div output where the content will be displayed.
var reader = new FileReader();
reader.onload = (function(aImg) { return function(e) { aImg.src = e.target.result; }; })(img);
reader.readAsDataURL(file);
}
}
Here our loop handling the user-selected files looks at each file's type attribute to see if it's an image file (by doing a regular expression match on the MIME type string pattern "image/*"). For each file that is an image, we create a new img element. CSS can be used to establish any pretty borders or shadows and to specify the size of the image, so that doesn't need to be done here.
Each image has the CSS class obj added to it, making it easy to find in the DOM tree. We also add a file attribute to each image specifying the File for the image; this will let us fetch the images for actual upload later. We use Node.appendChild() to add the new thumbnail to the preview area of our document.
Next, we establish the FileReader to handle asynchronously loading the image and attaching it to the img element. After creating the new FileReader object, we set up its onload function and then call readAsDataURL() to start the read operation in the background. When the entire contents of the image file are loaded, they are converted into a data: URL which is passed to the onload callback. Our implementation of this routine sets the img element's src attribute to the loaded image which results in the image appearing in the thumbnail on the user's screen.
Using object URLs
Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1) introduces support for the DOM window.URL.createObjectURL() and window.URL.revokeObjectURL() methods. These let you create simple URL strings that can be used to reference any data that can be referred to using a DOM File object, including local files on the user's computer.
When you have a File object you'd like to reference by URL from HTML, you can create an object URL for it like this:
var objectURL = window.URL.createObjectURL(fileObj);
The object URL is a string identifying the File object. Each time you call window.URL.createObjectURL(), a unique object URL is created even if you've created an object URL for that file already. Each of these must be released. While they are released automatically when the document is unloaded, if your page uses them dynamically you should release them explicitly by calling window.URL.revokeObjectURL():
window.URL.revokeObjectURL(objectURL);
Example: Using object URLs to display images
This example uses object URLs to display image thumbnails. In addition, it displays other file information including their names and sizes.
The HTML that presents the interface looks like this:
<input type="file" id="fileElem" multiple accept="image/*" style="display:none" onchange="handleFiles(this.files)"> <a href="#" id="fileSelect">Select some files</a> <div id="fileList"> <p>No files selected!</p> </div>
This establishes our file <input> element as well as a link that invokes the file picker (since we keep the file input hidden to prevent that less-than-attractive user interface from being displayed). This is explained in the section Using hidden file input elements using the click() method, as is the method that invokes the file picker.
The handleFiles() method follows:
window.URL = window.URL || window.webkitURL;
var fileSelect = document.getElementById("fileSelect"),
fileElem = document.getElementById("fileElem"),
fileList = document.getElementById("fileList");
fileSelect.addEventListener("click", function (e) {
if (fileElem) {
fileElem.click();
}
e.preventDefault(); // prevent navigation to "#"
}, false);
function handleFiles(files) {
if (!files.length) {
fileList.innerHTML = "<p>No files selected!</p>";
} else {
fileList.innerHTML = "";
var list = document.createElement("ul");
fileList.appendChild(list);
for (var i = 0; i < files.length; i++) {
var li = document.createElement("li");
list.appendChild(li);
var img = document.createElement("img");
img.src = window.URL.createObjectURL(files[i]);
img.height = 60;
img.onload = function() {
window.URL.revokeObjectURL(this.src);
}
li.appendChild(img);
var info = document.createElement("span");
info.innerHTML = files[i].name + ": " + files[i].size + " bytes";
li.appendChild(info);
}
}
}
This starts by fetching the URL of the <div> with the ID fileList. This is the block into which we'll insert our file list, including thumbnails.
If the FileList object passed to handleFiles() is null, we simply set the inner HTML of the block to display "No files selected!". Otherwise, we start building our file list, as follows:
- A new unordered list (
<ul>) element is created. - The new list element is inserted into the
<div>block by calling itsNode.appendChild()method. - For each
Filein theFileListrepresented byfiles:- Create a new list item (
<li>) element and insert it into the list. - Create a new image (
<img>) element. - Set the image's source to a new object URL representing the file, using
window.URL.createObjectURL()to create the blob URL. - Set the image's height to 60 pixels.
- Set up the image's load event handler to release the object URL since it's no longer needed once the image has been loaded. This is done by calling the
window.URL.revokeObjectURL()method and passing in the object URL string as specified byimg.src. - Append the new list item to the list.
- Create a new list item (
Here is a live demo of the code above:
Example: Uploading a user-selected file
Another thing you might want to do is let the user upload the selected file or files (such as the images selected using the previous example) to a server. This can be done asynchronously very easily.
Creating the upload tasks
Continuing with the code that built the thumbnails in the previous example, recall that every thumbnail image is in the CSS class obj with the corresponding File attached in a file attribute. This allows us to select all of the images the user has chosen for uploading using Document.querySelectorAll(), like this:
function sendFiles() {
var imgs = document.querySelectorAll(".obj");
for (var i = 0; i < imgs.length; i++) {
new FileUpload(imgs[i], imgs[i].file);
}
}
Line 2 fetches a NodeList, called imgs, of all the elements in the document with the CSS class obj. In our case, these will be all of the image thumbnails. Once we have that list, it's trivial to go through it and create a new FileUpload instance for each. Each of these handles uploading the corresponding file.
Handling the upload process for a file
The FileUpload function accepts two inputs: an image element and a file from which to read the image data.
function FileUpload(img, file) {
var reader = new FileReader();
this.ctrl = createThrobber(img);
var xhr = new XMLHttpRequest();
this.xhr = xhr;
var self = this;
this.xhr.upload.addEventListener("progress", function(e) {
if (e.lengthComputable) {
var percentage = Math.round((e.loaded * 100) / e.total);
self.ctrl.update(percentage);
}
}, false);
xhr.upload.addEventListener("load", function(e){
self.ctrl.update(100);
var canvas = self.ctrl.ctx.canvas;
canvas.parentNode.removeChild(canvas);
}, false);
xhr.open("POST", "http://demos.hacks.mozilla.org/paul/demos/resources/webservices/devnull.php");
xhr.overrideMimeType('text/plain; charset=x-user-defined-binary');
reader.onload = function(evt) {
xhr.send(evt.target.result);
};
reader.readAsBinaryString(file);
}
The FileUpload() function shown above creates a throbber, which is used to display progress information, and then creates an XMLHttpRequest to handle uploading the data.
Before actually transferring the data, several preparatory steps are taken:
- The
XMLHttpRequest's uploadprogresslistener is set to update the throbber with new percentage information so that as the upload progresses the throbber will be updated based on the latest information. - The
XMLHttpRequest's uploadloadevent handler is set to update the throbber progress information to 100% to ensure the progress indicator actually reaches 100% (in case of granularity quirks during the process). It then removes the throbber since it's no longer needed. This causes the throbber to disappear once the upload is complete. - The request to upload the image file is opened by calling
XMLHttpRequest'sopen()method to start generating a POST request. - The MIME type for the upload is set by calling the
XMLHttpRequestfunctionoverrideMimeType(). In this case, we're using a generic MIME type; you may or may not need to set the MIME type at all depending on your use case. - The
FileReaderobject is used to convert the file to a binary string. - Finally, when the content is loaded the
XMLHttpRequestfunctionsend()is called to upload the file's content.
sendAsBinary method which was previously used in the example above is considered deprecated as of Gecko 31 (Firefox 31 / Thunderbird 31 / SeaMonkey 2.28); use the standard send(Blob data) method instead.Handling the upload process for a file, asynchronously
<?php
if (isset($_FILES['myFile'])) {
// Example:
move_uploaded_file($_FILES['myFile']['tmp_name'], "uploads/" . $_FILES['myFile']['name']);
exit;
}
?><!DOCTYPE html>
<html>
<head>
<title>dnd binary upload</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<script type="text/javascript">
function sendFile(file) {
var uri = "/index.php";
var xhr = new XMLHttpRequest();
var fd = new FormData();
xhr.open("POST", uri, true);
xhr.onreadystatechange = function() {
if (xhr.readyState == 4 && xhr.status == 200) {
alert(xhr.responseText); // handle response.
}
};
fd.append('myFile', file);
// Initiate a multipart/form-data upload
xhr.send(fd);
}
window.onload = function() {
var dropzone = document.getElementById("dropzone");
dropzone.ondragover = dropzone.ondragenter = function(event) {
event.stopPropagation();
event.preventDefault();
}
dropzone.ondrop = function(event) {
event.stopPropagation();
event.preventDefault();
var filesArray = event.dataTransfer.files;
for (var i=0; i<filesArray.length; i++) {
sendFile(filesArray[i]);
}
}
}
</script>
</head>
<body>
<div>
<div id="dropzone" style="margin:30px; width:500px; height:300px; border:1px dotted grey;">Drag & drop your file here...</div>
</div>
</body>
</html>
Example: Using object URLs to display PDF
Object URLs can be used for other things than just images! They can be used to display embedded PDF files or any other resources that can be displayed by the browser.
In Firefox, to have the PDF appear embedded in the iframe (rather than proposed as a downloaded file), the preference pdfjs.disabled must be set to false .
<iframe id="viewer">
And here is the change of the src attribute:
var obj_url = window.URL.createObjectURL(blob);
var iframe = document.getElementById('viewer');
iframe.setAttribute('src', obj_url);
window.URL.revokeObjectURL(obj_url);
Example: Using object URLs with other file types
You can manipulate files of other formats the same way. Here is how to preview uploaded video:
var video = document.getElementById('video');
var obj_url = window.URL.createObjectURL(blob);
video.src = obj_url;
video.play()
window.URL.revokeObjectURL(obj_url);
Specifications
- File upload state (HTML 5 working draft)
- File API