Usando a File API adicionada ao DOM em HTML5, agora é possível para conteúdo web solicitar ao usuário para selecionar arquivos locais, e então ler o conteúdo desses arquivos. Essa seleção pode ser feita usando um elemento HTML <input>
ou por arrastar e soltar.
Se você quiser usar a DOM File API através de extensões ou outro código chrome, você pode; Na verdade, há algumas funcionalidades adicionais para se estar ciente. Veja Usando a DOM File API em código chrome para detalhes.
Selecionando arquivos usando HTML
Selecionar um único arquivo para uso com a File API é simples:
<input type="file" id="input" onchange="handleFiles(this.files)">
Quando um usuário seleciona um arquivo, a função handleFiles()
é chamada com um objeto FileList
contendo o objeto File
representando o arquivo selecionado pelo usuário.
If you want to let the user select multiple files, simply use the multiple
attribute on the input
element:
<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.
Using hidden file input elements using the click() method
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.
Consider this 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:
var fileSelect = document.getElementById("fileSelect"), fileElem = document.getElementById("fileElem"); fileSelect.addEventListener("click", function (e) { if (fileElem) { fileElem.click(); } e.preventDefault(); // prevent navigation to "#" }, false);
Obviously you can style the new button for opening the file picker as you wish.
Dynamically adding a change listener
If your input field was created using a JavaScript library such as jQuery, you'll need to use element.addEventListener()
to add the change
event listener, like this:
var inputElement = document.getElementById("inputField"); 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 looks up the file list instead of accepting a parameter, since event listeners added in this way can't accept an input parameter.
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);
Selecting files using drag and drop
You can also let the user drag and drop files into your web application.
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:
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.
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, then pull the file list out of it, passing that to handleFiles()
. From this point on, handling the files is the same whether the user used the input
element or drag and drop.
Getting information about selected files
The FileList
object provided by the DOM lists all 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:
var numFiles = files.length;
Individual File
objects can be retrieved by simply accessing the list as an array:
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.
There are three attributes provided by the File
object that contain useful information about the file.
-
name
- The file's name as a read-only string. This is just the file name, and does not include any path information.
-
size
- The size of the file in bytes as a read-only 64-bit integer.
-
type
-
The MIME type of the file as a read-only string, or
""
if the type couldn't be determined.
Example: Showing file(s) size
The following example shows a possible use of the size
property:
<!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>
Example: Showing thumbnails of user-selected images
Let's say you're developing the next great photo-sharing web site, and want to use HTML5 to display thumbnail previews of images before the user actually uploads them. Simply 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 (!file.type.match(imageType)) { continue; } var img = document.createElement("img"); img.classList.add("obj"); img.file = file; preview.appendChild(img); 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 string "image.*
"). For each file that is an image, we create a new img
element. CSS can be used to establish any pretty borders, shadows, and to specify the size of the image, so that doesn't even need to be done here.
Each image has the CSS class obj
added to it, to make them 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 actually uploading later. Finally, we use Node.appendChild()
to add the new thumbnail to the preview area of our document.
Then we establish the FileReader
to handle actually asynchronously loading the image and attaching it to the img
element. After creating the new FileReader
object, we set up its onload
function, 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 simply 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.
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. You can view the example live.
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 UI from being displayed. This is explained above 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 { var list = document.createElement("ul"); 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(e) { 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); } fileList.appendChild(list); } }
This starts by fetching the URL of the <div>
with the ID fileList
. This is the block into which we'll insert out file list, including thumbmails.
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 itselement.appendChild()
method. - For each
File
in theFileList
represented 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, passing in the object URL string as specified byimg.src
. - Append the new list item to the list.
- Create a new list item (
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 builds 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 lets us very easily select all 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 creates an array, called imgs
, of all the elements in the document with the CSS class obj
. In our case, these will be all the image thumbnails. Once we have that list, it's trivial to go through the list, creating 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", "https://demos.hacks.mozilla.org/paul/demos/resources/webservices/devnull.php"); xhr.overrideMimeType('text/plain; charset=x-user-defined-binary'); reader.onload = function(evt) { xhr.sendAsBinary(evt.target.result); }; reader.readAsBinaryString(file); }
The FileUpload()
function shown above creates a throbber, which is used to display progress information, then creates an XMLHttpRequest
to handle uploading the data.
Before actually transferring the data, several preparatory steps are taken:
- The
XMLHttpRequest
's uploadprogress
listener 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 uploadload
event handler is set to update the throbber with 100% as the progress information (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
XMLHttpRequest
functionoverrideMimeType()
. 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
FileReader
object is used to convert the file to a binary string. - Finally, when the content is loaded the
XMLHttpRequest
functionsendAsBinary()
is called to upload the file's content.
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) { // Handle response. 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>