How do I save and restore a File object in local s

问题:

I have an HTML5/javscript app which uses

<input type="file" accept="image/*;capture=camera" onchange="gotPhoto(this)">

to capture a camera image. Because my app wants to be runnable offline, how do I save the File (https://developer.mozilla.org/en-US/docs/Web/API/File) object in local storage, such that it can be retrieved later for an ajax upload?

I'm grabbing the file object from the using ...

function gotPhoto(element) { 
     var file = element.files[0];
     //I want to save 'file' to local storage here :-(
}

I can Stringify the object and save it, but when I restore it, it is no longer recognised as a File object, and thus can't be used to grab the file content.

I have a feeling it can't be done, but am open to suggestions.

fwiw My work around is to read the file contents at store time and save the full contents to local storage. This works, but quickly consumes local storage since each file is a 1MB plus photograph.

回答1:

You cannot serialize file API object.

Not that it helps with the specific problem, but ... Although I haven't used this, if you look here it seems that there are ways (althought not supported yet by most browsers) to store the offline image data to some files so as to restore them afterwards when the user is online (and not to use localstorage)

回答2:

Here is a workaround that I got working with the code below. I'm aware with your edit you talked about localStorage but I wanted to share how I actually implemented that workaround. I like to put the functions on body so that even if the class is added afterwards via AJAX the "change" command will still trigger the event.

See my example here: http://jsfiddle.net/x11joex11/9g8NN/

If you run the JSFiddle example twice you will see it remembers the image.

My approach does use jQuery. This approach also demonstrates the image is actually there to prove it worked.

HTML:

<input class="classhere" type="file" name="logo" id="logo" />
<div class="imagearea"></div>

JS:

$(document).ready(function(){
  //You might want to do if check to see if localstorage set for theImage here
  var img = new Image();                
  img.src = localStorage.theImage;

  $('.imagearea').html(img);

  $("body").on("change",".classhere",function(){
      //Equivalent of getElementById
      var fileInput = $(this)[0];//returns a HTML DOM object by putting the [0] since it's really an associative array.
      var file = fileInput.files[0]; //there is only '1' file since they are not multiple type.

      var reader = new FileReader();
      reader.onload = function(e) {
           // Create a new image.
           var img = new Image();

           img.src = reader.result;
           localStorage.theImage = reader.result; //stores the image to localStorage
           $(".imagearea").html(img);
       }

       reader.readAsDataURL(file);//attempts to read the file in question.
    });
});

This approach uses the HTML5 File System API's to read the image and put it into a new javascript img object. The key here is readAsDataURL. If you use chrome inspector you will notice the images are stored in base64 encoding.

The reader is Asynchronous, this is why it uses the callback function onload. So make sure any important code that requires the image is inside the onLoad or else you may get unexpected results.

回答3:

Convert it to base64 and then save it.

function gotPhoto(element) {
var file = element.files[0];
var reader = new FileReader()
reader.onload = function(base64) {
localStorage["file"] = base64;
}
reader.readAsDataURL(file);
}
// Saved to localstorage
function getPhoto() {
var base64 = localStorage["file"];
var base64Parts = base64.split(",");
var fileFormat = base64Parts[0].split(";")[1];
var fileContent = base64Parts[1];
var file = new File([fileContent], "file name here", {type: fileFormat});
return file;
}
// Retreived file object

回答4:

You could use this lib:

https://github.com/carlo/jquery-base64

then do something similar to this:

//Set file
var baseFile = $.base64.encode(fileObject);
window.localStorage.setItem("file",basefile);

//get file
var outFile = window.localStorage.getItem("file");

an other solution would be using json (I prefer this method) using: http://code.google.com/p/jquery-json/

//Set file
window.localStorage.setItem("file",$.toJSON(fileobject));

//get file
var outFile = $.evalJSON(window.localStorage.getItem("file"));

回答5:

I don't think that there is a direct way to Stringify and then deserialize the string object into the object of your interest. But as a work around you can store the image paths in your local storage and load the images by retrieving the URL for the images. Advantages would be, you will never run out of storage space and you can store 1000 times more files there.. Saving an image or any other file as a string in local storage is never a wise decision..

回答6:

create an object on the global scope

exp: var attmap = new Object();

after you are done with file selection, put your files in attmap variable as below,

attmap[file.name] = attachmentBody;
JSON.stringify(attmap)

Then you can send it to controller via input hidden or etc. and use it after deserializing.

(Map<String, String>)JSON.deserialize(attachments, Map<String,String>.class);

You can create your files with those values in a for loop or etc.

EncodingUtil.base64Decode(CurrentMapValue);

FYI:This solution will also cover multiple file selection