phonegap filereader onloadend doesn't work

问题

I'm using phonegap to take a picture and put it into a img-container.

navigator.camera.getPicture(onSuccess, onFail);

var onSuccess =  function(imageURI) {
        var pic = document.getElementById('picture');
        pic.style.display = 'block';
        pic.src = imageURI;
};

var onFail = function(message) {
        $('#infoField').val(message);
};

That works perfect. Now I want to load the picture with a format to be alble to send it over a websocket. Therefor I use the fileReader and set the type to dataUrl.

var reader = new FileReader();

reader.onloadend = function(evt) {
      $('textarea#textarea1').val("evt triggered");
      //var socket = io.connect(addr);
      //socket.emit('mobilePicture', "works");
};

reader.readAsDataURL(document.getElementById('picture').src);

Unfortunately nothing happens. The onloadend event isn't triggered anyhow. Any ideas?

回答1:

Try to re-arrange your code like this:

var onSuccess =  function(imageURI) {
        var pic = document.getElementById('picture');
        pic.style.display = 'block';
        pic.src = imageURI;

        var reader = new FileReader();

        reader.onloadend = function(evt) {
              $('textarea#textarea1').val("evt triggered");
              //var socket = io.connect(addr);
              //socket.emit('mobilePicture', "works");
        };

        reader.readAsDataURL(imageURI);
};

var onFail = function(message) {
        $('#infoField').val(message);
};

navigator.camera.getPicture(onSuccess, onFail);

You have to pass a file object to the file.reader not just a path to the file.

Edit / update:
To find a file on your filesystem by a path first you will need to create a fileEntry using the fileSystem:

Example

window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, gotFS, fail);

function gotFS(fileSystem) {
    fileSystem.root.getFile("readme.txt", {create: true, exclusive: false}, gotFileEntry, fail);
}

function gotFileEntry(fileEntry) {
    //fileEntry
    alert("Got the fileEntry!");
    reader.readAsDataURL(fileEntry);
    ...
}

回答2:

Using readAsDataURL may still result in corrupt images particularly with iOS. After hours of testing and research I can confirm another users findings on using readAsBinaryString() which changes handling the onloadend code too.

Reference post is at: Cordova - Reading Large Image corrupts image

To expand on @benka example and incorporate the command iOS is happy with the code might look like:

var onSuccess =  function(imageURI) {
var pic = document.getElementById('picture');
pic.style.display = 'block';
pic.src = imageURI;

var reader = new FileReader();

reader.onloadend = function(evt) {
  $('textarea#textarea1').val("evt triggered");
  //var socket = io.connect(addr);
  //socket.emit('mobilePicture', "works");
  var base64 = window.btoa(evt.target.result); // Send this to a server.
  var image.src = "data:image/jpeg;base64," + base64 ; // Use if you need a html src reference.
};

  reader.readAsBinaryString(imageURI);
};

var onFail = function(message) {
  $('#infoField').val(message);
};

navigator.camera.getPicture(onSuccess, onFail);

来源：https://stackoverflow.com/questions/20073302/phonegap-filereader-onloadend-doesnt-work