Ajax Binary Response

Question

Hi I'm wondering if there's anyway to stream a binary response in AJAX? This would be an ultimate solution otherwise I would need to realize the binary image to a file then stream that file to the user with a different URL.

new Ajax.Request('/viewImage?id=123', {
  // request returns a binary image inputstream
  onSuccess: function(transport) {
      // text example
      // alert(transport.responseText)

      // QUESTION: is there a streaming binary response?
      $('imgElem').src = transport.responseBinary;
  },
  onFailure: function(transport) {
      // handle failure
  }
});

Answer 1

It might not be possible to stream binary data, but you can use Ajax to retrieve binary data.

This is possible using one of two methods: Javascript Typed Arrays or an XMLHttpResponse overrideMimeType hack. Have a read of a good article on MDN – these examples are taken from there: Sending and Receiving Binary Data

The Typed Array method looks like this:

var oReq = new XMLHttpRequest();
oReq.open("GET", "/myfile.png", true);
oReq.responseType = "arraybuffer";

oReq.onload = function (oEvent) {
  var arrayBuffer = oReq.response; // Note: not oReq.responseText
  if (arrayBuffer) {
    var byteArray = new Uint8Array(arrayBuffer);
    for (var i = 0; i < byteArray.byteLength; i++) {
      // do something with each byte in the array
    }
  }
};

oReq.send(null);

Typed Arrays are not supported in IE < 10, Firefox < 4, Chrome < 7, Safari < 5.1 and Opera < 11.6, and mobile support is shaky but improving.

The second method uses an XMLHttpRequest method called overrideMimeType to allow the binary data to be passed through unmodified.

var req = new XMLHttpRequest();
req.open('GET', '/myfile.png', false);
// XHR binary charset opt by Marcus Granado 2006 [http://mgran.blogspot.com]
req.overrideMimeType('text\/plain; charset=x-user-defined');
req.send(null);
if (req.status != 200) return '';
// do stuff with req.responseText;

You get an unparsed binary string, upon which you can use var byte = filestream.charCodeAt(x) & 0xff; to retrieve a specific byte.

Answer 2

This is an extension to Tom Ashworth's response (which helped to put me on the right track with the issue I was facing). This allows you to just get the filestream (FileStreamResult if you are using asp.net mvc) and set it to the img src, which is cool.

var oReq = new XMLHttpRequest();
oReq.open("post", '/somelocation/getmypic', true );        
oReq.responseType = "blob";
oReq.onload = function ( oEvent )
{
    var blob = oReq.response;
    var imgSrc = URL.createObjectURL( blob );                        
    var $img = $( '<img/>', {                
        "alt": "test image",
        "src": imgSrc
    } ).appendTo( $( '#bb_theImageContainer' ) );
    window.URL.revokeObjectURL( imgSrc );
};
oReq.send( null );

The basic idea is that the data is returned untampered with, it is placed in a blob and then a url is created to that object in memory. See here and here. Note supported browsers.