Send a JPEG Blob with AJAX on Android
Having to upload a JPEG image as a raw Blob on Android 4.0+ revealed a couple of bugs.
If you have direct access to the specific JPEG image, when the image is on the same domain as the rest of your app, or you are in an environment without cross-origin restrictions, such as a PhoneGap webview, you can use the XHR2
responseType property to specify the format of the returned data.
XMLHttpRequest 2 support was introduced in Android 3.0.
Get image from
If you have to get the image data from a
canvas, you should be able to get the dataURL, write it to an
ArrayBuffer and then write it to a Blob. In the near future we’ll be able to use the
toBlob() method, which will return the image data as a Blob. Right now, to be “future friendly”, we can detect support for
toBlob() and use Sebastian Tschan’s canvas.toBlob() polyfill if there’s no native support for it.
Unfortunately, if you need the image data as a JPEG you’ll hit an Android bug.
toDataURL method allows you to specify the type of the returned
data:, so you should be able to specify the ‘image/jpeg’ type, and get a JPEG image. While this works on most desktop browsers, Android always returns the image as a PNG.
Here’s a simple test for the
canvas.toDataURL('image/jpeg') method, demonstrating the bug. While this will work on desktop browsers, testing it on Android 3+ will always return
You can now convert the image data to a Blob.
Android introduced support for Blobs in 3.0, using the now deprecated
BlobBuilder interface, and has kept it the same up to the latest version (4.2 as of November 2012).
According to chromestatus.com, Chrome for Android also has support only for the
BlobBuilder interface right now.
This isn’t really a problem, since we can check support for
BlobBuilder, and create the Blob using the supported interface.
canvas.toBlob polyfill adds an additional function called
dataURLtoBlob, which can convert a data url to a Blob. We can use this method on the manually-encoded JPEG image data.
Sending the Blob with AJAX
XHR2 allows us to send additional formats with AJAX beside FormData, like Blob or ArrayBuffer. We can simply use the
send method with our Blob as the parameter.
While the request works as expected on supporting desktop browsers, there seems to be a bug in Android that sends the request completely empty. Here’s a test for
xhr.send(blob) using GitHub’s API, which sends both a Blob and an ArrayBuffer.
On supporting desktop browsers, you’ll get the proper response from the API, while on Android you’ll get an error for the Blob-based request, because the API request doesn’t send the required parameters, sending the request without any payload.
Weirdly, the work-around is to send an
ArrayBuffer, instead of a Blob, which will have the exact same effect. You can convert a Blob to an
ArrayBuffer using the
FileReader object. Check the
str2ab_blobreader function in the example above to see how you use it exactly.
I’ve opened bug reports for these bugs on the Android bug tracker:
- Browser: canvas.toDataURL(‘image/jpeg’) returns image/png
- Browser: Trying to send a Blob with XHR2 sends the request with an empty body
And also on the Device-Bugs issue collection:
- canvas.toDataURL(‘image/jpeg’) returns image/png on Android
- Trying to send a Blob with XHR2 sends the request with an empty body on Android 4+
Please help me get the attention of the Android developers, so that they can get these bugs fixed for newer releases.