How to read valid base64 file in Android? (cordova)

Question

I need to read a PDF from the filesystem in Android in order to send it to a server. I cannot seem to read valid data however.

I have tried readAsDataURL as it appears to be the fastest. However the value returned (after removing the MIME type) is invalid base64.

      // read the file from the filesystem
  window.resolveLocalFileSystemURL(path,
    function (fileEntry) {
      fileEntry.file(function (file) {
        var reader = new FileReader();

        reader.onloadend = function (evt) {

          // test base64 is valid
          var patt = /^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/;
          var b64 = evt.target.result.split(",", 2)[1];
          console.log("is valid base64? " + patt.test(b64)); // false!

          var bytes = atob(b64); // Failed to execute 'atob' on 'Window': The string to be decoded is not correctly encoded.
        };

        //reader.readAsText(file); 
        reader.readAsDataURL(file);
      },
        function (err) {
          console.error(err);
        });
    }, function (err) {
      console.error(err);
    });

I have also tried readAsText and then converted this into base64 myself, but this is incredibly slow for large PDF files and the data when translated back is not valid.

Why is the base64 from readAsDataURL not valid? I have tried on multiple Android devices (with and without crosswalk). Using latest version of the file plugin.

Answer 1

It is a known and reported issue of the File plugin of Cordova.

FileReader readAsDataURL have issues when base 64 encoding large files. The process used is the following:

• base64 encode each 256K chunk.
• Append any base64 encoded chunk of 256K.
• Return the result.

The result is not a valid base64 encoded string but is instead a string composed of multiple valid base64 encoded string appended.

One workaround instead of decoding the result is to split the data in part of 256K chunk, decode each chunk individually and then you'll be able to re-encode the whole string.

Regarding the issue , it's tagged as affecting Android 4.4.2 but the problem still exists in Android 5 & 6. Regarding the resolution, it's still not assigned to anyone.