从XmlHttpRequest转换Blob，而无需将其完全加载到内存中

Question

I want to implement a client-side file decryption in the browser. It takes decrypted files from a server and should decrypt it in the browser, presenting a Save As dialog to save the decrypted file. It should also work for large files (1 GB or more).

I have the following strategy in mind:

• Download the file using XMLHttpRequest with responseType = 'blob' .
• Decrypt by transforming the Blob given by XMLHttpRequest .
• Provide the decrypted Blob as a objectURL to the user.

The decryption could work like a stream transformation, it reads chunks from the downloaded Blob, decrypts the data and writes into the output Blob.

However as far as I can tell this will only work with current browsers if you can load the whole file into memory (you need to store the complete decrypted blob in memory before you can create an objectURL ). It seems that no kind of chunked reading/writing is supported by the current Blob , XMLHttpRequest and createObjectURL interface as described on the Mozilla Developer Network . A Blob is immutable and there doesn't seem to be an streaming support for binary data in browsers.

Is there any way to implement this with current browsers?

Answer 1

After getting a Blob with responseType = 'blob'; , you can use blob.slice(start, end) to get a subview and use the FileReader API to grab the data associated with this subview.