Download Utils

Download Utils downloads large files in Unity reliably, in the Editor and in players. A download continues where it stopped after a network failure, a domain reload or an Editor restart, and it never leaves a broken file that looks complete.

Why this exists

UnityWebRequest with a DownloadHandlerFile is fine for a 5 MB file. For a 4 GB model it is not enough:

• A dropped connection loses everything received so far. The next attempt starts at byte 0.
• In the Editor, a domain reload (a script recompile, entering Play mode) stops every running async method. A long download stops halfway, without an error.
• The file is written at the target path while it downloads. A failed or cancelled download can leave a truncated file that the next run treats as complete.
• Two Editors, or two packages, that download into the same cache file overwrite each other.
• Resume logic, server validation, size checks, file locks and persistent state are a few hundred lines of code that is easy to get subtly wrong. Download Utils is that code, with tests.

Resumable downloads

• Writes to <target>.downloading and continues with HTTP Range requests.
• Resumes only when it can prove that the bytes on disk belong to the same file: same URL, and the server’s ETag or Last-Modified value sent back with If-Range. If the file on the server changed, it starts over in the same request.
• Handles servers that ignore Range, reject the range (416) or report another size: it starts over once instead of corrupting the file.

The target file changes only when the download is complete

• The target is replaced only after the received size matches the size from the server and your optional expected size.
• A wrong expected size fails as soon as the response headers arrive, before the body downloads.
• Error responses are never written to the file.

Persistent Editor operations

• A queue of work saved on disk. It continues after a domain reload or an Editor restart.
• An entry in the Background Tasks window, with progress and a cancel button.
• Your item code only does the work. It never saves or resumes state itself.

File locks

• FileLock.AcquireAsync keeps two downloads of the same file apart, in one Editor or in two. It waits without blocking the main thread.

A small API

• One method downloads one URL to one file: ResumableDownloader.DownloadFileAsync.
• Progress in bytes, reported at most every 100 ms. Cancellation with a CancellationToken; the partial file stays for the next call.
• Errors name the URL and the cause (WebRequestException with the response code).
• Generic: no assumptions about the host, the file format or the caller. You own the URLs, the cache layout and the validation.
• The download core is runtime-safe. Only the Editor integration (Background Tasks, the Editor update loop) is in the Editor assembly.
• Full C# source.

What it does not do

• No content hash check. If you have a hash, check the file after the download.
• No archive unpacking, no authentication flows, no parallel chunked downloads of one file.

Requirements

• Unity 6000.3 LTS or later.

Documentation | GitHub