Swift API v2 Example of uploading a file using filesUploadSession(Start, Append, Finish)

Each call to filesUploadSessionStart, filesUploadSessionAppend, or filesUploadSessionFinish uploads all of the data you give to it. So in your case, if you supply 1 GB, it will attempt to upload all of that data at once, before response is called. That will likely fail though, so you should instead only upload a piece at a time. E.g., you could use chunks of 4 MB, where you supply 4 MB to start, another 4 MB to append repeatedly as necessary, and then the remainder to finish.