I've recently been integrating Background Transfer Service into an application so that the user is able to download files in the background.
I did most of my building/testing on an old iPhone 6 device that was running iOS 9.3.5 (I try to keep my previous device back a version of iOS). Background Transfer Service works great on this device. I am able to put 100+ download tasks into the queue and they all finish and report progress as expected after sending the application into the background and then re-opening the application.
This also appears to work great on a new 5th generation iPad running iOS 10.3.
Where things get weird is on my iPhone 7 Plus which is running iOS 10.3.1. The downloads kick off fine, progress is reported as expected in the didWriteData method, however if I background the application and wait ~10 seconds and re-open the application the progress never catches back up to what progress would have been done in the background and the progress never increments at all (it stays where it was when it was backgrounded). The didFinishDownloading method does end up getting called however it appears to wait until the very end of all the download tasks completions for that method to fire for all of the remaining tasks. So the didWriteData, didFinishDownloading, didCompleteWithError, etc. all happen right at the end in one burst.
I have however had a few instances where my iPhone 7 Plus device running iOS 10.3.1 did show progress after bringing the app back into the foreground however those instances I could could on one hand. More often than not (9 times out of 10) the progress is never reported on this device after re-opening the application.
I am at a bit of a loss and am wondering if there is a known bug we should be looking out for, and if so when we could expect a fix? Or if there are currently any known work arounds. From my testing/debugging I cannot get progress to work in my application on the iPhone 7 Plus running 10.3.1. I also had a co-worker test this as well and they also experienced this.
In fact they went a step further and opened the Netflix application on their 10.3.1 iPhone 7 Plus device and began downloading a movie. Progress incremented as expected, they then backgrounded the application and waited ~20 seconds and re-opened the application, only to find the progress get stuck where it was when the app was initially backgrounded and it never moves. After a given amount of time of the progress not moving the download does eventually finish (the progress indicator disappears) and the Netflix movie is downloaded and able to be played. This is the same behavior I see above in my application.
Any help with this would be greatly appreciated.
Thanks in advance!