Working Download Files on iOS

[janrenz]

Thanks to marcelgarus for helping me out here

[github-actions]

Flutter Analyze found 0 issues ✅

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[janrenz]

There still seems to be an issue (race condition?) with the SQLITE DB:
execute query: SELECT * FROM task WHERE task_id = "org.schulcloud.download.background.1583189560.930137.1" ORDER BY id DESC LIMIT 1
open DB successfully
database is locked
Load task successfully

[MarcelGarus]

Of course, we'll still have to fix that error.
The code seems fine, but we'll probably wait for the new version of the flutter_downloader package, which I'm currently working on.