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Basically, you would change the policy if you’ve prompted the user accordingly or allow them to set
behavior through your settings. For example, if the user has a setting to disallow downloads or uploads
on a metered network, your apps would set the general costPolicy to unrestrictedOnly. If you know
you’re on a network where roaming charges would apply and the user has consented to a transfer,
you’d want to change the costPolicy of that individual operation to always. Otherwise the API would
not perform the transfer because doing so on a roaming network is disallowed by default.
When a transfer is blocked by policy, the operation’s progress.status property will contain
BackgroundTransferStatus.pausedCostedNetwork.
Grouping Transfers
The group property that’s found in BackgroundDownloader, BackgroundUploader, DownloadOperation, and
UploadOperation is a simple string that tags a transfer as belonging to a particular group. The property
can be set only through BackgroundDownloader and BackgroundUploader; you would set this prior to
creating a series of individual operations. In those operations, the group property is available but
read-only.
The purpose of grouping is so that you can selectively enumerate and control related transfers, as
we’ll see in the next section. For example, a photo app that organizes pictures into albums or album
pages can present a UI through which the user can pause, resume, or cancel the transfer of an entire
album, rather than working on the level of individual files. The group property makes the
implementation of this kind of experience much easier, as the app doesn’t need to maintain its own
grouping structures.
The group has no bearing on the transfers themselves; it is not communicated to the server upload
page.
Suspend, Resume, and Restart with Background Transfers
At the beginning of this section I mentioned that background transfers will continue while an app is
suspended and paused if the app is terminated by the system. Because apps will be terminated only in
low-memory conditions, it’s appropriate to also pause background transfers.
When an app is resumed from the suspended state, it can check on the status of pending transfers by
using the BackgroundDownloader.getCurrentDownloadsAsync and
BackgroundUploader.getCurrent-UploadsAsync methods. In both cases two variants of the methods
exist: one that enumerates all transfers, and one that enumerates those belonging to a specific group (as
matches the group properties in the operations).
The list that comes back from these methods is a vector of DownloadOperation and UploadOperation
objects, and, as always, the vector can be addressed as an array. Code to iterate over the list looks like
this:
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