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e found on the SkyDrive reference, on the Skydrive core reference (which includes a list of supported
file types), and in the PhotoSky sample. A backgrounder on this and other Windows Live services can
also be found on the Building Windows 8 blog post entitled Extending "Windows 8" apps to the cloud
with SkyDrive.
By now you probably have a number of other questions forming in your mind about how roaming
actually works: “How often is data synchronized?” “How do I manage different versions?” “What else
should I know?” These are good questions, and fortunately there are good answers!
• Assuming there’s network connectivity, an app’s roaming state is roamed within 30 minutes on
an active machine. It’s also roamed immediately when the user logs on or locks the machine.
Locking the machine is always the best way to force a sync to the cloud. Note that if the cloud
service is only aware of the user (that is, a Microsoft account) having only one device,
synchronization with the cloud service happens only about once per day. When the service is
aware that the user has multiple machines, it begins synchronizing within the 30-minute period;
if the app is uninstalled on all but one machine, synchronization reverts to the longer period.
• When saving roaming state, you can write values whenever you like, such as when those settings
are changed. You don’t need to worry about writing settings as a group because Windows has a
built-in debounce period to combine changes together and reduce overall network traffic.
• If you have a group of settings that really must be roamed together, manage these as a
composite setting in your roamingSettings container.
• With files you create within the roamingFolder, these will not be roamed so long as you have the
file open for writing (that is, as long as you have an open stream). It’s a good idea to make sure
that all streams are closed when the app is suspended.
• Windows allows each app to have up to 8K worth of “high priority” settings that will be roamed
within one minute, thereby allowing apps on multiple devices to stay much more closely in sync.
To use this, create a single or composite setting in the root of your roamingSettings with the
name HighPriority—that is, roamingSettings.values["HighPriority"] (a container with this
name will roam normally). So long as you keep the size of this setting below 8K, it will be roamed
within a minute of being changed; if you exceed that size, it will be roamed with normal priority.
See Scenario 6 of the Application data sample for a demonstration.
• On a trusted PC, systemwide user settings like the Start page configuration are automatically
roamed independent of app. This also includes encrypted credentials saved by apps in the
credential locker; apps should never attempt to roam passwords. Apps that create secondary
tiles (as we’ll see in Chapter 13) can indicate whether such tiles should be copied to a new device
when the app is installed.
• When there are multiple app data versions in use by the same app (with multiple app versions, of
course), Windows will manage each version of the app data separately, meaning that newer app
data won’t be roamed to devices with apps that use older app data versions. In light of this, it’s a
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