Sharing UserDefaults between extensions

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You cannot use UserDefaults.standard to share data between a host app and its app extension. You instead have to create a shared container with UserDefaults(suiteName:) to share data.

Even though an app extension bundle is nested within its containing
app’s bundle, the running app extension and containing app have no
direct access to each other’s containers.

To enable data sharing, use Xcode or the Developer portal to enable
app groups for the containing app and its contained app extensions.
Next, register the app group in the portal and specify the app group
to use in the containing app.

After you enable app groups, an app extension and its containing app
can both use the NSUserDefaults API to share access to user
preferences. To enable this sharing, use the initWithSuiteName: method
to instantiate a new NSUserDefaults object, passing in the identifier
of the shared group.

For more, refer to: https://developer.apple.com/library/content/documentation/General/Conceptual/ExtensibilityPG/ExtensionScenarios.html#//apple_ref/doc/uid/TP40014214-CH21-SW1

How to use App Groups: https://github.com/pgpt10/Today-Widget

Standard or SuiteName?

Use standard one for data that is only for Host App. Use suiteName for data that you want to share between Extension and Host App. Just don’t persist the same data in both of them. Avoid data redundancy. Use both of them according to the context.

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