RUMM-2299 Core Context Application State Publisher

[maxep]

What and why?

Publish application state to Datadog Context.

How?

Implement a AppStatePublisher that complies to ContextValuePublisher and provide AppStateHistory to the context.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference
• Add CHANGELOG entry for user facing changes

Custom CI job configuration (optional)

• Run unit tests
• Run integration tests
• Run smoke tests

[ncreated]

Awesome 🚀. I left few non-blocking feedbacks 🙌.

[ncreated]

nit/ The more often I see this in different publishers, the more I think it should be called startPublishing(to:) to better express what it should do 💭. Just a note, I'm totally fine with keeping existing name (after all, it is well explained in the protocol comment).

[maxep]

That would work, current name also correctly reflect what it should do, at the protocol level, IMO. publisher.publish(to receiver:) Lets keep it as is for now 👍

[ncreated]

nit/ As we have a nice doc string now (🏅), I think it's worth adding in that it needs to be called on the main thread, because it calls application. applicationState transitively:

private static var currentApplicationState: UIApplication.State {
   UIApplication.managedShared?.applicationState ?? .active // fallback to most expected state
}

WDYT?

Suggested change

	/// Creates a Application state publisher for publishing application state
	/// history.
	///
	/// - Parameters:
	/// - initialState: The initial application state.
	/// - queue: The queue for publishing the history.
	/// - dateProvider: The date provider for the Application state snapshot timestamp.
	/// - notificationCenter: The notification center where this publisher observes `UIApplication` notifications.
	init(
	initialState: AppState,
	queue: DispatchQueue = AppStatePublisher.defaultQueue,
	dateProvider: DateProvider = SystemDateProvider(),
	notificationCenter: NotificationCenter = .default
	) {
	/// Creates a Application state publisher for publishing application state
	/// history.
	///
	/// **Note**: It must be called on the main thread.
	///
	/// - Parameters:
	/// - initialState: The initial application state.
	/// - queue: The queue for publishing the history.
	/// - dateProvider: The date provider for the Application state snapshot timestamp.
	/// - notificationCenter: The notification center where this publisher observes `UIApplication` notifications.
	init(
	initialState: AppState,
	queue: DispatchQueue = AppStatePublisher.defaultQueue,
	dateProvider: DateProvider = SystemDateProvider(),
	notificationCenter: NotificationCenter = .default
	) {

[maxep]

Oh indeed, but this should be documented in the extension below, the one that uses UIKit.
I didn't know about threading constraint on UIApplication/applicationState. This is a good argument to make the subscription/assignment thread-safe as you mentioned here so we can dispatch the ApplicationStatePublisher init and subscription on the main thread!

[ncreated]

Ah, indeed it has a consequence on subscriptions (array) thread-safety. Good call 👍.