Try to reduce time taken to get a trackable online

I noticed that on a stationary device, or on a simulator set to simulate
a static location, the first trackable added would come online almost
immediately, but subsequent ones would not come online or sometimes
would come online after a long delay.

This is because a trackable will not come online until, after the
trackable is added, we receive a location update from LocationService.
The frequency of location updates from PassiveLocationManager is
determined by that of CLLocationManager, which usually only emits
location updates when the device has moved a sufficient distance (where
“sufficient” is determined by the publisher’s resolution policy).

So, we provide a mechanism for the publisher to request a location
update from CLLocationManager independently of the device’s motion, and
we request a location update when any trackable (except for the first)
is added.

It’s probably worth also explaining how Android behaves in this scenario
and why. My notes here are based on the codebase at commit 3a0d833.

There are two different things that happen in the Android asset tracking
SDK, either of which has the side effect of causing the publisher to
receive a location update when a new trackable is added:

1. The publisher calls DefaultMapbox.changeResolution, which in turn
   ends up removing and re-adding all of (Google SDK)
   FusedLocationProviderClient’s location observers (via its
   requestLocationUpdates method). The documentation for
   FusedLocationProviderClient.requestLocationUpdates implies that
   observers will always receive a location update after this method is
   called (although it might be an old location).

2. (Only in the case where the added trackable is set as the active
   trackable – i.e. when it is added using .track and not .add)

   The publisher calls DefaultMapbox.clearRoute, which calls (Mapbox
   SDK) MapboxNavigation.setNavigationRoutes. Through some process that I
   haven’t looked into, this ends up calling (Mapbox SDK)
   MapboxTripSession.kt’s navigatorObserver’s onStatus, which makes an
   explicit call to updateLocationMatcherResult, which emits a location
   update.

I think that the solution I’ve implemented for iOS turns Android side
effect 1 into an explicit behaviour.

Closes #425.

Sources/AblyAssetTrackingPublisher/DefaultPublisher.swift

@@ -263,12 +263,18 @@ extension DefaultPublisher {
         ablyPublisher.subscribeForChannelStateChange(trackable: event.trackable)
 
         trackables.insert(event.trackable)
-        //Start updating location only after the first trackable
         if (trackables.count == 1){
+            //Start updating location only after the first trackable
             locationService.startRecordingLocation()
             locationService.startUpdatingLocation()
+        } else {
+            // We do this to increase the chances of receiving an enhanced location update for this trackable, so that the trackable can move into the online connection state as quickly as possible (see the hasSentAtLeastOneLocation check inside handleConnectionStateChange).
+
+            // If we did not do this, then (since startUpdatingLocation has already been called on the location service) in the case of a slowly-moving or stationary device, the trackable would not move into the online connection state until the device had moved a distance deemed worthy (by the resolution policy) of a new location update.
+            locationService.requestLocationUpdate()
         }
         resolveResolution(trackable: event.trackable)
+
         hooks.trackables?.onTrackableAdded(trackable: event.trackable)
         event.completion.handleSuccess()
         duplicateTrackableGuard.finishAddingTrackableWithId(event.trackable.id, result: .success)

Sources/AblyAssetTrackingPublisher/Services/DefaultLocationService.swift

@@ -72,6 +72,16 @@ class DefaultLocationService: LocationService {
         locationManager.systemLocationManager.stopUpdatingLocation()
     }
 
+    func requestLocationUpdate() {
+        /*
+         From reading the ``CLLocationManager/startUpdatingLocation`` documentation, this seems to be the only programmatic way to provoke it into emitting a location event:
+
+        > Calling this method several times in succession does not automatically result in new events being generated. Calling stopUpdatingLocation() in between, however, does cause a new initial event to be sent the next time you call this method.
+        */
+        locationManager.systemLocationManager.stopUpdatingLocation()
+        locationManager.systemLocationManager.startUpdatingLocation()
+    }
+
     enum LocationHistoryTemporaryStorageConfiguration {
         private static let fileManager = FileManager.default
 

Sources/AblyAssetTrackingPublisher/Services/LocationService.swift

@@ -19,4 +19,6 @@ protocol LocationService: AnyObject {
     func startRecordingLocation()
     func stopRecordingLocation(completion: @escaping ResultHandler<LocationRecordingResult?>)
     func changeLocationEngineResolution(resolution: Resolution)
+    /// Requests that the location service emit a new location update as soon as possible. The location service will make a best effort to ensure that, shortly after this method is called, its delegate receives a ``LocationServiceDelegate/locationService(sender:didUpdateRawLocationUpdate)`` and ``LocationServiceDelegate/locationService(sender:didUpdateEnhancedLocationUpdate)`` event.
+    func requestLocationUpdate()
 }

Tests/PublisherTests/DefaultPublisher/DefaultPublisherTests.swift

@@ -289,6 +289,39 @@ class DefaultPublisherTests: XCTestCase {
         XCTAssertEqual(publisher.activeTrackable, trackable)
     }
 
+    func testAddSecondTrackable_callsRequestLocationUpdateOnLocationService() {
+        ablyPublisher.connectCompletionHandler = { completion in  completion?(.success) }
+        let expectation = XCTestExpectation()
+        expectation.expectedFulfillmentCount = 2
+
+        // When tracking a trackable
+        publisher.add(trackable: trackable) { result in
+            switch result {
+            case .success:
+                expectation.fulfill()
+            case .failure:
+                XCTFail("Failure callback shouldn't be called")
+            }
+        }
+
+        // It should not yet have called requestLocationUpdate on the location service
+        XCTAssertFalse(locationService.requestLocationUpdateCalled)
+
+        // And then adding another trackable
+        publisher.add(trackable: Trackable(id: "TestAddedTrackableId1")) { result in
+            switch result {
+            case .success:
+                expectation.fulfill()
+            case .failure:
+                XCTFail("Failure callback shouldn't be called")
+            }
+        }
+        wait(for: [expectation], timeout: 5.0)
+
+        // It should call requestLocationUpdate on the location service
+        XCTAssertTrue(locationService.requestLocationUpdateCalled)
+    }
+
     func testAdd_track_error() {
         let errorInformation = ErrorInformation(type: .publisherError(errorMessage: "Test AblyPublisherService error"))
         ablyPublisher.connectCompletionHandler = { completion in  completion?(.failure(errorInformation)) }

Tests/Support/AblyAssetTrackingPublisherTesting/Mocks/MockLocationService.swift

@@ -40,4 +40,9 @@ public class MockLocationService: LocationService {
         stopRecordingLocationParamCompletion = completion
         stopRecordingLocationCallback?(completion)
     }
+
+    public var requestLocationUpdateCalled = false
+    public func requestLocationUpdate() {
+        requestLocationUpdateCalled = true
+    }
 }