Calculation methods call completion handlers on inconsistent queues

[1ec5]

As of #382, sometimes calculate(_:completionHandler:) and calculateRoutes(matching:completionHandler:) call the completion handler on the URLSession delegate queue:

mapbox-directions-swift/Sources/MapboxDirections/Directions.swift

Lines 160 to 173 in be90d21

	guard let response = possibleResponse, ["application/json", "text/html"].contains(response.mimeType) else {
	completionHandler(nil, nil, .invalidResponse)
	return
	}
	guard let data = possibleData else {
	completionHandler(nil, nil, .noData)
	return
	}
	if let error = possibleError {
	completionHandler(nil, nil, .unknown(response: possibleResponse, underlying: error, code: nil, message: nil))
	return
	}

whereas sometimes these methods call the completion handler on the main queue:

mapbox-directions-swift/Sources/MapboxDirections/Directions.swift

Lines 181 to 183 in be90d21

	DispatchQueue.main.async {
	completionHandler(nil, nil, apiError)
	}

mapbox-directions-swift/Sources/MapboxDirections/Directions.swift

Lines 196 to 198 in be90d21

	DispatchQueue.main.async {
	completionHandler(result.waypoints, routes, nil)
	}

mapbox-directions-swift/Sources/MapboxDirections/Directions.swift

Lines 200 to 203 in be90d21

	DispatchQueue.main.async {
	let bailError = Directions.informativeError(code: nil, message: nil, response: response, underlyingError: error)
	completionHandler(nil, nil, bailError)
	}

The methods should consistently call the completion handler from a single queue, and we should document which queue that is. Previously, in v0.30.0, these methods always called the completion handler on the main queue:

mapbox-directions-swift/Sources/MapboxDirections/MBDirections.swift

Lines 257 to 259 in 65857d1

	DispatchQueue.main.async {
	errorHandler(apiError)
	}

mapbox-directions-swift/Sources/MapboxDirections/MBDirections.swift

Lines 263 to 265 in 65857d1

	DispatchQueue.main.async {
	completionHandler(json)
	}

As it is, a developer doesn’t know whether their completion handler implementation needs to dispatch to the main queue or stay on the current queue. So, for example, a completion handler that presents a UIAlertController would result in a crash like this:

#0	(null) in __exceptionPreprocess ()
#1	(null) in objc_exception_throw ()
#2	(null) in _AssertAutolayoutOnAllowedThreadsOnly ()
#3	(null) in -[NSISEngine withBehaviors:performModifications:] ()
#4	(null) in -[UIView(UIConstraintBasedLayout) _calculatedSystemLayoutSizeFittingSize:withHorizontalFittingPri... ()
#5	(null) in -[UIView(AdditionalLayoutSupport) _systemLayoutSizeFittingSize:withHorizontalFittingPriority:vert... ()
#6	(null) in -[_UIAlertControllerView _minimumSizeForAllActions] ()
#7	(null) in -[_UIAlertControllerView _itemSizeForHorizontalLayout:visualStyleRequiresActionRepresentationToFi... ()
#8	(null) in -[_UIAlertControllerView _canLayOutActionsHorizontally] ()
#9	(null) in -[_UIAlertControllerView _actionLayoutDirectionChanged] ()
#10	(null) in -[_UIAlertControllerView _updateStyleForIdiomChange:] ()
#11	(null) in -[_UIAlertControllerView _setVisualStyle:] ()
#12	(null) in -[UIAlertController loadView] ()
#13	(null) in -[UIViewController loadViewIfRequired] ()
#14	(null) in -[UIViewController view] ()
#15	(null) in -[UIViewController _setPresentationController:] ()
#16	(null) in -[UIViewController _presentViewController:modalSourceViewController:presentationController:animat... ()
#17	(null) in -[UIViewController _presentViewController:withAnimationController:completion:] ()
#18	(null) in __63-[UIViewController _presentViewController:animated:completion:]_block_invoke ()
#19	(null) in -[UIViewController _performCoordinatedPresentOrDismiss:animated:] ()
#20	(null) in -[UIViewController _presentViewController:animated:completion:] ()
#21	(null) in -[UIViewController presentViewController:animated:completion:] ()

/cc @mapbox/navigation-ios

[1ec5]

Oh cool, I hadn’t noticed #410, which fixes this issue.

[1ec5]

Fixed in #410.