Why would the Swift Movesense-API Subscription Callback Operation Be Nil? - swift

Question

I'm building a Swift integration with Movesense's api as found here: https://bitbucket.org/suunto/movesense-mobile-lib/src/master/IOS/
Following the examples given in the Movesense Showcase application, I've subscribed to the heart rate notifications:
let request = MovesenseRequest(resourceType: .heartRate, method: .subscribe,
parameters: nil)
let operation = device.sendRequest(request, observer: self)
I'm positive that my device is connected and working because I can retrieve energy and temperature information using .get methods without any problem.
The problem is that I don't receive any heart rate responses from the Movesense API. Stepping through the api, my I get stuck in the send request method where both the onCompletion and onEvent callbacks have an operation that is nil, so it dies without sending a response to my app.
internal func sendRequest(_ request: MovesenseRequest, serial: String,
observer: Observer) -> MovesenseOperation? {
guard let mds = self.mdsWrapper,
let jsonDecoder = self.jsonDecoder else {
let error = MovesenseError.integrityError("MovesenseConnection::sendRequest error.")
delegate?.onConnectionError(error)
return nil
}
let resourcePath = "\(serial)/\(request.path)"
let onCancel = {
switch request.method {
case .subscribe: mds.doUnsubscribe(resourcePath)
default: return
}
}
let operation = MovesenseOperationFactory.create(request: request,
observer: observer,
jsonDecoder: jsonDecoder,
onCancel: onCancel)
// Decode response with the MovesenseOperation instance
let onCompletion = { [connectionQueue, weak operation] (_ response: MDSResponse) in
dump(response)
guard let operation = operation else { return }
connectionQueue.async {
operation.handleResponse(status: response.statusCode, header: response.header,
data: response.bodyData)
}
}
switch request.method {
case .get: mds.doGet(resourcePath, contract: request.contract, completion: onCompletion)
case .put: mds.doPut(resourcePath, contract: request.contract, completion: onCompletion)
case .post: mds.doPost(resourcePath, contract: request.contract, completion: onCompletion)
case .del: mds.doDelete(resourcePath, contract: request.contract, completion: onCompletion)
case .unsubscribe: mds.doUnsubscribe(resourcePath)
case .subscribe:
let onEvent = { [connectionQueue, weak operation] (_ event: MDSEvent) in
dump(event)
guard let operation = operation else {
mds.doUnsubscribe(resourcePath)
return
}
connectionQueue.async {
operation.handleEvent(header: event.header,
data: event.bodyData)
}
}
mds.doSubscribe(resourcePath, contract: request.contract,
response: onCompletion, onEvent: onEvent)
}
return operation
}
I know the API works because the Movesense Showcase code in the repository works just fine, but I don't know what I missed in setting up the request so the operation wouldn't be nil every single time. What might cause that error?

Answer 1

I eventually found my own answer. The problem was with the weak reference. I wasn’t storing that operation anywhere, so when the callback was called, the operation was always nil because the reference was garbage collected. The solution was to store the operation returned by the subscribe request somewhere and then remove the reference when you unsubscribe. I personally used a map to store my references, but any method to preserve the reference would be fine.