CMSensorRecorder data reliably flowing to Kinesis

I've refactored the iPhone side of the code a bit to better represent background processing of the sensor data. The good thing is that WCSession:sendMessage handles iPhone background processing properly. This is at the expense of having to handle reachability errors in code. The checkpoint of code is here.

Now the flow is roughly:

• On Watch
• CMSensorRecorder is activated and is recording records locally regardless of the application state of the Watch
• When the sensor application is in the foreground, a thread attempts to dequeue data form the recorder
• And when this data is received a WCSession:sendMessage is used to send the data to the iPhone in the background
• Iff a valid reply comes back from this message, the CMSensorRecorder fetch position is updated to fetch the next unprocessed sensor data
• On iPhone
• A background thread is always ready to receive messages from the Watch
• Those messages are saved to a local Kinesis queue
• A timer based flush will submit this Kinesis queue to AWS
• AWS credentials from Cognito are now manually refreshed by checking the credentials expire time
• The send and submit kinesis calls are now asynchronous tasks

So this is pretty close to continuous feed on the iPhone side.

Some areas of durability to re-explore next:

• How to build a Watch dequeue that can run when the application isn't in foreground?
• Is there another way for WCSession to send to a background task other than sendMessage?
• How reliable is the sendMessage call?
• When the iPhone is out of range
• When the iPhone is locked
• When it is busy running another application
• I do see some transient 'not paired' exceptions when sending volume
• While this does allow for automatic background processing, is there a simpler way of transferring data that doesn't require the application handling reachability errors?
• How reliable is the Kinesis send-retry when the iPhone can't reach AWS?

I will next be building more quantitative checks of the actual data sent through the system to understand where data get sent more than once, or where it is lost.