I have a scenario where I need to retrieve multiple sets of data from HealthKit -- body temperature, weight, and blood pressure. I need all 3 before I can continue processing because they're going to end up in a PDF.
My naive first approach is going to be run one, then in the HKSampleQuery's resultsHandler call the second, then in that resultsHandler call the third. That feels kind of -- I don't know -- it feels like I'm missing something.
Is there a better way or is the naive approach reasonable?
You should try to run the queries in parallel for better performance. In the completion handler for each one, call a common function that notes a query has completed. In that common function, when you determine that all of the queries have finished then you can proceed to the next step.
One simple approach to tracking the completion of the queries in the common function is to use a counter, either counting up from zero to the number of queries, or down from the number of total queries to zero.
Since HealthKit query handlers are called on an anonymous background dispatch queue, make sure you synchronize access to your counter, either by protecting it with a lock or by modifying the counter on a serial dispatch queue that you control, such as the main queue.