Simple VC to retrieve all heart rate records in an iPhone and store them in CSV files

ViewController.swift

/*
 Steps:
 1. Create a new project using template: 'Single View Application'.
 2. Enable HealthKit capabilities for the app.
 3. Add these properties to 'Info.plist':
    - 'NSHealthShareUsageDescription'
    - 'NSHealthUpdateUsageDescription'
 4. Copy & paste the content of this file into 'ViewController.swift'.
 4. Run the app in your iPhone.
 5. Using Xcode, download the container for this app.
 6. Open the container. Inside folder 'Documents', there is a bunch of CSV
    files with all your heart rate records until the current moment. Each
    file is suffixed with the date of the records inside. Each record is a
    tuple with the following format:

    <Time when the record was saved>,<Heart rate in bpm>
 
 NOTICE: The timezone for all records is GMT.
 */

import HealthKit
import UIKit

class ViewController: UIViewController {

    let healthStore = HKHealthStore()
    let heartRateType = HKObjectType.quantityType(forIdentifier: .heartRate)!

    override func viewDidLoad() {
        super.viewDidLoad()

        if !HKHealthStore.isHealthDataAvailable() {
            print("Health data is NOT available")

            return
        }

        healthStore.requestAuthorization(toShare: nil,
                                         read: [heartRateType],
                                         completion: { [weak self] (success, error) in
                                            guard success else {
                                                print("Request auth. Error:", error!)

                                                return
                                            }

                                            print("Autherization granted")

                                            self?.queryHeartRateSamples()
        })
    }


    func queryHeartRateSamples() {
        let predicate = HKQuery.predicateForSamples(withStart: Date(timeIntervalSince1970: 0),
                                                    end: Date())
        let query = HKSampleQuery(sampleType: heartRateType,
                                  predicate: predicate,
                                  limit: HKObjectQueryNoLimit,
                                  sortDescriptors: nil,
                                  resultsHandler: { [weak self] (_, results, error) in
                                    guard results != nil else {
                                        print("Query. Error:", error!)

                                        return
                                    }

                                    print("Samples retrieved")

                                    self?.processHeartRateSamples(results as! [HKQuantitySample])
        })

        healthStore.execute(query)
    }

    func processHeartRateSamples(_ heartRateSamples: [HKQuantitySample]) {
        let gmtZone = TimeZone(abbreviation: "GMT")!

        var calendar = Calendar(identifier: .gregorian)
        calendar.timeZone = gmtZone

        let formatter = DateFormatter()
        formatter.timeZone = gmtZone
        formatter.dateFormat = "yyyy-MM-dd HH:mm:ss"

        var csv: FileHandle?
        var csvDate = Date(timeIntervalSince1970: 0)

        for sample in heartRateSamples {
            let order = calendar.compare(sample.startDate, to: csvDate, toGranularity: .day)
            if order != ComparisonResult.orderedSame {
                csv?.closeFile()

                csvDate = sample.startDate

                let csvURL = csvFileURLWithDate(csvDate)
                print("Build \(csvURL.lastPathComponent)")

                try? "Time,BPM\n".write(to: csvURL, atomically: false, encoding: .utf8)

                csv = try? FileHandle(forWritingTo: csvURL)
                csv?.seekToEndOfFile()
            }

            let date = sample.startDate
            let heartRate = sample.quantity.doubleValue(for: HKUnit(from: "count/min"))

            csv?.write("\(formatter.string(from: date)),\(heartRate)\n".data(using: .utf8)!)
        }

        csv?.closeFile()

        print("Done")
    }

    func csvFileURLWithDate(_ date: Date) -> URL {
        let dirs = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)
        let docURL = URL(fileURLWithPath: dirs.first!)

        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "yyyyMMdd"

        let filename = "heartRate-\(dateFormatter.string(from: date)).csv"

        return URL(fileURLWithPath: filename, relativeTo: docURL)
    }
}