ResearchKit activities controller managing a survey.

## ActivitiesViewController.swift
//
//  ActivitiesViewController.swift
//  MyStudyApp
//
//  Created by Vincent Tourraine on 2/8/16.
//  Copyright © 2016 Shazino. All rights reserved.
//

import UIKit
import ResearchKit


class ActivitiesViewController: UIViewController, ORKTaskViewControllerDelegate {

    let QuestionStepIdentifier = "yes-no-step"

    @IBAction func startSurvey(sender: UIButton) {
        let introStep = ORKInstructionStep(identifier: ORKInstruction0StepIdentifier)
        introStep.title = NSLocalizedString("Basic Survey", comment: "")
        introStep.text  = NSLocalizedString("This is a basic survey.", comment: "")

        let step = ORKQuestionStep(identifier: QuestionStepIdentifier)
        step.title = NSLocalizedString("Do you feel good?", comment: "")
        step.answerFormat = ORKBooleanAnswerFormat()

        let completionStep = ORKOrderedTask.makeCompletionStep()

        let task = ORKOrderedTask(identifier: "yes-no-task", steps:[introStep, step, completionStep])

        let viewController = ORKTaskViewController(task: task, taskRunUUID: nil)
        viewController.delegate = self
        presentViewController(viewController, animated:true, completion:nil)
    }


    // MARK: Task view controller delegate

    func taskViewController(taskViewController: ORKTaskViewController, didFinishWithReason reason: ORKTaskViewControllerFinishReason, error: NSError?) {
        print("Task run UUID: \(taskViewController.taskRunUUID.UUIDString)")

        if reason == .Completed {
            if let stepResult = taskViewController.result.stepResultForStepIdentifier(QuestionStepIdentifier),
                let stepResults = stepResult.results,
                let stepFirstResult = stepResults.first,
                let booleanResult = stepFirstResult as? ORKBooleanQuestionResult,
                let booleanAnswer = booleanResult.booleanAnswer {
                    print("Result for question: \(booleanAnswer.boolValue)")
            }
        }

        let durationFormatter = NSNumberFormatter()
        durationFormatter.maximumFractionDigits = 2

        if let results = taskViewController.result.results {
            for result in results {
                guard let startDate = result.startDate,
                let endDate = result.endDate
                    else { continue }

                let duration = endDate.timeIntervalSinceDate(startDate)
                let formattedDuration = durationFormatter.stringFromNumber(duration)
                print("Duration for step “\(result.identifier)”: \(formattedDuration!) s")
            }
        }

        dismissViewControllerAnimated(true, completion: nil)
    }
}