title: In Swift, does a protocol extension allow function bodies?
body: <p>I'm going through a tutorial and I noticed that the author extended their protocol called Activity and wrote the function's body in their code.  This does compile however I was under the impression that protocols only show method signatures or if it does implement the body then it'll be a mutating function.  The code below doesn't use mutating on one of its functions but it still runs and WORKS!  Can someone explain the phenomena or confirm that protocol extensions can have method bodies?</p>
  
  <pre><code>import CareKit
  import SwiftyJSON
  
  enum ActivityType: String {
          case Intervention
          case Assessment
  }
  
  enum ScheduleType: String {
      case Weekly
      case Daily
  
  }
  
  enum StepFormat : String {
      case Scale
      case Quantity
  }
  
  protocol Activity {
  
      var identifier : String  { get set}
      var groupIdentifier : String  { get set}
      var title : String  { get set}
      var colour : UIColor?  { get set}
      var text : String  { get set}
      var startDate : Date  { get set}
      var schedule : [NSNumber]  { get  set}
      var scheduleType : ScheduleType  { get set}
      var instructions : String?   { get set}
      var imageURL : NSURL?   { get set}
      var activityType: ActivityType  { get set}
      var medication : Medication?  { get set}
  
      init()
      init(json: JSON)
      func createCareKitActivity() -&gt; OCKCarePlanActivity
  
  }
  
  extension Activity {
  
      //  A mutating function to allow Acticities or Assessments to intialiser base properties
      mutating func parseActivityFields(json: JSON) {
          self.identifier = json["identifier"].string!
          self.groupIdentifier = json["group_identifier"].string!
          self.title = json["title"].string!
          self.text = json["text"].string!
  
          let colourString = json["color"].string!
          self.colour = UIColor.colorWithString(colourString)
  
          if let instructionString = json["instructions"].string {
              self.instructions = instructionString
          }
  
          if let imageString = json["imageURL"].string {
              let componentsOfString = imageString.components(separatedBy: ".")
  
              if let pathForResource = Bundle.main.path(forResource: componentsOfString[0], ofType: componentsOfString[1]){
                  self.imageURL = NSURL(fileURLWithPath: pathForResource)
              }
          }
  
          self.startDate = dateFromString(string: json["startdate"].string!)!
          self.scheduleType = ScheduleType(rawValue: json["scheduletype"].string!)!
  
          self.schedule = json["schedule"].string!.components(separatedBy: ",").map ( {
              NSNumber(value: Int32($0)!)
          })
  
          if let medication = json["medication"].string,
              let medicationImageString = json["medicationimage"].string {
  
              let componentsOfString = medicationImageString.components(separatedBy: ".")
              let pathForResource = Bundle.main.path(forResource: componentsOfString[0], ofType: componentsOfString[1])
  
              self.medication = Medication.init(medication: medication, imageURL: NSURL.init(fileURLWithPath: pathForResource!))
          }
  
      }
  
      init(json: JSON) {
  
          self.init()
  
          self.parseActivityFields(json: json)
  
      }
  
  
      func createCareKitActivity() -&gt; OCKCarePlanActivity{
  
          //creates a schedule based on the internal values for start and end dates
          let startDateComponents = NSDateComponents(date: self.startDate, calendar: NSCalendar(calendarIdentifier: NSCalendar.Identifier.gregorian)! as Calendar)
  
          let activitySchedule: OCKCareSchedule!
  
          switch self.scheduleType {
          case .Weekly :
              activitySchedule = OCKCareSchedule.weeklySchedule(withStartDate: startDateComponents as DateComponents, occurrencesOnEachDay: self.schedule)
  
          case .Daily:
              activitySchedule = OCKCareSchedule.dailySchedule(withStartDate: startDateComponents as DateComponents, occurrencesPerDay: self.schedule[0].uintValue)
  
          }
  
          let activity = OCKCarePlanActivity.intervention(
              withIdentifier: identifier,
              groupIdentifier: nil,
              title: title,
              text: text,
              tintColor: colour,
              instructions: instructions,
              imageURL: imageURL as? URL,
              schedule: activitySchedule,
              userInfo: ["medication": medication], optional: false)
  
          return activity
  
      }
  }
  </code></pre>