什么时候在Swift中调用mapView：viewForAnnotation？

但是下面的代码只调用一次mapView：viewForAnnotation.我有几个注释,但“视图调用”只打印一次.我想这与线程有关？

import UIKit
import MapKit
import CoreLocation

class ViewController: UIViewController,UITextFieldDelegate,MKMapViewDelegate,CLLocationManagerDelegate {

    @IBOutlet var searchtext: UITextField!
    @IBOutlet var map: MKMapView!

    var locationManager = CLLocationManager()

    override func viewDidLoad() {
        super.viewDidLoad()

        self.searchtext.delegate = self

        locationManager.delegate = self
        self.map.delegate = self

        locationManager.desiredAccuracy = kCLLocationAccuracyBest
        locationManager.requestWhenInUseAuthorization()
        locationManager.startUpdatingLocation()
    }

    func textFieldDidBeginEditing(textField: UITextField!) {    //delegate method
        var allAnnotations = self.map.annotations
        self.map.removeAnnotations(allAnnotations)
    }

    func textFieldShouldReturn(textField: UITextField!) ->Bool {
        textField.resignFirstResponder()

        //...

        let session = NSURLSession.sharedSession()

        var task = session.dataTaskWithURL(url!,completionHandler: { (date,response,error) -> Void in
            if (error != nil) {
                println(error)
            }else {
                var placenum = 0
                //placenum is to see if all the places are in the visible rect. If so I will use showAnnotations to zoom (in) to the best-fitted view show them. If not,I will only show these in the visible rect
                for place in places {
                    //...
                    if (regionContains(self.map.region,coordinate)) {
                        placenum = placenum+1
                    }
                    self.map.addAnnotation(annotation)
                }

                if (placenum==places.count) {
                    self.map.showAnnotations(self.map.annotations,animated: true)
                }
            }

        })

        task.resume()

        return true
    }

    func locationManager(manager: CLLocationManager!,didUpdateLocations locations: [AnyObject]!) {

        //...
        self.map.setRegion(region,animated: false)

        locationManager.stopUpdatingLocation()
    }

    func mapView(mapView: MKMapView!,viewForAnnotation annotation: MKAnnotation!) -> MKAnnotationView! {
        println("view called")

        return nil
    }