swift - 什么时候是mapView :viewForAnnotation called in Swift?

Question

根据 this post每当您调用 addAnnotation 方法时，都会调用 mapView:viewForAnnotation

但是下面的代码只调用了一次mapView:viewForAnnotation。我有几个注释，但“ View 调用”只打印了一次。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
    }

Answer 1

简而言之，当注释落在 map View 的可见部分时，将调用 mapView(_:viewFor:)。即，(a) 注释已添加到 map View 并且它属于 region 或 (b) region 更改，使得以前不存在的注释现在可见。不用说，如果您设置 map View 的委托(delegate)(以编程方式或在 Interface Builder 中)，此方法也只会被调用。

顺便说一句，dataTask(with:completionHandler:) 的完成处理程序不会在主线程上调用。因此，任何 UI 更新都必须显式分派(dispatch)回主线程，例如

DispatchQueue.main.async {
    for place in places {
        //...
        placenum = placenum + 1
        self.map.addAnnotation(annotation)
    }

    self.map.showAnnotations(self.map.annotations, animated: true)
}

我建议确保与 map View 的交互发生在主线程上，如上所示。

顺便说一句，请记住，如果您在 map 上显示用户位置，这本身会导致 mapView(_:viewFor:) 被调用。因此，如果您看到它只被调用一次，您可能需要确认 annotation 是 MKUserLocation 还是您添加的注释之一。