如何計算用戶移動時從用戶位置到註釋的距離

Question

我有一個用戶位置（藍點）和註釋mapView。當選擇註釋時，我將文本設置爲distLabel - 「距離點4.0％m的距離」。如何在用戶移動時更新該文本標籤？

didSelectAnnotationView：

- (void)mapView:(MKMapView *)mapView didSelectAnnotationView:(MKAnnotationView *)view 
{ 

    CLLocation *pinLocation = 
     [[CLLocation alloc] initWithLatitude: 
         [(MyAnnotation*)[view annotation] coordinate].latitude 
           longitude: 
         [(MyAnnotation*)[view annotation] coordinate].longitude]; 
    CLLocation *userLocation = 
     [[CLLocation alloc] initWithLatitude: 
          self.mapView.userLocation.coordinate.latitude    
           longitude: 
           self.mapView.userLocation.coordinate.longitude];   
    CLLocationDistance distance = [pinLocation distanceFromLocation:userLocation]; 

    [distLabel setText: [NSString stringWithFormat:@"Distance to point %4.0f m.",  
                distance]]; 
} 

我知道有一個功能didUpdateToLocation，但我怎麼能與didSelectAnnotationView使用它呢？

- (void)locationManager:(CLLocationManager *)manager 
    didUpdateToLocation:(CLLocation *)newLocation 
      fromLocation:(CLLocation *)oldLocation 
{ 
    //Did update to location 
} 

Answer 1

地圖視圖具有selectedAnnotations屬性，您可以在didUpdateToLocation方法用來告訴從中獲取其標註的距離。

（順便說一下，如果你正在使用的地圖視圖的userLocation，您可能需要使用地圖視圖的didUpdateUserLocation委託方法，而不是didUpdateToLocation這是一個CLLocationManager委託方法。）

在委託方法，你可以檢查是否有任何當前選定的註釋，如果是，則顯示與該註釋的距離（否則稱爲「未選擇註釋」）。

您可能需要編寫一個可從didSelectAnnotationView和didUpdateUserLocation中調用的常用方法以減少代碼重複。

例如：

-(void)updateDistanceToAnnotation:(id<MKAnnotation>)annotation 
{ 
    if (annotation == nil) 
    { 
     distLabel.text = @"No annotation selected"; 
     return; 
    } 

    if (mapView.userLocation.location == nil) 
    { 
     distLabel.text = @"User location is unknown"; 
     return; 
    } 

    CLLocation *pinLocation = [[CLLocation alloc] 
     initWithLatitude:annotation.coordinate.latitude 
       longitude:annotation.coordinate.longitude]; 

    CLLocation *userLocation = [[CLLocation alloc] 
     initWithLatitude:mapView.userLocation.coordinate.latitude 
       longitude:mapView.userLocation.coordinate.longitude]; 

    CLLocationDistance distance = [pinLocation distanceFromLocation:userLocation]; 

    [distLabel setText: [NSString stringWithFormat:@"Distance to point %4.0f m.", distance]]; 
} 

-(void)mapView:(MKMapView *)mapView didUpdateUserLocation:(MKUserLocation *)userLocation 
{ 
    if (mapView.selectedAnnotations.count == 0) 
     //no annotation is currently selected 
     [self updateDistanceToAnnotation:nil]; 
    else 
     //first object in array is currently selected annotation 
     [self updateDistanceToAnnotation:[mapView.selectedAnnotations objectAtIndex:0]]; 
} 

- (void)mapView:(MKMapView *)mapView didSelectAnnotationView:(MKAnnotationView *)view 
{  
    [self updateDistanceToAnnotation:view.annotation]; 
}