How can I limit user actions based on their proximity to a fixed point in iOS?

I 开发者_运维问答guess what I am trying to do should be simple, but don’t know how to search for it!

What I wanna do is: the user will vote for a music, but the user can only vote if he is 2km away from a fixed location (lat,lon).

Can you suggest some links that can help me implement this?!

Cheers.

Something like this...

@interface YourLocationViewController : UIViewController <CLLocationManagerDelegate> 
CLLocationManager *locationManager;

...

/**
 * Start tracking updates for location. 
 * Call this from viewLoad or something.
 */
-(void) trackUpdates {

    self.locationManager = [[[CLLocationManager alloc] init] autorelease];
    self.locationManager.delegate = self;

    /* Pinpoint our location with the following accuracy:
     *
     *     kCLLocationAccuracyBestForNavigation  highest + sensor data
     *     kCLLocationAccuracyBest               highest     
     *     kCLLocationAccuracyNearestTenMeters   10 meters   
     *     kCLLocationAccuracyHundredMeters      100 meters
     *     kCLLocationAccuracyKilometer          1000 meters 
     *     kCLLocationAccuracyThreeKilometers    3000 meters
     */
    self.locationManager.desiredAccuracy = kCLLocationAccuracyNearestTenMeters;

    /* Notify changes when device has moved x meters.
     * Default value is kCLDistanceFilterNone: all movements are reported.
     */
    self.locationManager.distanceFilter = 10.0f;

    // update location
    if ([CLLocationManager locationServicesEnabled]){
        [self.locationManager startUpdatingLocation];
    }
}

- (void)locationManager:(CLLocationManager *)manager 
    didUpdateToLocation:(CLLocation *)newLocation
           fromLocation:(CLLocation *)oldLocation {
    CLLocationDistance meters = [newLocation distanceFromLocation:fixedPoint];
    if (meters>2000){
        // drink a shot
    }
}