objective-c MKMapView center on user location

问题:

I'm trying to zoom into the user location as the center reference for the screen. I have this code:

MainViewController.h

#import <UIKit/UIKit.h>
#import "FlipsideViewController.h"
#import <MapKit/MapKit.h>
#import <CoreLocation/CoreLocation.h>

IBOutlet MKMapView *mapView;

@interface MainViewController : UIViewController <FlipsideViewControllerDelegate, MKMapViewDelegate> {
   MKMapView *mapView;
}
@property (nonatomic, retain) IBOutlet MKMapView *mapView;

MainViewController.m

@implementation MainViewController
@synthesize mapView;

 - (void)viewDidLoad {
    [super viewDidLoad];
    mapView = [[MKMapView alloc]
           initWithFrame:self.view.bounds
           ];
    mapView.showsUserLocation = YES;
    mapView.mapType = MKMapTypeHybrid;
    mapView.delegate = self;
    [self.view addSubview:mapView];
}

- (void)mapView:(MKMapView *)mapView didUpdateUserLocation:(MKUserLocation *)userLocation {
    MKCoordinateRegion region;
    MKCoordinateSpan span;
    span.latitudeDelta = 0.005;
    span.longitudeDelta = 0.005;
    CLLocationCoordinate2D location;
    location.latitude = userLocation.coordinate.latitude;
    location.longitude = userLocation.coordinate.longitude;
    region.span = span;
    region.center = location;
    [mapView setRegion:region animated:YES];
 }

Now I'm only getting a build warning on the last line [mapView setRegion:region animated:YES] stating: 'local declaration of 'mapView' hides instance variable'

回答1:

When you do mapView.showsUserLocation = YES;, you ask it to retrieve the user location. This doesn't happen instantly. As it takes time, the map view notifies its delegate that a user location is available via the delegate method mapView:didUpdateUserLocation. So you should adopt the MKMapViewDelegate protocol and implement that method. You should move all your zooming-in code to this method.

Setting the delegate

- (void)viewDidLoad {
    [super viewDidLoad];
    mapView = [[MKMapView alloc]
           initWithFrame:CGRectMake(0, 
                                    0,
                                    self.view.bounds.size.width, 
                                    self.view.bounds.size.height)
           ];
    mapView.showsUserLocation = YES;
    mapView.mapType = MKMapTypeHybrid;
    mapView.delegate = self;
    [self.view addSubview:mapView];
}

Updated delegate method

- (void)mapView:(MKMapView *)aMapView didUpdateUserLocation:(MKUserLocation *)aUserLocation {
    MKCoordinateRegion region;
    MKCoordinateSpan span;
    span.latitudeDelta = 0.005;
    span.longitudeDelta = 0.005;
    CLLocationCoordinate2D location;
    location.latitude = aUserLocation.coordinate.latitude;
    location.longitude = aUserLocation.coordinate.longitude;
    region.span = span;
    region.center = location;
    [aMapView setRegion:region animated:YES];
}

回答2:

In your interface you forgot to inherit MapViewDelegate -

#import <MapKit/MapKit.h>

@interface MainViewController : UIViewController <FlipsideViewControllerDelegate, MKMapViewDelegate> 
{
   MKMapView *mapView;
}
@property (nonatomic, retain) IBOutlet MKMapView *mapView;

Rest seems fine.