仅在获取用户位置后提取 JSON 数据

Question

我在 viewDidLoad 上调用方法时遇到问题，但它不会返回任何数据。如果我将该方法添加到一个按钮，并等到绘制位置并对其进行缩放，则数据被正确拉出。我最初的想法是，一旦用户允许定位，它就会拉入数据，而不必等待点击按钮来调用方法。

代码如下：

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.mapView.delegate = self;
    [self.mapView setShowsUserLocation:YES];

    // Instantiating location object
    locationManager = [[CLLocationManager alloc] init];

    [locationManager setDelegate:self];

    // Setting some parameters for the location object
    [locationManager setDistanceFilter:kCLDistanceFilterNone];
    [locationManager setDesiredAccuracy:kCLLocationAccuracyBest];

    // THIS IS THE METHOD NOT WORKING UNLESS ADDED TO A BUTTON, AND WAITING 
    // FOR IT TO ZOOM, WHICH I ASSUME READ THE USERS PLOTTED LOCATION?
    [self queryGooglePlaces:@"food|restaurant"];

    // Do any additional setup after loading the view.
}
- (void)mapView:(MKMapView *)mv didAddAnnotationViews:(NSArray *)views
{
    MKCoordinateRegion region;
    region = MKCoordinateRegionMakeWithDistance(locationManager.location.coordinate, 10*METERS_PER_MILE, 10*METERS_PER_MILE);

    [mv setRegion:region animated:YES];
}
-(void)mapView:(MKMapView *)mapView regionDidChangeAnimated:(BOOL)animated {
    //Get the east and west points on the map so you can calculate the distance (zoom level) of the current map view.
    MKMapRect mRect = self.mapView.visibleMapRect;
    MKMapPoint eastMapPoint = MKMapPointMake(MKMapRectGetMinX(mRect), MKMapRectGetMidY(mRect));
    MKMapPoint westMapPoint = MKMapPointMake(MKMapRectGetMaxX(mRect), MKMapRectGetMidY(mRect));

    //Set your current distance instance variable.
    currentDist = MKMetersBetweenMapPoints(eastMapPoint, westMapPoint);

    //Set your current center point on the map instance variable.
    currentCentre = self.mapView.centerCoordinate;
}
-(void) queryGooglePlaces: (NSString *) googleType
{
    NSString *url = [NSString stringWithFormat:@"https://maps.googleapis.com/maps/api/place/search/json?location=%f,%f&radius=%@&types=%@&sensor=true&key=%@", currentCentre.latitude, currentCentre.longitude, [NSString stringWithFormat:@"%i", currentDist], googleType, kGOOGLE_API_KEY];

    NSURL *url2 = [NSURL URLWithString:[url stringByAddingPercentEscapesUsingEncoding: NSUTF8StringEncoding]];

    // Formulate the string as an URL object
    NSURL *googleRequestURL = url2;

    // Retrieve the results of the URL
    dispatch_async(kBgQueue, ^{
        NSData *data = [NSData dataWithContentsOfURL: googleRequestURL];
        [self performSelectorOnMainThread:@selector(fetchedData:)withObject:data waitUntilDone:YES];
    });
}
-(void)fetchedData:(NSData *)responseData {
    //parse out the json data
    NSError* error;
    NSDictionary* json = [NSJSONSerialization
                          JSONObjectWithData:responseData

                          options:kNilOptions
                          error:&error];

    //The results from Google will be an array obtained from the NSDictionary object with the key "results".
    NSArray* places = [json objectForKey:@"results"];

    //Write out the data to the console.
    NSLog(@"Google Data: %@", places);
}

Answer 1

我猜currentCentre 的值在 viewDidLoad 时间为零，因此缺乏食物和餐馆。

你将不得不等待你的用户位置通过钩子返回一个有效值

- (void)mapView:(MKMapView *)amapView didUpdateUserLocation:(MKUserLocation *)userLocation

可能

- (void)mapView:(MKMapView *)amapView didUpdateUserLocation:(MKUserLocation *)userLocation {

  currentCentre = userLocation.location.coordinate;

  if(thisIsNotTheFirstTime == NO) {

     [self queryGooglePlaces:@"food|restaurant"];
     thisIsNotTheFirstTime = YES;
  }

}

thisIsNotTheFirstTime 是一个 ivar。如果你想消除双重否定，你必须明确地初始化它。