如何比较CurrentLocation和GPS坐标，如果相等则显示消息？

Question

我正在尝试创建一个最终会在特定位置振动的应用程序。这周我刚开始学习Objective-C，发现这个教程对初始测试很有用：http://www.appcoda.com/how-to-get-current-location-iphone-user/

而不是振动，我正在测试它，以便标签在指定的坐标上变化。但是，当我运行应用程序时，它不会更改标签。在xCode的底部调试输出栏中，它显示了坐标，它们与我想要的坐标是一样的，但它并没有将标签"For Latitude Value“更改为"Success”。下面是当前的代码：

//  ViewController.m

#import "ViewController.h"
@import CoreLocation;

@interface ViewController ()<CLLocationManagerDelegate>
@property (strong, nonatomic) CLLocationManager *locationManager;
@property (strong, nonatomic) CLGeocoder *geocoder;
@property (strong, nonatomic) CLPlacemark *placemark;

@end

@implementation ViewController {
CLLocationManager *locationManager;
}

- (void)requestAlwaysAuthorization{

}

 - (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    locationManager = [[CLLocationManager alloc] init];
    if ([locationManager respondsToSelector:@selector(requestAlwaysAuthorization)]) {
    [locationManager requestAlwaysAuthorization];
    }
}

- (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray *)locations
{
NSLog(@"%@", [locations lastObject]);
}

- (IBAction)getCurrentLocation:(id)sender {
locationManager.delegate = self;
locationManager.desiredAccuracy = kCLLocationAccuracyBest;
[locationManager startUpdatingLocation];
}

#pragma mark - CLLocationManagerDelegate

- (void)locationManager:(CLLocationManager *)manager didFailWithError:(NSError *)error
{
    NSLog(@"didFailWithError: %@", error);
UIAlertView *errorAlert = [[UIAlertView alloc]
                           initWithTitle:@"Error" message:@"Failed to Get Your Location" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];
[errorAlert show];
}

- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation
{
NSLog(@"didUpdateLocations: %@", newLocation);
CLLocation *currentLocation = newLocation;

if ((currentLocation.coordinate.longitude == 51.50998000)&&(currentLocation.coordinate.latitude == -0.13370000)) {
    self.longitudeLabel.text = [NSString stringWithFormat:@"success"];
}
}

- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}

@end

如果有帮助的话，这里是.h：

//  ViewController.h

#import <UIKit/UIKit.h>
#import <AudioToolbox/AudioToolbox.h>
#import <CoreLocation/CoreLocation.h>

@interface ViewController : UIViewController <CLLocationManagerDelegate>

@property (weak, nonatomic) IBOutlet UILabel *latitudeLabel;
@property (weak, nonatomic) IBOutlet UILabel *longitudeLabel;
@property (weak, nonatomic) IBOutlet UILabel *addressLabel;
- (IBAction)getCurrentLocation:(id)sender;

@end

我也尝试过这样做：

if (currentLocation != nil) {
    longitudeLabel.text = [NSString stringWithFormat:@"%.8f", currentLocation.coordinate.longitude];
    latitudeLabel.text = [NSString stringWithFormat:@"%.8f", currentLocation.coordinate.latitude];
}

就像他们在教程中所做的那样，它应该更改标签以显示GPS坐标，但它们也不会更改。同样，它只是在调试栏中显示GPS坐标。

有什么办法解释为什么这个不起作用吗？如果这个问题已经在某处被报道了，我很抱歉，我已经花了几个小时在网上搜索和尝试解决方案，但由于我是新手，我可能不知道要搜索的正确术语。任何建议都是非常感谢的！

Answer 1

我不建议为了相等而比较两个双精度值。从理论上讲，它们永远不会相等。一种更好的方法是检查该位置是否足够接近您的参考位置。下面是我的代码库中的一个示例。这是我在实用函数中使用的一个C文件。在这里，它处理浮点值，但您也可以对双精度值执行相同的操作。

int relativelyClose(CGFloat value, CGFloat reference, CGFloat precision) 
{
    return fabs(value - reference) / reference < range;
}

int relativelyClosePoints(CGPoint point, CGPoint r, CGFloat precision) 
{
    CGFloat dx = point.x -r.x;
    CGFloat dy = point.y-r.y;
    CGFloat distance = sqrt(dx*dx + dy*dy);
    CGFloat reflength = sqrt(r.x*r.x + r.y*r.y);
    return (distance < reflength * precision);
}

此外，为了确保设置标签值是在主线程上执行的，您可以像这样包装这些调用：

dispatch_async(dispatch_get_main_queue(), ^ {
    longitudeLabel.text = ...;
    latitudeLabel.text = ....;
});