watchValueForKeyPath不起作用

Question

這是我第一次在iOS項目中使用KVO，我想通過GPS定位后觀察兩個UILabel的變化，並不斷通知我。 當我運行項目時，我得到了2個標簽，它們顯示了日期，並且它們都隨着時間而變化，但是observeValueForKeyPath不起作用。

@implementation GpsViewController{
    CLLocationManager *locationManager;
    CLLocation *crnLoc;
}

@synthesize gpsView;
@synthesize gpsModel;

- (void)viewDidLoad 
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    gpsView = [[GpsView alloc] initWithFrame:CGRectMake(0, 0, widthtScreen,heightScreen)];
    [self.view addSubview:gpsView];
    [self initLocation];
}

- (void)initLocation 
{
    locationManager = [[CLLocationManager alloc] init];
    locationManager.delegate = self;
    locationManager.desiredAccuracy = kCLLocationAccuracyBest;
    [locationManager requestWhenInUseAuthorization];
    [locationManager requestAlwaysAuthorization];
    [locationManager startUpdatingLocation];
}

- (void)locationManager:(CLLocationManager *)manager didFailWithError:(NSError *)error
{
    [self showAlert];
}

- (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray *)locations
{
    crnLoc = [locations lastObject];
    NSLog(@"position long : %@",[NSString stringWithFormat:@"%.8f",crnLoc.coordinate.longitude]);
    NSLog(@"position lat : %@",[NSString stringWithFormat:@"%.8f",crnLoc.coordinate.latitude]);
    gpsView.longitudeLabel.text = [NSString stringWithFormat:@"%.8f",crnLoc.coordinate.longitude];
    [gpsView.longitudeLabel addObserver:self forKeyPath:@"long" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];
    gpsView.latitudeLabel.text = [NSString stringWithFormat:@"%.8f",crnLoc.coordinate.latitude];
    [gpsView.latitudeLabel addObserver:self forKeyPath:@"lat" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];
 }

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if([keyPath isEqualToString:@"long"]) {
        NSLog(@"observe1");
    }
    if([keyPath isEqualToString:@"lat"]) {
        NSLog(@"observe2");
    }
}

Answer 1

應該更像這樣：

[gpsView.longitudeLabel addObserver:self forKeyPath:@"text" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];

和在

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context;

您的對象是相應的標簽，因此您可以檢查

if object == gpsView.longitudeLabel

Answer 2

您不能只在forKeyPath內部創建任何字符串。 這確定了要觀察的變量的名稱，因此，如果要觀察文本的變化，可以將“ text”用作forKeyPath 。