尝试调用注释时出现NSInvalidArgumentException

Question

注释在地图上显示正常,但是当我点击一个时,我会引发此异常:

[NSNull length]: unrecognized selector sent to instance
'NSInvalidArgumentException', reason: '-[NSNull length]: unrecognized selector sent to instance 

我的相关代码是这样的:

- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation {

    static NSString *identifier = @"MyLocation";   
    if ([annotation isKindOfClass:[MyLocation class]]) {

        MKPinAnnotationView *annotationView = (MKPinAnnotationView *) [mapView2 dequeueReusableAnnotationViewWithIdentifier:identifier];
        if (annotationView == nil) {
            annotationView = [[MKPinAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:identifier];
        } else {
            annotationView.annotation = annotation;
        }

        annotationView.enabled = YES;
        annotationView.canShowCallout = YES;

        annotationView.pinColor = MKPinAnnotationColorRed;
        return annotationView;
    }

    return nil;    
}

并配置注释:

NSNumber * latitude = [[row objectAtIndex:21]objectAtIndex:1];
        NSNumber * longitude = [[row objectAtIndex:21]objectAtIndex:2];
        NSString * crimeDescription = [row objectAtIndex:17];
        NSString * address = [row objectAtIndex:13];

        CLLocationCoordinate2D coordinate;
        coordinate.latitude = latitude.doubleValue;
        coordinate.longitude = longitude.doubleValue;            
        MyLocation *annotation = [[MyLocation alloc] initWithName:crimeDescription address:address coordinate:coordinate] ;
        [mapView2 addAnnotation:annotation];

并且在MyLocation持有注释的类中:

- (id)initWithName:(NSString*)name address:(NSString*)address coordinate:(CLLocationCoordinate2D)coordinate {
    if ((self = [super init])) {
        _name = [name copy];
        _address = [address copy];
        _coordinate = coordinate;
    }
    return self;
}

Answer 1

当您点击注释视图时,它会尝试获取length注释的内容,title以便它知道调出的宽度.

您获得的异常表示该title注释的某种方式设置为NSNull对象而不是NSString.

一个NSNull对象不具有length这样你会得到"无法识别的选择"异常的方法.

您还没有展示如何title在MyLocation类中实现,但我假设它在创建注释时返回_name您正在设置的crimeDescription(来自row数组).

不知何故,row数组中的值有一个NSNull而不是一个NSString.

如果你不需要的NSNull值,你可以更改设置代码row数组值,因此不会把NSNullS(它可以把NSString好像@"Unknown"代替).

另一种选择是,你可以修改getter方法为title在MyLocation类,并将它返回一个"未知" NSString,如果_name是的NSNull.

例如:

-(NSString *)title
{
    if ([_name isKindOfClass:[NSNull class]]) 
        return @"Unknown";
    else
        return _name;
}