如何将 MKOverlay 对象存储到文件中以便稍后检索

Question

我正在编写一个 iOS 应用程序来记录您的旅行路径，然后将其存储起来以便以后检索。大部分绘制路径的代码都是基于示例代码Breadcrumb。

现在我正在添加功能来保存绘制的叠加层。最好的方法是什么？我可以使用CoreData，但我不想对绘制的叠加层做太多事情，而不是稍后再次检索它。我目前尝试了一个简单的NSArray。我将 CrumbPath 对象转换为 NSData 并将其存储在数组中。后来我检索它并将其转换回 CrumbPath。但是，我似乎做错了什么。

@interface CrumbPath : NSObject <MKOverlay>
{
    MKMapPoint *points;
    NSUInteger pointCount;
    NSUInteger pointSpace;

    MKMapRect boundingMapRect;

    pthread_rwlock_t rwLock;
}

- (id)initWithCenterCoordinate:(CLLocationCoordinate2D)coord;
- (MKMapRect)addCoordinate:(CLLocationCoordinate2D)coord;

@property (readonly) MKMapPoint *points;
@property (readonly) NSUInteger pointCount;

@end

我像这样保存一个 CrumbPath 对象“crumbs”：

NSData *pointData = [NSData dataWithBytes:crumbs.points length:crumbs.pointCount * sizeof(MKMapPoint)];
[patternArray addObject:pointData];
[timeArray addObject:date];

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
// Create the full file path by appending the desired file name
NSString *patternFile = [documentsDirectory stringByAppendingPathComponent:@"patterns.dat"];
NSString *timeFile = [documentsDirectory stringByAppendingPathComponent:@"times.dat"];

//Save the array
[patternArray writeToFile:patternFile atomically:YES];
[timeArray writeToFile:timeFile atomically:YES];

稍后像这样检索它以显示在表格中：

NSData *pointData = [appDelegate.patternArray objectAtIndex:indexPath.row];
MKMapPoint *points = malloc(pointData.length);
(void)memcpy([pointData bytes], points, sizeof(pointData));

并再次构造路径：

crumbs = [[CrumbPath alloc] initWithCenterCoordinate:MKCoordinateForMapPoint(points[0])];
for (int i = 1; i <= pointCount; i++) {
    [crumbs addCoordinate:MKCoordinateForMapPoint(points[i])];
}    
[map addOverlay:crumbs];

但是，我收到一个错误：'NSInvalidArgumentException', reason: '-[NSConcreteData isEqualToString:]: unrecognized selector sent to instance

Answer 1

就个人而言，我倾向于这样做：

1. 首先，我倾向于保存我传递给MKPolygon 实例方法polygonWithCoordinates 的CLLocationCoordinate2D 坐标的C 数组（而不是MKMapPoint C 数组）。如果您更愿意使用MKMapPoints，您可以修改此代码，但我更喜欢一种我可以从外部检查并理解的格式（即CLLocationCoordinate2D 的纬度和经度值）。所以，让我们假设你用这样的一行代码来创建你的MKPolygon：

   MKPolygon* poly = [MKPolygon polygonWithCoordinates:coordinates count:count];
   

   因此，您可以像这样保存坐标：

   [self writeCoordinates:coordinates count:count file:filename];
   

   其中writeCoordinates:count:filename:定义如下：

   - (void)writeCoordinates:(CLLocationCoordinate2D *)coordinates count:(NSUInteger)count file:(NSString *)filename
   {
       NSData *data = [NSData dataWithBytes:coordinates length:count * sizeof(CLLocationCoordinate2D)];
       [data writeToFile:filename atomically:NO];
   }
   

   然后你可以从文件中创建MKPolygon：

   MKPolygon* poly = [self polygonWithContentsOfFile:filename];
   

   其中polygonWithContentsOfFile 定义为：

   - (MKPolygon *)polygonWithContentsOfFile:(NSString *)filename
   {
       NSData *data = [NSData dataWithContentsOfFile:filename];
       NSUInteger count = data.length / sizeof(CLLocationCoordinate2D);
       CLLocationCoordinate2D *coordinates = (CLLocationCoordinate2D *)data.bytes;
   
       return [MKPolygon polygonWithCoordinates:coordinates count:count];
   }
   

2. 或者，您可以将上述两个方法替换为：

   const NSString *kLatitudeKey = @"latitude";
   const NSString *kLongitudeKey = @"longitude";
   
   - (void)writeCoordinates:(CLLocationCoordinate2D *)coordinates count:(NSUInteger)count file:(NSString *)filename
   {
       NSMutableArray *array = [NSMutableArray arrayWithCapacity:count];
   
       for (NSUInteger i = 0; i < count; i++)
       {
           CLLocationCoordinate2D coordinate = coordinates[i];
           [array addObject:@{kLatitudeKey:@(coordinate.latitude), kLongitudeKey:@(coordinate.longitude)}];
       }
   
       [array writeToFile:filename atomically:NO];
   }
   
   - (MKPolygon *)polygonWithContentsOfFile:(NSString *)filename
   {        
       NSArray *array = [NSArray arrayWithContentsOfFile:filename];
       NSUInteger count = [array count];
       CLLocationCoordinate2D coordinates[count];
   
       for (NSUInteger i = 0; i < count; i++)
       {
           NSDictionary *dictionary = array[i];
           coordinates[i].latitude = [dictionary[kLatitudeKey] doubleValue];
           coordinates[i].longitude = [dictionary[kLongitudeKey] doubleValue];
       }
   
       return [MKPolygon polygonWithCoordinates:coordinates count:count];
   }
   

显然，您希望添加错误检查代码以确保 writeToFile 和 dataWithContentsOfFile（或 arrayWithContentsOfFile）成功，但希望这能给您带来想法。