stringRelativePath

NSString+RelativePaths.m

//
//  NSString+RelativePaths.m

#import "NSString+RelativePaths.h"


/**
 
 Returns a path to resourcePath, relative to anchorPath.
 
 @param resourcePath trailing path components, possibly forming a valid absolute path
 @param anchorPath trailing path components, of the anchorPath.
 
 This is for constructing a path to a resource relative to some other anchorPath,
 for functions like +[UIImage imageNamed:] which only accept paths relative to a certain
 anchor like the bundle root.
 
 For example:
 NSString * absPath = @"/absolute/path/to/my/image.png";
 UIImage * imageAbs = [UIImage imageNamed:absPath]; // does NOT work
 
 NSString * relPath = PathRelativeToAnchorPath(absPath,[[NSBundle mainBundle] bundlePath]);
 UIImage * imageRel = [UIImage imageNamed:relPath]; // works
 
 Code adapted from:
 http://stackoverflow.com/questions/6539273/objective-c-code-to-generate-a-relative-path-given-a-file-and-a-directory
 */
static NSString* PathRelativeToAnchorPath(NSString * resourcePath, NSString * anchorPath)
{
  NSArray *pathComponents = [resourcePath pathComponents];
  NSArray *anchorComponents = [anchorPath pathComponents];
  
  NSInteger componentsInCommon = MIN([pathComponents count], [anchorComponents count]);
  for (NSInteger i = 0, n = componentsInCommon; i < n; i++) {
    if (![[pathComponents objectAtIndex:i] isEqualToString:[anchorComponents objectAtIndex:i]]) {
      componentsInCommon = i;
      break;
    }
  }
  
  NSUInteger numberOfParentComponents = [anchorComponents count] - componentsInCommon;
  NSUInteger numberOfPathComponents = [pathComponents count] - componentsInCommon;
  
  NSMutableArray *relativeComponents = [NSMutableArray arrayWithCapacity:
                                        numberOfParentComponents + numberOfPathComponents];
  for (NSInteger i = 0; i < numberOfParentComponents; i++) {
    [relativeComponents addObject:@".."];
  }
  [relativeComponents addObjectsFromArray:
   [pathComponents subarrayWithRange:NSMakeRange(componentsInCommon, numberOfPathComponents)]];
  return [NSString pathWithComponents:relativeComponents];
}

@implementation NSString (RelativePaths)

- (NSString*)stringWithPathRelativeTo:(NSString*)anchorPath {
  return PathRelativeToAnchorPath(self, anchorPath);
}

@end