Writing attributed strings line-by-line to an RTF file - objective-c

Question

I'm trying to write a series of attributed strings to an RTF file, line-by-line at various points in my application sit is running (you can think of this as log data, only with attributes). The file is created just fine and it would appear that all the data is being written to the file, but when I open the RTF file only the first line written to the file appears. I suspect that there's something written to the file in the first write that essentially end the RTF file, but I'm not quite sure what that is.
- (void) writeLineWithSizeAndStyle: (NSString *) line : (CGFloat)fontSize : (NSFontTraitMask) traits {
NSMutableAttributedString *attrString = [[NSMutableAttributedString alloc] initWithString:line];
NSFileHandle *fileHandle = [NSFileHandle fileHandleForUpdatingAtPath:filePath];
NSFontManager *fm = [NSFontManager sharedFontManager];
NSRange range = [line rangeOfString:line];
NSFont *font = [NSFont systemFontOfSize:fontSize];
font = [fm convertFont:font toHaveTrait:traits];
[attrString addAttribute: NSFontAttributeName value:font range:range];
NSData *rtfData = [attrString RTFFromRange:NSMakeRange(0, attrString.length) documentAttributes:#{}];
// Go the the end of the file, write the data, and close the file
[fileHandle seekToEndOfFile];
[fileHandle writeData: rtfData];
[fileHandle closeFile];
}
Is there something I need to to in converting each attributed string to NSData that effectively tells RFT not to terminate the file upon writing?
Thanks!
Update:
Here's the code for solution 2 I proposed in the comments section. This operates on an instance variable in the class object (playByPlay) that simply appends each new line onto the entire "log". This works great, but again is only really a solution for sufficiently small files.
- (void) writeLineWithSizeAndStyle: (NSString *) line : (CGFloat)fontSize : (NSFontTraitMask) traits {
NSFileHandle *fileHandle = [NSFileHandle fileHandleForUpdatingAtPath:filePath];
NSMutableAttributedString *attrString = [[NSMutableAttributedString alloc] initWithString:line];
NSRange range = [line rangeOfString:line];
NSFontManager *fm = [NSFontManager sharedFontManager];
NSFont *font = [NSFont systemFontOfSize:fontSize];
font = [fm convertFont:font toHaveTrait:traits];
[attrString addAttribute: NSFontAttributeName value:font range:range];
[playByPlayText appendAttributedString:attrString];
NSData *rtfData = [playByPlayText RTFFromRange:NSMakeRange(0, playByPlayText.length) documentAttributes:#{}];
[fileHandle writeData: rtfData];
[fileHandle closeFile];
}