生成唯一标识，可以作为文件或路径名。

GUID

+ (NSString *)GUID
{
	CFUUIDRef theUUID = CFUUIDCreate(NULL);

	CFStringRef theString = CFUUIDCreateString(NULL, theUUID);

	NSString *unique = [NSString stringWithString:(__bridge id)theString];

	CFRelease(theString); CFRelease(theUUID); // Cleanup CF objects

	return unique;
}