title: how to use sortUsingFunction:context
body: <p>appdata.items is NSMutableArray.</p>
  
  <p>I connot compile This code. 
  Error code is "prop.173 has an incomplete type".</p>
  
  <pre><code>NSInteger compareInfo(id aInfo1, id aInfo2, void *context){
    NSDate* info1 = [aInfo1 objectAtIndex:2];
    NSDate* info2 = [aInfo2 objectAtIndex:2];
    return [info1 compare:info2];
  }
  
  -(void)saveData{
    NSData* data = [[NSMutableData alloc] init];
    appdata.items = [appdata.items sortUsingFunction:compareInfo context:NULL];
    NSKeyedArchiver* archiver = [[NSKeyedArchiver alloc] initForWritingWithMutableData:data];
    [archiver encodeObject:appdata forKey:DATAKEY];
    [archiver finishEncoding];
    [data writeToFile:[self dataFilePath] atomically:YES];
    [archiver release];
    [data release];
  }
  </code></pre>