How to update node values and add nodes in xml file?

In my iOS application I can read data from xml code to read xml::

-(NSString *)readxmlFile:(NSString *)fileName ElementName:(NSString *)Element Poisson:(NSInteger *)Index {
    xmlelement = Element;
    NSString *filePath = [[NSBundle mainBundle] pathForResource:fileName ofType:@"xml"]; 

    NSData *myData = [NSData dataWithContentsOfFile:filePath];  
    NSString *str,*docStr;

    if (myData) {  
        str = [[NSString alloc] initWithData:myData encoding:NSASCIIStringEncoding];

    } 
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];

    // the path to write file
    NSString *appFile = [documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.xml",fileName]];
    NSError *error = nil;
    BOOL fileExists = [[NSFileManager defaultManager] fileExistsAtPath:appFile];
    NSData *myData1 = [NSData dataWithContentsOfFile:appFile];
    if(fileExists)
    {
        NSLog(@"file exist in document");

        if (myData1) {  
            docStr = [[NSString alloc] initWithData:myData1 encoding:NSASCIIStringEncoding];
            xmlparser=[[NSXMLParser alloc] initWithData: myData1];
            [xmlparser setDelegate:self];
            [xmlparser parse];
            NSLog(@"Data>>%@",xmlData);
        } 
    }
    else {
        NSLog(@"file does not exist");
        xmlparser=[[NSXMLParser alloc] initWithData: myData1];
        [xmlparser setDelegate:self];
        [xmlparser parse];
        NSLog(@"Data>>%@",xmlData);
        BOOL success = [str writeToFile:appFile  atomically:YES encoding:NSUTF8StringEncoding error:&error];
        if (!success) {
            NSLog(@"Error: %@", [error userInfo]);
        }
        else {
            NSLog(@"File write is successful");

        }
        NSData *myData1 = [NSData dataWithContentsOfFile:appFile];
        if (myData1) {  
            docStr = [[NSString alloc] initWithData:myData1 encodin开发者_Python百科g:NSASCIIStringEncoding];
        } 
    }
     return [xmlData objectAtIndex:Index];//pasre data from xml based on node name and save in xmlData 
}

but unable to write xml (add nodes and update node values), please guide to me how to add nodes and update node values in xml file.

NSXMLParser is just a parser. It doesn't write XML. It's great for dealing with streams of XML but not so great if you get the whole chunk at once.

Google has written a great XML reading/writing class called GDataXmlNode. Here is a blog post about how to use it. http://www.raywenderlich.com/725/how-to-read-and-write-xml-documents-with-gdataxml

It's much nicer than using NSXMLParser.