如何在iPhone SDK中将NSData转换为字符串？

// Implement this method in your code to do something with the image.
- (void)useImage:(UIImage*)theImage
{
    /*
     turning the image into a NSData object
     getting the image back out of the UIImageView
     setting the quality to 90
     */

    NSData *imageData = UIImageJPEGRepresentation(theImage, 90);
    // setting up the URL to post to


    NSString *urlString=[SiteUrl    stringByAppendingString:@\"iphone_upload_photo.php?type=colleague&token=\"];
    urlString=[urlString stringByAppendingString:PublicToken];





    // setting up the request object now
    NSMutableURLRequest *request = [[[NSMutableURLRequest alloc] init] autorelease];
    [request setURL:[NSURL URLWithString:urlString]];
    [request setHTTPMethod:@\"POST\"];

    /*
     add some header info now
     we always need a boundary when we post a file
     also we need to set the content type

     You might want to generate a random boundary.. this is just the same

     */
    NSString *boundary = [NSString stringWithString:@\"---------------------------14737809831466499882746641449\"];
    NSString *contentType = [NSString stringWithFormat:@\"multipart/form-data; boundary=%@\",boundary];
    [request addValue:contentType forHTTPHeaderField: @\"Content-Type\"];

    /*
     now lets create the body of the post
     */
    NSMutableData *body = [NSMutableData data];
    [body appendData:[[NSString stringWithFormat:@\"\\r\\n--%@\\r\\n\",boundary] dataUsingEncoding:NSUTF8StringEncoding]];
    [body appendData:[[NSString stringWithString:@\"Content-Disposition: form-data; name=\\\"userfile\\\"; filename=\\\"ipodfile.jpg\\\"\\r\\n\"] dataUsingEncoding:NSUTF8StringEncoding]];
    [body appendData:[[NSString stringWithString:@\"Content-Type: application/octet-stream\\r\\n\\r\\n\"] dataUsingEncoding:NSUTF8StringEncoding]];
    [body appendData:[NSData dataWithData:imageData]];
    [body appendData:[[NSString stringWithFormat:@\"\\r\\n--%@--\\r\\n\",boundary] dataUsingEncoding:NSUTF8StringEncoding]];
    // setting the body of the post to the reqeust
    [request setHTTPBody:body];

    // now lets make the connection to the web
    NSData *returnData = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];
    NSString *returnString = [[NSString alloc] initWithData:returnData encoding:NSUTF8StringEncoding];

    [self LoadContent];



}