use of undeclared identifier objective-c - ios

Question

I am getting the use of undeclared identifier 'data' in the didreceiveresponse method. I have imported the Model header file which has the array and data declared. I found that if I declare them inside the viewcontroller.h file the error goes away. What is the cause of this problem?
#import "ViewController.h"
#import "DetailViewController.h"
//#import "Model.h"
#import "Model.h"
#interface ViewController ()
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
[UIApplication sharedApplication].networkActivityIndicatorVisible = YES;
NSURL *url = [NSURL URLWithString:#"https://s3.amazonaws.com/jon-hancock-phunware/nflapi-static.json"];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
[[NSURLConnection alloc] initWithRequest:request delegate:self];
// Do any additional setup after loading the view, typically from a nib.
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
data = [[NSMutableData alloc]init];
}
This is the model header file.
#import <UIKit/UIKit.h>
#interface Model : UIViewController
//NSArray *bars;
//NSMutableData *data;
#property (nonatomic, retain) NSArray *bars;
#property (nonatomic, retain) NSMutableData *data;
#end
This fixes the error defining the variables inside the viewcontroller.h file.
import
#interface ViewController : UIViewController {
IBOutlet UITableView *mainTableView;
NSArray *bars;
NSMutableData *data;
}
#end

Answer 1

It's Objective-C not Swift. You access properties with self.data or _data. It doesn't know well enough to look to your global variables unless you're explicit about it. Also, I would lean towards self.data over directly accessing _data so the setter gets called. In this case, it's almost certainly not going to be an issue but setting ivars directly with an underscore can mess up key value observation and might have unexpected consequences that are difficult to debug.