iOS/iPhone and Timer (NSTimer, Objective-C)

NSTimer *timer;
timer = [NSTimer scheduledTimerWithTimeInterval:1.00 target:self selector:@selector(setCommand) userInfo:nil repeats:YES];

- (void) setCommand
{

}

[timer invalidate];

//
//  ViewController.h
//  timerApp
//
//  Created by Weerachai on 12/8/55 BE.
//  Copyright (c) 2555 Weerachai. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController

{
    IBOutlet UILabel *lblResult;
    
    NSTimer *timer;
}

- (IBAction)btnStart:(id)sender;
- (IBAction)btnStop:(id)sender;

@end

//
//  ViewController.m
//  timerApp
//
//  Created by Weerachai on 12/8/55 BE.
//  Copyright (c) 2555 Weerachai. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()
{
    int i;
}

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
	// Do any additional setup after loading the view, typically from a nib.
}

- (IBAction)btnStart:(id)sender {
    timer = [NSTimer scheduledTimerWithTimeInterval:1.00 target:self selector:@selector(setCommand) userInfo:nil repeats:YES];
}

- (void) setCommand
{
    i = i + 1;
    
    NSMutableString *result;
	result = [NSString stringWithFormat:@"Number = %d",i];
    
    lblResult.text = result;
}

- (IBAction)btnStop:(id)sender {
    [timer invalidate];
}


- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (void)dealloc {
    [lblResult release];
    [super dealloc];
}

@end