ios – 仅在arc或gc模式中允许的弱性质的合成

plz帮我删除这个错误

plz建议我一些很好的ios开发人员教程

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

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

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

@end


#import <UIKit/UIKit.h>

@interface ViewController : UIViewController
@property (weak,nonatomic) IBOutlet UITextField *txtUsername;
@property (weak,nonatomic) IBOutlet UITextField *txtPassword;
- (IBAction)loginClicked:(id)sender;
- (IBAction)backgroundClick:(id)sender;


@end