UITextField becomeFirstResponder does not work in UIControlEventEditingDidEndOnExit

I have a UITextField and UILabel. When user entered some text to textField and tap "Enter" on keyboard I want to save text to the label then clear textField and make it first responder again. But something is going wrong. becomeFirstResponder does not work.

Here is the code of ViewController:

@interface ViewController ()

@property (weak, nonatomic) IBOutlet UITextField *textField;
@property (weak, nonatomic) IBOutlet UILabel *label;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    [self.textField addTarget:self action:@selector(saveResult:) 
        forControlEvents:UIControlEventEditingDidEndOnExit];
}

-(void) saveResult: (id) sender {
    self.label.text = self.textField.text;
    self.textField.text = @"";
    [self.textField becomeFirstResponder];
}

@end

via Chebli Mohamed