UITextFiled限制输入字母并且输入的字母大写

代码：

- (void)viewDidLoad {
    [super viewDidLoad];
    
//    NSArray *myArr = [NSArray arrayWithObjects:@"1.jpg",@"2.jpg",@"3.jpg",@"4.jpg",@"5.jpg",@"6.jpg",@"7.jpg", nil];
//
//    NSUInteger count = [myArr count];
//    for (int index = 0 ; index<count; index++) {
//        [self layoutSubImage:myArr[index] withX:50*(index%3) withY:50*(index/3)];
//    }
    
    _aField.delegate = self;
    [_aField addTarget:self action:@selector(textFieldChanged:) forControlEvents:UIControlEventEditingChanged];
}

- (void)textFieldChanged:(UITextField *)aTextField{
    aTextField.text = [aTextField.text uppercaseString];
}

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string{
    NSCharacterSet *set = [[NSCharacterSet characterSetWithCharactersInString:ALLCHARACTERS] invertedSet];
    NSString *upperStr = [string uppercaseString];
    NSString *filted = [[upperStr componentsSeparatedByCharactersInSet:set] componentsJoinedByString:@""];
    NSLog(@"string>>>%@,fileted>>>%@",string,filted);
    BOOL allowed = [upperStr isEqualToString:filted];
    return allowed;
}