1.只让输入的字母变大写

• 方法一

- (void)viewDidLoad {
    [super viewDidLoad];
    
    _plateTextField = [[UITextField alloc] init];
    _plateTextField.delegate = self;
    [_plateTextField addTarget:self action:@selector(textFieldChanged:) forControlEvents:UIControlEventEditingChanged];
}

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

• 方法二

- (void)viewDidLoad {
    [super viewDidLoad];
    
    _plateTextField = [[UITextField alloc] init];
    _plateTextField.delegate = self;
}
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    
    if ([self isBlankString:string]) {
        return YES;
    }
    char commitChar = [string characterAtIndex:0];
    if (commitChar > 96 && commitChar < 123) {
     //小写变成大写
    NSString * uppercaseString = string.uppercaseString;
    NSString * str1 = [textField.text substringToIndex:range.location];
    NSString * str2 = [textField.text substringFromIndex:range.location];
    textField.text = [NSString stringWithFormat:@"%@%@%@",str1,uppercaseString,str2];
    
        return NO;
    }
}

// 判断字符串是否为空
- (BOOL) isBlankString:(NSString *)string {
    if (string == nil || string == NULL) {
        return YES;
    }
    if ([string isKindOfClass:[NSNull class]]) {
        return YES;
    }
    if ([[string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] length]==0) {
        return YES;
    }
    return NO;
}

2.限制只能输入字母 + 输入后自动变大写 + 限制输入字母个数

#define kAlphaNum @"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"

- (void)viewDidLoad {
    [super viewDidLoad];
    
    _plateTextField = [[UITextField alloc] init];
    _plateTextField.delegate = self;
    [_plateTextField addTarget:self action:@selector(textFieldChanged:) forControlEvents:UIControlEventEditingChanged];
}

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

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    
    NSUInteger proposedNewLength = textField.text.length - range.length + string.length;
    /* 你想限制输入的字母个数 */
    NSInteger textLength = 5;
    
    if ([textField isEqual:self.plateTextField]) {
        NSCharacterSet *cs;
        cs = [[NSCharacterSet characterSetWithCharactersInString:kAlphaNum] invertedSet];
        NSString *filtered = [[string componentsSeparatedByCharactersInSet:cs] componentsJoinedByString:@""]; 
        BOOL canChange = [string isEqualToString:filtered];
        
        return proposedNewLength>textLength ? NO: canChange;
    }
    return YES;
}