static int MaxLength = 16;

- (void)viewDidLoad {
    [super viewDidLoad];

    [self.myTextField addTarget:self action:@selector(textFieldDidChange:) forControlEvents:UIControlEventEditingChanged];
}

- (void)textFieldDidChange:(UITextField *)textField{
    //輸入中文，即textField.textInputMode.primaryLanguage為：zh-Hans（簡體）、zh-Hant（繁體）
 
的時候，如果有佔位拼音就退出，不做計算
    if (textField.markedTextRange != nil) {
        return;
    }
    NSString *tempString = textField.text;
//　　去掉首尾的空格和回車
//  　tempString = [tempString stringByTrimmingCharactersInSet:NSCharacterSet.whitespaceAndNewlineCharacterSet];
    if (tempString.length > MaxLength) {
        tempString 
 
= [tempString substringToIndex:MaxLength];
    }
    textField.text = tempString;
}

- (void)dealloc {
    [NSNotificationCenter.defaultCenter removeObserver:self];
}