1、初始化時註冊通知notification：

[[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(textFiledEditChanged:)
                                                name:@"UITextFieldTextDidChangeNotification"
                                              object:_textFiled];

2、 實現監聽方：

#define kMaxLength 40
 

-(void)textFiledEditChanged:(NSNotification *)obj{

    UITextField *textField = (UITextField *)obj.object;
    NSString *toBeString = textField.text;
    // 鍵盤輸入模式
    NSString *lang = textField.textInputMode.primaryLanguage;;

    if ([lang isEqualToString:@"zh-Hans"]) {
        UITextRange *selectedRange = [textField markedTextRange];
        // 簡體中文輸入，包括簡體拼音，健體五筆，簡體手寫
 

        UITextPosition *position = [textField positionFromPosition:selectedRange.start offset:0];
        if (!position) {
            if (toBeString.length > kMaxLength) {
                textField.text = [toBeString substringToIndex:kMaxLength];
            }
        }
    }
    else{
        if
 
 (toBeString.length > kMaxLength) {
            textField.text = [toBeString substringToIndex:kMaxLength];
        }
    }
}

3、在dealloc裡登出掉監聽方法：

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self name:@"UITextFieldTextDidChangeNotification" object:_textFiled];
}

OK，輸入中文時可以限制輸入漢字的長度了。