对UITextFiled字数限制

[textField addTarget:self action:@selector(textFieldDidChange:) forControlEvents:UIControlEventEditingChanged];

- (void) textFieldDidChange:(UITextField *)textField
{
    NSInteger kMaxLength = 4;
    NSString *toBeString = textField.text;
    NSString *lang = [[UIApplication sharedApplication]textInputMode].primaryLanguage; //ios7之前使用[UITextInputMode currentInputMode].primaryLanguage
    if ([lang isEqualToString:@"zh-Hans"]) { //中文输入
        UITextRange *selectedRange = [textField markedTextRange];
        //获取高亮部分
        UITextPosition *position = [textField positionFromPosition:selectedRange.start offset:0];
        if (!position) {// 没有高亮选择的字，则对已输入的文字进行字数统计和限制
            if (toBeString.length > kMaxLength) {
                textField.text = [toBeString substringToIndex:kMaxLength];
//                [ToastUtils showHud:@"超过字数限制"];
            }
        }
        else{//有高亮选择的字符串，则暂不对文字进行统计和限制
        }
    }else{//中文输入法以外的直接对其统计限制即可，不考虑其他语种情况
        if (toBeString.length > kMaxLength) {
            textField.text = [toBeString substringToIndex:kMaxLength];
        }
    }
}