UITextFieldDelegate  協議。

     2.  在標頭檔案中，定義如下變數：

//限制使用者只能輸入字母和數字

#define kAlphaNum   @"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"

     3. 在實現檔案中：

       將 textFiled物件的delegate設定為self:

        m_accountField  =  [[UITextField  alloc]  initWithFrame:CGRectMake

m_accountField.borderStyle = UITextBorderStyleNone;

        m_accountField.font = fieldFont;

m_accountField.delegate  = self;

//UITextFieldDelegate
- (BOOL)textFieldShouldReturn:(UITextField *)textField;
{
    //使用者結束輸入
    [textField   resignFirstResponder];

    return  YES;
}

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string   // return NO to not change text
{
    NSCharacterSet *cs;
    cs = [[NSCharacterSet characterSetWithCharactersInString:kAlphaNum] invertedSet];
    NSString *filtered =
    [[string componentsSeparatedByCharactersInSet:cs] componentsJoinedByString:@""];
    BOOL basic = [string isEqualToString:filtered];
    return basic;
}