textField限制只能输入一定长度的字符

- (BOOL)textField:(UITextField )textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString )string; 
{  //string就是此时输入的那个字符textField就是此时正在输入的那个输入框返回YES就是可以改变输入框的值NO相反
if ([string isEqualToString:@"\n"])  //按会车可以改变
    { 
        return YES; 
    }

NSString * toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string]; //得到输入框的内容

if (self.myTextField == textField)  //判断是否时我们想要限定的那个输入框
{ 
    if ([toBeString length] > 20) { //如果输入框内容大于20则弹出警告
        textField.text = [toBeString substringToIndex:20]; 
        UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:nil message:@"超过最大字数不能输入了" delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil, nil] autorelease]; 
        [alert show]; 
        return NO; 
    } 
} 
return YES; 
 
}</pre>