uialertview 设置输入的位数限制

-(void)creatalter{

UIAlertView *alertview = [[UIAlertViewalloc]initWithTitle:@"请输入登录密码"message:nildelegate:selfcancelButtonTitle:@"取消"otherButtonTitles:@"确定",nil];
alertview.alertViewStyle =UIAlertViewStylePlainTextInput;
alertview.tag =826;
UITextField * textfield=(UITextField *) [alertviewtextFieldAtIndex:0];
textfield.tag=1200;
textfield.delegate =self;
[alertview show];

}
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range
replacementString:(NSString *)string;
{

if ([stringisEqualToString:@"\n"])
{
returnYES;
}
NSString * toBeString = [textField.textstringByReplacingCharactersInRange:rangewithString:string];

if (textField.tag==1200)
{
if ([toBeStringlength] >
16) { //这里限定输入的位数
textField.text = [toBeStringsubstringToIndex:16];

returnNO;
}
}
returnYES;
}