UITextField限制字数的方法

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string;
{
if ([string isEqualToString:@"\n"])
{
return YES;
}
NSString * toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string];

if (self.myTextField == textField)
{
if ([toBeString length] > 20) {
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;
}