UITextField限制字数的方法

在输入东西的时候，如果想限制最大字数，可以用下面方法

- (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;
}