UITableView的分割线长短的控制

UITableView的默认的cell的分割线左边没有顶满，而右边却顶满了。这样显示很难看。我需要让其左右两边都是未顶满状态，距离是20像素

// code1
    if ([self.tableView respondsToSelector:@selector(setSeparatorInset:)]) {
        [self.tableView setSeparatorInset:UIEdgeInsetsMake(0, 20, 0, 20)];
    }

// code2 
    if ([self.tableView respondsToSelector:@selector(setLayoutMargins:)]) {
        [self.tableView setLayoutMargins:UIEdgeInsetsMake(0, 20, 0, 20)];
    }

添加UITableView的一个代理方法：

- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath
{
    if ([cell respondsToSelector:@selector(setSeparatorInset:)])
    {
        [cell setSeparatorInset:UIEdgeInsetsMake(0, 15, 0, 15)];
    }
    if ([cell respondsToSelector:@selector(setLayoutMargins:)])
    {
        [cell setLayoutMargins:UIEdgeInsetsMake(0, 15, 0, 15)];
    }
}

ode1处代码： 
定制cell分割线的frame

code2处代码： 
-layoutMargins returns a set of insets from the edge of the view’s bounds that denote a default spacing for laying out content. 
If preservesSuperviewLayoutMargins is YES, margins cascade down the view tree, adjusting for geometry offsets, so that setting the left value of layoutMargins on a superview will affect the left value of layoutMargins for subviews positioned close to the left edge of their superview’s bounds 
If your view subclass uses layoutMargins in its layout or drawing, override -layoutMarginsDidChange in order to refresh your view if the margins change.