UITableView的分割線長短的控制

UITableView的預設的cell的分割線左邊沒有頂滿，而右邊卻頂滿了。這樣顯示很難看。我需要讓其左右兩邊都是未頂滿狀態，距離是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的一個代理方法：

- (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處代碼： 
定製cell分割線的frame

code2處代碼： 
-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.