title: Q:get a view's constrains returns empty array
body: <p>First I show my <code>cell</code> , my label has several <code>constaints</code>:</p>
  
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  <p><a href="https://i.stack.imgur.com/LsWJd.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LsWJd.png" alt="constrains"></a></p>
  
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  <p>In my <code>- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath</code> method.</p>
  
  <p>I get the <code>label</code>'s constrains :</p>
  
  <p><code>NSArray *cons = (((NoticeAnnouncementCell*)cell).mainTitle).constraints;</code></p>
  
  <p>but I get a empty array.</p>