title: How to handle relative positioning in iOS automatically?
body: <p>I want to position a sub UIView (<code>sub_UIView</code>) as a subview of parent UIView (<code>parent_UIView</code>). When the <code>parent_UIView</code> resizing/moving, the <code>sub_UIView</code> would stay the relative position of the <code>parent_UIView</code>. For example, the <code>sub_UIView</code> is at the bottom right corner of the <code>parent_UIView</code>. When the <code>parent_UIView</code> is moving, the <code>sub_UIView</code> would stay at the bottom right corner of the <code>parent_UIView</code> automatically. </p>
  
  <p>I managed to do this manually by updating the frame of the <code>sub_UIView</code> when the <code>parent_UIView</code>'s frame moving, but how to do this automatically in iOS? are there any properties for this? (similar to <code>autoresizingmask</code> for resizing subviews) </p>