title: How to position a child view relative to containing view size?
body: <p>I want to be able to position my child view 25% the size of the super view from the top.  </p>
  
  <pre><code>NSLayoutConstraint *topPositionConstraint = [NSLayoutConstraint  constraintWithItem:_containerView 
                                                                            attribute:NSLayoutAttributeTop 
                                                                            relatedBy:NSLayoutRelationEqual 
                                                                               toItem:_childView 
                                                                            attribute:NSLayoutAttributeHeight 
                                                                           multiplier:0.25f 
                                                                             constant:0.0f];
  </code></pre>
  
  <p>However, right now I'm getting the following exception: </p>
  
  <pre><code>'NSInvalidArgumentException', reason: '*** +[NSLayoutConstraint constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:]: Invalid pairing of layout attributes'
  </code></pre>
  
  <p>Why does the error occur and how can I achieve what I want?</p>