title: myView.frame.origin.x = value; does not work - But why?
body: <p>I know that I can't use this:</p>
  
  <pre><code>myView.frame.origin.x = 25.0;
  </code></pre>
  
  <p>and that I have to use this instead:</p>
  
  <pre><code>CGRect myFrame = myView.frame;
  myFrame.origin.x = 25.0;
  myView.frame = myFrame;
  </code></pre>
  
  <p>And I'm doing it all the time, but I don't know why I must do it that way. I would like to fill that gap in my understanding. Can someone explain ?  </p>
  
  <p>Nowadays Xcode gives you "Expression not assignable". Some time ago you got a compile error "Lvalue required as left operand of assignment".</p>