title: How to change readonly property in category in objective-c
body: <p>In my *.h file I have <code>foo</code> property that is readonly.<br>
  I update that property win some other public method by using <code>_foo = _foo + 1;</code>    </p>
  
  <p>Now I have category on that *.h file.<br>
  I need to update <code>foo</code> property from category.  </p>
  
  <p>If I use <code>_foo</code> then I got <code>Use of undeclared identifier '_foo''</code><br>
  If I use <code>self.foo = 5</code> then I got <code>Assignment to readonly property</code>   </p>
  
  <p>I know that I can fix this by setting <code>foo</code> property as readwrite, but I would like to avoid that.</p>
  
  <p><strong>Question</strong><br>
  How to solve it ?<br>
  Also is it possible to set property as readonly from outside the class, but as readwrite from within the class and category ?<br>
   That would solve this problem.</p>