title: What does ?attr/ mean on Android?
body: <p>I am working on an example about Support Library and Toolbar, this is the code of the layout on the Android documentation</p>
  
  <pre><code>&lt;android.support.v7.widget.Toolbar
      android:id="@+id/my_toolbar"
      android:layout_width="match_parent"
      android:layout_height="?attr/actionBarSize"
      android:background="?attr/colorPrimary"
      android:elevation="4dp"
      android:theme="@style/ThemeOverlay.AppCompat.ActionBar"
      app:popupTheme="@style/ThemeOverlay.AppCompat.Light"/&gt;
  </code></pre>
  
  <p>It is the first time I see these <code>?attr</code> and I have no clue about what they mean or where are these values stored. Are these custom or are they predefined on the Android framework?</p>
  
  <p>Reference: <a href="https://developer.android.com/training/appbar/setting-up.html" rel="noreferrer">Android Toolbar Documentation</a></p>