title: How to declare a constant in swift that can be used in objective c
body: <p>if I declare the swift constant as a <strong>global constant</strong> like:</p>
  
  <pre><code>let a = "123"
  </code></pre>
  
  <p>but the <code>a</code> cannot be found in <strong>objective c</strong>.</p>
  
  <p>How to solve this?</p>