title: iOS, decrease icon badge number
body: <p>I use </p>
  
  <pre><code>[[UIApplication sharedApplication] setApplicationIconBadgeNumber:[UIApplication sharedApplication].applicationIconBadgeNumber -1];
  </code></pre>
  
  <p>to decrement the icon badge count, but actually it gets cleared. when I press the home button, the badge count is not there.</p>
  
  <p>BUT, if I try a fixed value like this </p>
  
  <pre><code>[[UIApplication sharedApplication] setApplicationIconBadgeNumber:4];
  </code></pre>
  
  <p>, the badge count is there as expected.</p>
  
  <p>So, the question is: Why is </p>
  
  <pre><code>[[UIApplication sharedApplication] setApplicationIconBadgeNumber:[UIApplication sharedApplication].applicationIconBadgeNumber -1];
  </code></pre>
  
  <p>not working?</p>