title: Angularjs suitable moment to launch a http request when app start
body: <p>I just started to learn Angularjs 2 days ago. A question have confused me for all the 2 days.</p>
  
  <p>I need to make a http request to the server to get some data when the app start, but I cannot find the suitable moment to do this.</p>
  
  <p>I've tried to make a <code>controller</code>, which calls <code>$http.get()</code>. But It doesn't work. It seems that the controller won't be instantiated if it's never used in the html (not sure about this). </p>
  
  <p>I also tried to find other ways, but I only found $http which is used for http request. And <code>$http</code> only appears in the <code>controller</code>.</p>
  
  <p>Maybe I need use other Angularjs methods? Or, I should do the <em>instantiate</em> action manually?</p>