title: Why does Android Studio 3.1.1 always display Unresolved reference when I reopen a project for the first time?
body: <p>I have <code>import info.dodata.mirror.R</code>, I always get the warning information <code>Unresolved reference: R</code> when I reopen a project for the first time, just like the following image.</p>
  
  <p>I have to click the menu  Build -> Rebuild project to rebuild the project, or I close the project and reopen it again in Android Studio 3.1.1 to resolve the problem.</p>
  
  <p>Both my office PC and home PC with windows 7 x64 meet the same problem. How can I fix it? Thanks!</p>
  
  <p><strong>Image</strong></p>
  
  <p><a href="https://i.stack.imgur.com/0xITd.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0xITd.png" alt="enter image description here"></a></p>
  
  <p><strong>To ice1000:</strong></p>
  
  <p>Thanks! 
  Do you mean that I should click Refresh All Gradle Projects button?
  <a href="https://i.stack.imgur.com/LNGgL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LNGgL.png" alt="enter image description here"></a></p>