title: How to put new placeholder resources into Android Studio project ("tools:sample" resources)?
body: <h2>Background</h2>
  
  <p>Sometimes, you wish to put some placeholders to be shown only on the IDE, in layout files.</p>
  
  <p>As an example, you can use this:</p>
  
  <pre><code>&lt;ImageView tools:src="@tools:sample/avatars" ... /&gt;
  </code></pre>
  
  <p>And get this in the preview:</p>
  
  <p><a href="https://i.stack.imgur.com/RAOMy.png" rel="noreferrer"><img src="https://i.stack.imgur.com/RAOMy.png" alt="enter image description here"></a></p>
  
  <p>Such files are not part of the APK you get when you build your app, so it's safe to use just for developing.</p>
  
  <p>This is what I was told from <a href="https://issuetracker.google.com/issues/37127749#comment3" rel="noreferrer"><strong>here</strong></a>:</p>
  
  <blockquote>
    <p>With sample data in 3.0, you can now have placeholder images that are
    not part of the compiled apk. You just need a sampledata directory in
    your project with a subdirectory containing all the images you want to
    use as placeholders. You can refer those images from "tools"
    attributes. Also, there are predefined stock images like
    @sample/avatars or @sample/background/scenic</p>
  </blockquote>
  
  <h2>The problem</h2>
  
  <p>I can't find how to add more of such images into the project (so that they will only be used in the IDE, and not a part of the APK), and if there is a way to put other resources, other than images.</p>
  
  <p>In fact I can't find the docs of this feature.</p>
  
  <h2>What I tried</h2>
  
  <p>I tried to put an image on "res/sampledata" and tried on "res/sample", but I wasn't able to reach it in both cases.</p>
  
  <h2>The questions</h2>
  
  <ol>
  <li>What's even the name of this feature?</li>
  <li>How can I put the image file into the project, and use it as a placeholder this way ? In which folder?</li>
  <li>Is it possible to add more images to be used this way?</li>
  <li>Is it possible to add other resources? Layouts? Strings?</li>
  <li>Are there more capabilities of this new feature?</li>
  </ol>