title: How to get the buildDir of a subproject in Gradle?
body: <p>In <code>gradle.build</code> you can reference the <code>buildDir</code> and use it as a source or destination. How can I reference the <code>buildDir</code> of a subproject from the root <code>build.gradle</code> file?</p>
  
  <p>(And a similar supplementary question, how can I reference subprojects in a Copy task? I have tried ':subproject_name/some_directory' but it errors).</p>