title: When does my class need a build() method?
body: <p>I understand the general approach to building UI layouts using Flutter. However, I'm still unclear which classes or UI widgets require a .build() method when I'm creating my own vs. using the defaults generated by the project.</p>
  
  <p>I haven't found a clear explanation yet - even in the Flutter tutorials. They all seem to just gloss over how "the build method takes a BuildContext" and then go on to the next subject without explaining further.</p>
  
  <p>Does anyone have a succinct explanation of the build method and when it is or isn't needed? And more specifically: what does it actually do?</p>