title: When does a LifecycleRegistry instance start listening to LifecycleOwner's lifecycle changes?
body: <p>I've started learning architecture components, but can't find one thing.</p>
  
  <p><a href="https://developer.android.com/reference/android/arch/lifecycle/LifecycleFragment.html" rel="nofollow noreferrer">LifecycleFragment</a> just creates a new <a href="https://developer.android.com/reference/android/arch/lifecycle/LifecycleRegistry.html" rel="nofollow noreferrer">LifecycleRegistry</a> object, which does not start observing the fragment's lifecycle.</p>
  
  <p>I guess the LifecycleRegistry object starts listening to the fragment's lifecycle when we, for example, put it into <a href="https://developer.android.com/reference/android/arch/lifecycle/LiveData.html#observe(android.arch.lifecycle.LifecycleOwner,%20android.arch.lifecycle.Observer%3CT%3E)" rel="nofollow noreferrer">LiveData.observe()</a> as first param, but I haven't found any proof of this in source code.</p>
  
  <p>Question: When and how does a <a href="https://developer.android.com/reference/android/arch/lifecycle/LifecycleRegistry.html" rel="nofollow noreferrer">LifecycleRegistry</a> object start to observe a fragment's lifecycle and refresh <code>LifecycleRegistry.mState</code>?</p>