title: Android MediatorLiveData observer
body: <p>I'm a bit confused on why the following code doesn't work:</p>
  
  <pre><code>MutableLiveData&lt;String&gt; mutableTest = new MutableLiveData&lt;&gt;();
  MediatorLiveData&lt;String&gt; mediatorTest = new MediatorLiveData&lt;&gt;();
  mediatorTest.addSource(mutableTest, test -&gt; {
      Timber.d(test);
  });
  mutableTest.setValue("bla!");
  </code></pre>
  
  <p>This code seems straightforward, however the debugger doesn't enter the callback and nothing is logged to the console... </p>
  
  <p>Edit: shouldn't this work then? </p>
  
  <pre><code>    MutableLiveData&lt;String&gt; mutableTest = new MutableLiveData&lt;&gt;();
      MediatorLiveData&lt;String&gt; mediatorTest = new MediatorLiveData&lt;&gt;();
      mediatorTest.observe(loginActivity, str -&gt; Timber.d(str));
      mediatorTest.addSource(mutableTest, str -&gt; Timber.d(str));
      mutableTest.setValue("bla!");
  </code></pre>