setValue and postValue on MutableLiveData in UnitTest : <p>I try to test some methods on my Application but I get an error when calling mutablelivedata.postValue. Here is a snippet and the error message:</p>

<pre><code>@Test
public void callStartScreenRepository(){
    Observer&lt;User&gt; userObserver = mock(Observer.class);
    startScreenViewModel.returnUser().observeForever(userObserver);
    maennlich = new User();
    maennlich.setVorname("Christian");
    MutableLiveData&lt;User&gt; userTest = new MutableLiveData&lt;&gt;();
    userTest.postValue(maennlich);
    when(startScreenRepository.userWebService("testUser")).thenReturn(userTest);
    verify(userObserver).onChanged(maennlich);
}
</code></pre>

<p>java.lang.RuntimeException: Method getMainLooper in android.os.Looper not mocked. See <a href="http://g.co/androidstudio/not-mocked" rel="noreferrer">http://g.co/androidstudio/not-mocked</a> for details.</p>

<pre><code>at android.os.Looper.getMainLooper(Looper.java)
at android.arch.core.executor.DefaultTaskExecutor.postToMainThread(DefaultTaskExecutor.java:48)
at android.arch.core.executor.AppToolkitTaskExecutor.postToMainThread(AppToolkitTaskExecutor.java:100)
at android.arch.lifecycle.LiveData.postValue(LiveData.java:277)
at android.arch.lifecycle.MutableLiveData.postValue(MutableLiveData.java:28)
at com.example.cfrindt.foodtracking.ViewModels.StartScreenViewModelTest.callStartScreenRepository(StartScreenViewModelTest.java:102)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:117)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:42)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:262)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:84)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)
</code></pre>

<p>I receive this error no matter if I am using setValue() or postValue().
How can I make it work?</p>
