setValue and postValue on MutableLiveData in UnitTest

Question

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:

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

java.lang.RuntimeException: Method getMainLooper in android.os.Looper not mocked. See http://g.co/androidstudio/not-mocked for details.

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) 

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

Answer 1

First thing is that you need to work with the InstantTaskExecutorRule, that means:

@Rule public InstantTaskExecutorRule instantExecutorRule = new InstantTaskExecutorRule(); 

This will allow to work with MutableLiveData instantly. Remember to include in your app's build.gradle the import:

testCompile "android.arch.core:core-testing:$rootProject.ext.arch_version" 

Then, you need to either define a JUnit Rule with a rule overriding RxSchedulers or within the @Before override the Schedulers as well using RxAndroidPlugins (if you are using RxJava)

RxSchedulersOverrideRule:

public class RxSchedulersOverrideRule implements TestRule {     @Override public Statement apply(final Statement base, Description description) {     return new Statement() {         @Override public void evaluate() throws Throwable {             RxAndroidPlugins.reset();             RxAndroidPlugins.setMainThreadSchedulerHandler(scheduler -> Schedulers.trampoline());          RxJavaPlugins.reset();         RxJavaPlugins.setIoSchedulerHandler(scheduler -> Schedulers.trampoline());         RxJavaPlugins.setComputationSchedulerHandler(scheduler -> Schedulers.trampoline());         RxJavaPlugins.setNewThreadSchedulerHandler(scheduler -> Schedulers.trampoline());          base.evaluate();          RxAndroidPlugins.reset();         RxJavaPlugins.reset();       }     };   } } 

And calling it in the test class:

@Rule public RxSchedulersOverrideRule rxSchedulersOverrideRule = new RxSchedulersOverrideRule(); 

The other option is calling within setup() in @Before:

RxAndroidPlugins.setInitMainThreadSchedulerHandler(schedulerCallable -> Schedulers.trampoline()); 

After using this you need to reset in @After's tearDownClass():

RxAndroidPlugins.reset();