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Improving Robolectric's Looper simulation

TL;DR: We'd love your feedback on improvements we've made to make Robolectric's Looper behavior more realistic. Try it out today by annotating your tests with @LooperMode(PAUSED) and let us know your experience!


Unlike on a real device, Robolectric shares a single thread for both UI operations and Test code. By default, Robolectric will execute tasks posted to Loopers synchronously inline. This causes Robolectric to execute tasks earlier than they would be on a real device. While in many cases this has no observable effect it can lead to bugs that are hard to track down.

Consider the code below. When run on the UI thread on a device, the assertion would pass.

List<String> events = new ArrayList<>();
new Handler(Looper.getMainLooper()).post(() -> events.add("after"));

assertThat(events).containsExactly("before", "between", "after").inOrder();

Robolectric’s default behavior is to process posted code synchronously and immediately, so the assertion fails with [before, after, between], which is clearly incorrect.

Robolectric’ current implementation is notoriously prone to deadlocks, infinite loops and other race conditions. Robolectric will duplicate each task posted to a Looper into a separate list stored in org.roboelectric.util.Scheduler. The Looper’s set of tasks and the Schedulers can get out of sync, causing hard-to-diagnose errors.


We’ve re-written Robolectric’s threading model and looper simulation in a way that we hope will address the deficiencies of the current behavior. It’s available to try out now by applying a @LooperMode(PAUSED) annotation to your test classes or methods. Some of the highlights of the PAUSED mode vs the existing ‘LEGACY’ mode include:

  • Tasks posted to the main looper are not automatically executed inline. Similar to the legacy paused IdleState, tasks posted to the main looper must be explicitly executed via ShadowLooper APIs. However, we’ve made a couple additional improvements in PAUSED mode that make this easier:
  • Robolectric will warn users if a test fails with unexecuted tasks in the main looper queue. This is a hint that the unexecuted behavior is important for the test case.
  • AndroidX Test APIs, like ActivityScenario, FragmentScenario and Espresso will automatically idle the main looper
  • Tasks posted to background loopers are executed in real threads. This will hopefully eliminate the need for hacks when trying to test code that asserts that it is running in a background thread

Using PAUSED LooperMode

To switch to PAUSED:

  • Use robolectric 4.3. In gradle add

    dependencies {
      testImplementation 'org.robolectric:robolectric:4.3'
  • Apply the LooperMode(PAUSED) annotation to your test package/class/method

  • Convert any background org.robolectric.util.Scheduler calls for controlling Loopers to shadowOf(looper)
  • Recommended, but not mandatory: Convert any foreground org.robolectric.util.Scheduler calls to shadowOf(getMainLooper()). The Scheduler APIs will be @deprecated and removed over time.
  • Convert any usages to or
  • Run your tests. If you see test failures like Main looper has queued unexecuted runnables, you may need to insert shadowOf(getMainLooper()).idle() calls to your test to drain the main Looper. Its recommended to step through your test code with a watch set on Looper.getMainLooper().getQueue() to see the status of the looper queue, to determine the appropriate point to add a shadowOf(getMainLooper()).idle() call.


import static android.os.Looper.getMainLooper;
import static org.robolectric.annotation.LooperMode.Mode.PAUSED;
import static org.robolectric.Shadows.shadowOf;

import org.robolectric.annotation.LooperMode;

public class MyTest {
  public void testCodeThatPosts() {
    List<String> events = new ArrayList<>();
    new Handler(Looper.getMainLooper()).post(() -> events.add("after"));

    // the 'after' task is posted, but has not been executed yet
    assertThat(events).containsExactly("before", "between").inOrder();
    // execute all tasks posted to main looper 

    assertThat(events).containsExactly("before", "between", "after").inOrder();

Take a look at Robolectric’s ShadowPausedAsyncTaskTest for an example of using PAUSED LooperMode and background tasks.


  • Animations: Use of Animations can cause delayed tasks to be posted to the main looper queue. You can use the ShadowLooper.idleFor or ShadowLooper.runToEndOfTasks APIs to execute these tasks.


Please let us know of any roadblocks to adopting PAUSED LooperMode. We’d like to make it the default mode for tests in the next release, and thus your feedback would be most welcome.