Cannot invoke observe on a background thread

Question

I just upgraded to AndroidX and after doing that all my request. I make in background thread do not run but come up with this error.

private void getContactsList() {
    Task.callInBackground((Callable<Void>) () -> { 
     mainActivityViewModel.geContacts(contactListRequest).observe(MainActivity.this, new Observer<ContactListResponse>() {
            @Override
            public void onChanged(@Nullable ContactListResponse contactListResponse) {
                if(contactListRequest != null){
                    System.out.println("Successful");
                }
            }
        });
        return null;
    }).continueWith((Continuation<Void, Void>) task -> {
        if (task.isFaulted()) {
            Log.e(TAG, "find failed", task.getError());
        }
        return null;
    });
}



java.lang.IllegalStateException: Cannot invoke observe on a background thread
    at androidx.lifecycle.LiveData.assertMainThread(LiveData.java:443)
    at androidx.lifecycle.LiveData.observe(LiveData.java:171)
    at com.qucoon.rubies.MainActivity.lambda$getContactsList$12(MainActivity.java:887)
    at com.qucoon.rubies.-$$Lambda$MainActivity$qPAxeGqyWT-wT3M7e8stM1rX2gQ.call(lambda)
    at bolts.Task$4.run(Task.java:357)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
    at java.lang.Thread.run(Thread.java:818)

Check your import of `Observer` and maybe from `MV` you should you `postValue(...)`? This method post from other thread and `setValue(...)` from the main thread. — ardiien, Feb 15 '19 at 00:41
code looks fine. you need to post more relevant code. the threading part where you set contactList — Raghunandan, Feb 15 '19 at 00:42
`Task.callInBackground((Callable) () ...` Maybe here hides the problem? **`Cannot invoke observe on a background thread`** — ardiien, Feb 15 '19 at 00:52
It was working before i upgraded to Android X @YaroslavOvdiienko — QuCoon Team, Feb 15 '19 at 01:04
if you are using viemodel and observing livedata that should happen on the main thread. if you want to get the contact list that should happen on a different thread. you would then postvalue to livedata and observere those changes in your activity. https://developer.android.com/topic/libraries/architecture/livedata — Raghunandan, Feb 15 '19 at 03:17
@Raghunandan Yes thanks I applied what you said and it works fine — QuCoon Team, Feb 15 '19 at 12:42

score 16 · Answer 1 · edited Nov 15 '19 at 14:59

16

You need to execute your code in the main thread try to call getContactsList() via a handler:

Handler handler = new Handler(Looper.getMainLooper());
    handler.post(new Runnable() {
        public void run() {
            getContactsList()
        }
    });

edited Nov 15 '19 at 14:59

robsiemb

6,157
7
32
46

answered Nov 15 '19 at 14:38

Hamza Wakrim

161
1
3

score 5 · Answer 2 · answered Dec 10 '19 at 11:05

5

If for Kotlin and Coroutines (with AndroidX lifecycle library 2.2.0), the following might do:

lifecycleScope.launch {
   withContext(Dispatchers.Main) {
      // Do something
   }
}

Alternatively if not for lifecycleScope and the Lifecycle 2.2.0 library, you can define your own CoroutineScope and use it as so.

answered Dec 10 '19 at 11:05

benChung

366
4
9

Nice, this help me figure out my problem. You can also specified the dispatcher for launch builder `launch(Dispatchers.Main) {...}` because the default one without specified is `Dispatchers.Default`. – MohammadL Aug 27 '20 at 17:11
I had to add `implementation "androidx.lifecycle:lifecycle-runtime-ktx:2.3.0"` to my build.gradle so that `lifecycleScope` became available in my lifecycleOwner component – hannojg Feb 27 '21 at 08:32

score 2 · Answer 3 · answered Jul 10 '20 at 08:01

Annotate your test method with @UiThreadTest

Example:

@Test @UiThreadTest
fun test_onViewCreated() {
    val view = View(appContext)
    doNothing().`when`(spyObject).initEssentials(view)

    spyObject.onViewCreated(view, null)

    verify(spyObject).init(view)
}

This worked for me

score 0 · Answer 4 · answered Oct 19 '21 at 00:27

0

java 11:

var handler = new Handler(Looper.getMainLooper());
    handler.post(() ->getContactsList());

lambdas are available in java 8 though. var is introduced in java 11.

answered Oct 19 '21 at 00:27

Ryan

356
1
6
26

Cannot invoke observe on a background thread

4 Answers4