Multiple calls to set LiveData is not observed

Question

I have recently seen a weird issue that is acting as a barrier to my project. Multiple calls to set the live data value does not invoke the observer in the view.

It

Answer 1

You should call viewModel.fetchFirstThree() after Activity's onStart() method. for example in onResume() method.

Because in LiveData the Observer is wrapped as a LifecycleBoundObserver. The field mActive set to true after onStart().

class LifecycleBoundObserver extends ObserverWrapper implements GenericLifecycleObserver {

    @Override
    boolean shouldBeActive() {
        return mOwner.getLifecycle().getCurrentState().isAtLeast(STARTED);// return true after onStart()
    }
    @Override
    public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
        if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
            removeObserver(mObserver);
            return;
        }
        activeStateChanged(shouldBeActive());// after onStart() change mActive to true
    }
}

When the observer notify the change it calls considerNotify, before onStart it will return at !observer.mActive

 private void considerNotify(ObserverWrapper observer) {
    if (!observer.mActive) {// called in onCreate() will return here.
        return;
    }
    if (!observer.shouldBeActive()) {
        observer.activeStateChanged(false);
        return;
    }
    if (observer.mLastVersion >= mVersion) {
        return;
    }
    observer.mLastVersion = mVersion;
    //noinspection unchecked
    observer.mObserver.onChanged((T) mData);
}