LiveDataTestUtil.java

/* Copyright 2019 Google LLC.	
   SPDX-License-Identifier: Apache-2.0 */
public class LiveDataTestUtil {
    public static <T> T getOrAwaitValue(final LiveData<T> liveData) throws InterruptedException {
        final Object[] data = new Object[1];
        final CountDownLatch latch = new CountDownLatch(1);
        Observer<T> observer = new Observer<T>() {
            @Override
            public void onChanged(@Nullable T o) {
                data[0] = o;
                latch.countDown();
                liveData.removeObserver(this);
            }
        };
        liveData.observeForever(observer);
        // Don't wait indefinitely if the LiveData is not set.
        if (!latch.await(2, TimeUnit.SECONDS)) {
            throw new RuntimeException("LiveData value was never set.");
        }
        //noinspection unchecked	
        return (T) data[0];
    }
}

import androidx.annotation.VisibleForTesting
import androidx.lifecycle.LiveData
import androidx.lifecycle.Observer
import java.util.concurrent.CountDownLatch
import java.util.concurrent.TimeUnit
import java.util.concurrent.TimeoutException


@VisibleForTesting(otherwise = VisibleForTesting.NONE)
fun <T> LiveData<T>.getOrAwaitValue(
    time: Long = 2,
    timeUnit: TimeUnit = TimeUnit.SECONDS,
    afterObserve: () -> Unit = {}
): T {
    var data: T? = null
    val latch = CountDownLatch(1)
    val observer = object : Observer<T> {
        override fun onChanged(o: T?) {
            data = o
            latch.countDown()
            this@getOrAwaitValue.removeObserver(this)
        }
    }
    this.observeForever(observer)

    try {
        afterObserve.invoke()

        // Don't wait indefinitely if the LiveData is not set.
        if (!latch.await(time, timeUnit)) {
            throw TimeoutException("LiveData value was never set.")
        }

    } finally {
        this.removeObserver(observer)
    }

    @Suppress("UNCHECKED_CAST")
    return data as T
}