RxSchedulersOverrideRule.java

package in.mvpstarter.sample.util;

import org.junit.rules.TestRule;
import org.junit.runner.Description;
import org.junit.runners.model.Statement;

import java.util.concurrent.Callable;


import io.reactivex.Scheduler;
import io.reactivex.android.plugins.RxAndroidPlugins;
import io.reactivex.functions.Function;
import io.reactivex.plugins.RxJavaPlugins;
import io.reactivex.schedulers.Schedulers;
/**
 * NOTE: You MUST use this rule in every test class that targets app code that uses RxJava.
 * Even when that code doesn't use any scheduler. The RxJava {@link Schedulers} class is setup
 * once and caches the schedulers in memory. So if one of the test classes doesn't use this rule
 * by the time this rule runs it may be too late to override the schedulers. This is really not
 * ideal but currently there isn't a better approach.
 * More info here: https://github.com/ReactiveX/RxJava/issues/2297
 * <p>
 * This rule registers SchedulerHooks for RxJava and RxAndroid to ensure that subscriptions
 * always subscribeOn and observeOn Schedulers.immediate().
 * Warning, this rule will reset RxAndroidPlugins and RxJavaPlugins before and after each test so
 * if the application code uses RxJava plugins this may affect the behaviour of the testing method.
 */
public class RxSchedulersOverrideRule implements TestRule {
    
    public final Scheduler SCHEDULER_INSTANCE = Schedulers.trampoline();
    private Function<Scheduler, Scheduler> mSchedulerFunction = scheduler -> SCHEDULER_INSTANCE;
    private Function<Callable<Scheduler>, Scheduler> mSchedulerFunctionLazy = schedulerCallable
            -> SCHEDULER_INSTANCE;

    @Override
    public Statement apply(final Statement base, Description description) {
        return new Statement() {
            @Override
            public void evaluate() throws Throwable {
                RxAndroidPlugins.reset();
                RxAndroidPlugins.setInitMainThreadSchedulerHandler(mSchedulerFunctionLazy);

                RxJavaPlugins.reset();
                RxJavaPlugins.setIoSchedulerHandler(mSchedulerFunction);
                RxJavaPlugins.setNewThreadSchedulerHandler(mSchedulerFunction);
                RxJavaPlugins.setComputationSchedulerHandler(mSchedulerFunction);

                base.evaluate();

                RxAndroidPlugins.reset();
                RxJavaPlugins.reset();

            }
        };
    }
}