TrackCovid19/app/src/main/java/com/josh/trackcovid19v2/AppExecutors.java

package com.josh.trackcovid19v2;

import android.os.Handler;
import android.os.Looper;

import androidx.annotation.NonNull;

import java.util.concurrent.Executor;
import java.util.concurrent.Executors;

/**
 * Global executor pools for the whole application.
 * <p>
 * Grouping tasks like this avoids the effects of task starvation (e.g. disk reads don't wait behind
 * webservice requests).
 * <p>
 * Creating a class like this is a way for android apps to handle non-trivial threading.
 */
public class AppExecutors {

    // For Singleton instantiation
    private static final Object LOCK = new Object();
    private static AppExecutors sInstance;
    private final Executor diskIO;
    private final Executor mainThread;
    private final Executor networkIO;

    private AppExecutors(Executor diskIO, Executor networkIO, Executor mainThread) {
        this.diskIO = diskIO;
        this.networkIO = networkIO;
        this.mainThread = mainThread;
    }

    public static AppExecutors getInstance() {
        if (sInstance == null) {
            synchronized (LOCK) {
                sInstance = new AppExecutors(Executors.newSingleThreadExecutor(), Executors
                        .newFixedThreadPool(3), new MainThreadExecutor());
            }
        }
        return sInstance;
    }

    public Executor diskIO() {
        return diskIO;
    }

    public Executor mainThread() {
        return mainThread;
    }

    public Executor networkIO() {
        return networkIO;
    }

    private static class MainThreadExecutor implements Executor {
        private Handler mainThreadHandler = new Handler(Looper.getMainLooper());

        @Override
        public void execute(@NonNull Runnable command) {
            mainThreadHandler.post(command);
        }
    }
}