rxjava: queue scheduler with default idle job - scheduler

Question

I have a client server application and I'm using rxjava to do server requests from the client. The client should only do one request at a time so I intent to use a thread queue scheduler similar to the trampoline scheduler.
Now I try to implement a mechanism to watch changes on the server. Therefore I send a long living request that blocks until the server has some changes and sends back the result (long pull).
This long pull request should only run when the job queue is idle. I'm looking for a way to automatically stop the watch request when a regular request is scheduled and start it again when the queue becomes empty. I thought about modifying the trampoline scheduler to get this behavior but I have the feeling that this is a common problem and there might be an easier solution?

Answer 1

You can hold onto the Subscription returned by scheduling the long poll task, unsubscribe it if the queue becomes non-empty and re-schedule if the queue becomes empty.
Edit: here is an example with the basic ExecutorScheduler:
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
public class IdleScheduling {
static final class TaskQueue {
final ExecutorService executor;
final AtomicReference<Future<?>> idleFuture;
final Runnable idleRunnable;
final AtomicInteger wip;
public TaskQueue(Runnable idleRunnable) {
this.executor = Executors.newFixedThreadPool(1);
this.idleRunnable = idleRunnable;
this.idleFuture = new AtomicReference<>();
this.wip = new AtomicInteger();
this.idleFuture.set(executor.submit(idleRunnable));
}
public void shutdownNow() {
executor.shutdownNow();
}
public Future<?> enqueue(Runnable task) {
if (wip.getAndIncrement() == 0) {
idleFuture.get().cancel(true);
}
return executor.submit(() -> {
task.run();
if (wip.decrementAndGet() == 0) {
startIdle();
}
});
}
void startIdle() {
idleFuture.set(executor.submit(idleRunnable));
}
}
public static void main(String[] args) throws Exception {
TaskQueue tq = new TaskQueue(() -> {
while (!Thread.currentThread().isInterrupted()) {
try {
Thread.sleep(1000);
} catch (InterruptedException ex) {
System.out.println("Idle interrupted...");
return;
}
System.out.println("Idle...");
}
});
try {
Thread.sleep(1500);
tq.enqueue(() -> System.out.println("Work 1"));
Thread.sleep(500);
tq.enqueue(() -> {
System.out.println("Work 2");
try {
Thread.sleep(500);
} catch (InterruptedException ex) {
}
});
tq.enqueue(() -> System.out.println("Work 3"));
Thread.sleep(1500);
} finally {
tq.shutdownNow();
}
}
}