Scheduler

If you want to introduce multithreading into your cascade of Observable operators, you can do so by instructing those operators (or particular Observables) to operate on particular Schedulers.

Many of the RxJava Observable operators have varieties that take a Scheduler as a parameter. These instruct the operator to do some or all of its work on a particular Scheduler.

You can make an Observable act on a particular Scheduler by means of the observeOn or subscribeOn operators. observeOn instructs an Observable to call its observer's onNext, onError, and onCompleted methods on a particular Scheduler; subscribeOn takes this a step further and instructs the Observable to do all of its processing (including the sending of items and notifications to observers) on a particular Scheduler.

You can also split an operator that works on an Observable onto multiple threads with the parallel operator.

See also:

• RxJava Threading Examples
• Intro to Rx: Scheduling and Threading
• Rx Workshop: Schedulers

Varieties of Scheduler

You obtain a Scheduler from the factory methods described in the Schedulers class. The following table shows the varieties of Scheduler that are available to you by means of these methods:

Scheduler	purpose
Schedulers.computation( )	meant for computational work such as event-loops and callback processing; do not use this scheduler for I/O (use Schedulers.io( ) instead)
Schedulers.from(executor)	uses the specified `Executor` as a Scheduler
Schedulers.immediate( )	schedules work to begin immediately in the current thread
Schedulers.io( )	meant for I/O-bound work such as asynchronous performance of blocking I/O, this scheduler is backed by a thread-pool that will grow as needed; for ordinary computational work, switch to Schedulers.computation( )
Schedulers.newThread( )	creates a new thread for each unit of work
Schedulers.test( )	useful for testing purposes (see below)
Schedulers.trampoline( )	queues work to begin on the current thread after any already-queued work

Default Schedulers for RxJava Observable operators

Some Observable operators in RxJava have alternate forms that allow you to set which Scheduler the operator will use for (at least some part of) its operation. Others do not operate on any particular Scheduler, or operate on a particular default Scheduler. Those that have a particular default Scheduler include:

operator	Scheduler
buffer(timespan)	computation
buffer(timespan, count)	computation
buffer(timespan, timeshift)	computation
debounce(timeout, unit)	computation
delay(delay, unit)	computation
delaySubscription(delay, unit)	computation
interval	computation
parallel	computation
repeat	trampoline
repeatWhen	trampoline
replay(time, unit)	computation
replay(buffersize, time, unit)	computation
replay(selector, time, unit)	computation
replay(selector, buffersize, time, unit)	computation
retry	trampoline
sample(period, unit)	computation
skip(time, unit)	computation
skipLast(time, unit)	computation
take(time, unit)	computation
takeLast(time, unit)	computation
takeLast(count, time, unit)	computation
takeLastBuffer(time, unit)	computation
takeLastBuffer(count, time, unit)	computation
throttleFirst	computation
throttleLast	computation
throttleWithTimeout	computation
timeInterval	immediate
timeout(timeoutSelector)	immediate
timeout(firstTimeoutSelector, timeoutSelector)	immediate
timeout(timeoutSelector, other)	immediate
timeout(timeout, timeUnit)	computation
timeout(firstTimeoutSelector, timeoutSelector, other)	immediate
timeout(timeout, timeUnit, other)	computation
timer	computation
timestamp	immediate
window(timespan)	computation
window(timespan, count)	computation
window(timespan, timeshift)	computation

Using Schedulers

Aside from passing these Schedulers in to RxJava Observable operators, you can also use them to schedule your own work on Subscriptions. The following example uses the schedule( ) method of the Scheduler class to schedule work on the newThread Scheduler:

worker = Schedulers.newThread().createWorker();
worker.schedule(new Action0() {

    @Override
    public void call() {
        yourWork();
    }

});
// some time later...
worker.unsubscribe();

Recursive Schedulers

To schedule recursive calls, you can use schedule( ) and then schedule(this) on the Worker object:

worker = Schedulers.newThread().createWorker();
worker.schedule(new Action0() {

    @Override
    public void call() {
        yourWork();
        // recurse until unsubscribed (schedule will do nothing if unsubscribed)
        worker.schedule(this);
    }

});
// some time later...
worker.unsubscribe();

Checking or Setting Unsubscribed Status

Objects of the Worker class implement the Subscription interface, with its isUnsubscribed( ) and unsubscribe( ) methods, so you can stop work when a subscription is cancelled, or you can cancel the subscription from within the scheduled task:

Worker worker = Schedulers.newThread().createWorker();
Subscription mySubscription = worker.schedule(new Action0() {

    @Override
    public void call() {
        while(!worker.isUnsubscribed()) {
            status = yourWork();
            if(QUIT == status) { worker.unsubscribe(); }
        }
    }

});

The Worker is also a Subscription and so you can (and should, eventually) call its unsubscribe( ) method to signal that it can halt work and release resources:

worker.unsubscribe();

Delayed and Periodic Schedulers

You can also use a version of schedule( ) that delays your action on the given Scheduler until a certain timespan has passed. The following example schedules someAction to be performed on someScheduler after 500ms have passed according to that Scheduler's clock:

someScheduler.schedule(someAction, 500, TimeUnit.MILLISECONDS);

Another Scheduler method allows you to schedule an action to take place at regular intervals. The following example schedules someAction to be performed on someScheduler after 500ms have passed, and then every 250ms thereafter:

someScheduler.schedulePeriodically(someAction, 500, 250, TimeUnit.MILLISECONDS);

Testing Schedulers

A TestScheduler allows you to exercise fine-tuned manual control over how the scheduler’s clock behaves. This can be useful for testing interactions that depend on precise arrangements of actions in time. This scheduler has four additional methods:

advanceTimeTo(time,unit)

advances the Scheduler’s clock to a particular point in time

advanceTimeBy(time,unit)

advances the Scheduler’s clock forward by a particular amount of time

triggerActions( )

start any unstarted actions that have been scheduled for a time equal to or earlier than the present time according to the Scheduler’s clock

triggerActions(time) (the time unit is nanoseconds)

start any unstarted actions that have been scheduled for a time equal to or earlier than the specified time