Default Schedulers

Scheduling is a core part of writing any app that uses the Reactive Extensions, as all operations that are deferred (i.e. run on other threads or on the UI thread). Schedulers allow apps to control what context code runs in, and it is important that libraries that run code on other threads are scheduler-aware.

ReactiveUI provides two app-wide schedulers that should be used in-place of other schedulers such as the built-in Rx schedulers:

• RxApp.MainThreadScheduler - This scheduler executes on the UI thread. On XAML-based platforms, this is equivalent to Dispatcher.BeginInvoke.

• RxApp.TaskpoolScheduler - This scheduler executes code via the TPL taskpool. This is equivalent to Task.Run.

These global variables are global because they can be globally replaced in a unit test (see the "Testing schedulers" section below). This means, that in a unit test, you can make concurrent code (i.e. code that runs on many threads simultaneously) run synchronously and have the same result happen every time.

Objects that care about scheduling

• ReactiveCommand - CreateAsyncXYZ return results on RxApp.MainThreadScheduler.

• MessageBus - Message bus can optionally schedule messages to an arbitrary scheduler.

When should I care about scheduling

You should try to attempt to remove all sources of concurrency other than scheduling via RxApp. This isn't always possible, but threads created via "new Thread()" or "Task.Run" can't be controlled in a unit test. The most straightforward way to fix these is by replacing them with "Observable.Start":

Old:

var result = await Task.Run(() => {
    int number = ThisCalculationTakesALongTime();
    return number;
});

Dispatcher.BeginInvoke(new Action(() => DoAThing()));

New:

var result = await Observable.Start(() => {
    int number = ThisCalculationTakesALongTime();
    return number;
}, RxApp.TaskpoolScheduler);

RxApp.MainThreadScheduler.Schedule(() => DoAThing());

If you create a shared component, you should also consider allowing the scheduler being specified as an optional constructor parameter.

Testing schedulers

In a unit test runner, by default, the MainThreadScheduler runs code immediately instead of on the (non-existent) UI thread. The TaskpoolScheduler is left unchanged by default.

The best way to run under an alternate scheduler is via the With method, most often used with TestScheduler. This replaces both schedulers with the specified scheduler:

(new TestScheduler()).With(sched =>{
    // Code run in this block will have both RxApp.MainThreadScheduler
    // and RxApp.TaskpoolScheduler assigned to the new TestScheduler.
});