Simplify Usage of Your Reactive Service by Building an RxJS Operator to Manage It For You

Instructor: [0:00] We've built our service and it works great, but has a very, very simple API, a function you can call to tell it that a task has started and one to tell it that a task is completed.

[0:11] This is great because it opens up our service to be used in the largest amount of context with Observables, Promises, setTimeouts, fetch events, and other things we haven't even thought of. For the most popular use cases like Observables, we can provide additional extensions on top of it to make it even easier to use.

[0:32] As right now, anytime a developer wants to track an Observable, it has to remember to call this right before it starts and then this right when it completes.

[0:41] A much more idiomatic solution for an RxJS context is an Operator. We'd pipe it to an Observable and they would automatically mark a task as started whenever this is subscribed to and mark it as done whenever it completes, whenever the timer fires.

[0:57] Let's build an Operator. All an Operator is is a function that takes an outsource Observable and returns another Observable. If I declare an interval that emits every half a second and we just take the first two values from it and then we subscribe...I'll log next notifications as well as completion events.

[1:20] If I wanted to track this Observable with my newly created Operator, I could just add it on here. Now my source would just be this, but because the convention is to make these Operators configurable by calling them as you pass them in the pipe, we're going to wrap this in a function that returns our Operator function. I know we're not really passing anything to this, but we're just following the convention.

[1:48] I'll now import our task functions from our service. If you remember from earlier, the function we pass to the Observable constructor will get called any time somebody subscribes to this Observable. This is a perfect place to call our new taskStarted function.

[2:05] The second thing we want to do is we want to make sure this Operator passes all notifications it gets from the source downwards to its subscribers. To listen for notifications from the source, I'll just subscribe to it. Whenever somebody subscribes to our returned Observable, we'll get a reference to that subscriber here. Now I can just pass all the events from the source down to our subscriber.

[2:31] Finally, we need to make sure we unsubscribe from this whenever our Observable gets disposed of. I'll store a reference to the subscription. In the disposable function we learned about earlier, I'll just call unsubscribe on our subscription.

[2:46] Since we started the task here and the event that this Observable is disposed of, we also want to complete the task. Otherwise, we run the risk of the spinner staying on the screen forever. Let me just add some console.logs so we can see when taskStarted is getting called and when taskComplete is getting called. I'm also going to import all the RxJS tokens that we've missed.

[3:09] If I bring in the console, we can see that we correctly called taskStarted whenever we subscribed to it. We then get the two notifications from the interval. Because we only take the first two, we then get a complete notification.

[3:24] What's interesting is that even though we don't explicitly unsubscribe from this one, we still get our taskComplete invocation inside our disposal function. That's because whenever our Observer gets a complete notification, it's going to immediately unsubscribe from its source Observable, which is going to trigger the disposal function in our Operator. Awesome.

[3:48] Let me just remove the debugging statements and I'll export our Operator. Then I'll go back to my page. I'll remove any reference to our service and instead I'll just import the Operator. I'll add it to this Observable as well. Finally, I'll just remove all the extra invocations to the old service.

[4:12] If I try this out, I'll just click once on each one of these buttons. The spinner appears and as tasks are completed, eventually it's going to go away. Nice, it works. Now we have an Operator that can just be piped to any Observable in our app and it's going to enable spinner tracking capabilities on that Observable.