Create safe and predictable observable abstractions
Because we're thinking in terms of very isolated layers of abstractions, we're also looking to build well abstracted observables that make sense on their own. One way you could figure out if an observable can live on its own is: If I threw my initial requirement away, could this observable still be useful for something else? As part of building well designed abstractions, we need to assume they can be used in any context, and not just in the one we're focused on building at the moment. So we need to make them as predictable as possible to consumers. In this lesson, we'll ensure that the stream we built previously guards against situations where we have more task completions than starts, and also always gives an initial value.
Instructor: [00:00] Our current load count observable is great and useful. Anybody can subscribe to it and they'll get how many tasks are currently loading in their application. Because we can't assume who will subscribe to it, how it will be used, we need to make it work in a very predictable fashion, so it doesn't bring surprises to consumers.
[00:18] Good abstractions are as predictable as possible. If somebody subscribes to this, they won't get anything until this emits a value. Which is not right. We know that the count is 0initially so might as well omit 0and then start tracking properly as tasks begin to start and finish. This scenario is captured in our requirements, start from 0The nice thing about RxJS is that it usually flows like an English instruction. Let me just import the startWith operator. We want to start with a 0Because startWith now gives us the initial value, we don't really need the second argument to scan.
[00:57] To put that differently, if you don't give an initial value to scan, it will just let the first value it gets flow through it. Any consumers to this will get 0immediately, and this function won't even get called for that initial value. Once we start getting more values after that, it will start getting called and it will start adding values to the initial one.
[01:17] Another problem is what happens if for some reason we get way more task completions than tasks starts? I don't know why that would happen, but this would start going into the negative which, again, doesn't really make sense.
[01:29] Let's extract this into a variable and say that if it's smaller than 0return 0otherwise, just return the new actual amount. Nice. This doesn't get into the negative. If we do get to the scenario where we get way more completions than starts, this will just keep emitting 0over and over again which is not a huge problem.
[01:50] Again, as good abstraction builders, we want to be as predictable as possible to our consumers. I'll go up and import the distinctUntilChanged operator. I'll use it here to filter subsequent values which are equal.
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