Chain propagation of a RxSwift Variable to another Variable - rx-swift

Question

This is an example of our use case:
We have a selectedIndex and a list of items.
class FoosViewModel {
let selectedIndex = Variable<Int>(0)
let items: [Foo] = ... // assume that this is initialized properly
}
In reality, we often care about which item is selected instead of the index of the selected item. So we'll have code like this:
selectedIndex.asObservable().subscribe(onNext: { [weak self] index in
guard let self = self else { return }
let selectedItem = items[index]
// Do sth with `selectedItem` here
}
Notice that the value of selectedItem is always driven by selectedIndex. Therefore, we change the code to the following:
class FoosViewModel {
let selectedIndex = Variable<Int>(0)
let selectedItem = Variable<Int>(items[0])
let items: [Foo] = ... // assume that this is initialized properly
init() {
selectedIndex.asObservable().subscribe(onNext: { [weak self] index in
guard let self = self else { return }
self.selectedItem = items[index]
}
}
}
This seems to be a common enough use case. Do we have an existing operator in Rx that can map a Variable to another? Is there sth like this:
class FoosViewModel {
let selectedIndex = Variable<Int>(0)
let selectedItem = selectedIndex.map{ items[$0] }
let items: [Foo] = ... // assume that this is initialized properly
}

Answer 1

What you have done is created two bits of state that are dependent on each other. It would be better to just have one source of truth and a derivative which means that one should be implemented differently than the other. Assuming that selectedIndex is the source of truth, then I would expect to see:
class FoosViewModel {
let selectedIndex = Variable<Int>(0)
let selectedItem: Observable<Foo?>
let items: [Foo]
init(items: [Foo]) {
selectedItem = selectedIndex.asObservable().map { index in
index < items.count ? items[$0] : nil
}
self.items = items
}
}
Unlike in your attempt, there is no temptation for a user of this class to try to assign a new value to selectedItem (in fact, the code won't even compile if you try.) As a side benefit, there is no need to do the "weak self dance" either since the map doesn't refer to self at all. All of this works because you made items a let rather than a var (good for you!)
If you wanted to be able to add/remove items then things get a bit more complex...
class MutableFooViewModel {
let selectedIndex = Variable<Int>(0)
let selectedItem: Observable<Foo?>
let items = Variable<[Foo]>([])
init(items: [Foo]) {
items.value = items
let _items = self.items // this is done to avoid reference to `self` in the below.
selectedItem = Observable.combineLatest(
_items.asObservable(),
selectedIndex.asObservable()
) { items, index in
index < items.count ? items[index] : nil
}
}
}
The idea here is that Subjects (Variable is a kind of subject) should not be the first thing you think of when making an Observable that depends on some other observable. In this respect, they are only good for creating the initial observable. (RxCocoa is full of them.)
Oh and by the way, Variable had been deprecated.