Update API: Observable -> Single

[Andrew Breckenridge]

In #9 we talked about updating to the new RxSwift Single stream.

I'll take a shot at this next time I'm using the library, otherwise anyone else is more than welcome to make a PR 😄

[Shai Mishali]

This will probably require some restructuring due to the nature of the way this was built :)

[Evgeny Sureev]

I've converted library to Maybe, thus eliminated "cancelled" error.
Also I've added ability to select files using UIDocumentPickerViewController.

[Shai Mishali]

@evgeny-sureev Is there a PR associated with this ? Or did you just suggest an idea here ?

[Evgeny Sureev]

I can make PR by weekend, now all work is done in local copy.

[Shai Mishali]

I'm trying to think if Maybe is the correct behavior 🤔 It's not specific enough IMO

I'm thinking "if I would write this today, what would I do" and my initial thought is returning a Result-style enum with a Single:

typealias Image = UIImage
typealias Video = URL

protocol Pickable {}
extension Image: Pickable {}
extension Video: Pickable {}

enum PickerResult<Asset: Pickable> {
    case selected(Asset)
    case cancelled
}

And then accordingly return Observable from the methods, which would return either a Video or Image type, according to the request you made.

Would you mind not doing the PR for now until I give this some more thought? And also, what's your opinion on the proposed solution here @evgeny-sureev ?

[Evgeny Sureev]

Single with PickerResult:

logoTapGesture.rx.event
    .flatMap { [unowned self] _ in
        return self.picker.takePhoto()
    }
    .map { result -> UIImage in
        switch result {
            case let .selected(asset):
                return asset.0 // return original image
            case .cancelled:
                // So how do we abort here? Throw something? Or use flatmap with Observable.empty()?
        }
    }
    .bind(to: logoImageView.rx.image)
    .disposed(by: disposeBag)

Maybe:

logoTapGesture.rx.event
    .flatMap { [unowned self] _ in
        return self.picker.takePhoto()
    }
    .map { $0.0 } // return original image
    .bind(to: logoImageView.rx.image)
    .disposed(by: disposeBag)

Also we returning two images from photo actions, so typealias for Image must be tuple: typealias Image = (UIImage, UIImage?). But tuples cannot be extended to declare protocol conformance.

[Shai Mishali]

@evgeny-sureev The point isn't necessarily aborting. It's so you can have some side-effects related to cancelation.

Maybe could work as well, but I think nullability isn't describing the fact the operation was cancelled, necessarily.

An error (the state of it today) isn't good as well IMO, same as throwing - since it terminates the sequence (and outer sequences, at that). Since Cancellation is an expected result (and not a terminal error), it shouldn't emit an error event.

Regarding handling the separate variants, it would be exactly like a regular Result type.
There are many ways to handle a result type that aren't as convoluted as a full-blown switch. (like elements() and errors() from RxSwiftExt, for example)

I just think this needs some more though :)

[Evgeny Sureev]

Right, with my approach you'll need to use something like amb(self.picker.takePhoto, Observable.just(defaultPicture)) for side effects on cancelation.
Of course, if side effects relatively simple, they all could fit into subscribe(onCompleted:).

[Shai Mishali]

@evgeny-sureev amb in this situation will always emit the .just, as its the first to emit between the two streams - so this would still not solve what you're trying to do.

I still think the PickerResult approach better, but I'll think this more thoroughly throughout the weekend and put some notes here.