RxSwift - optional issue on binding to tableView - ios

Question

I have an UITableView and a users variable whose signature is like:
let users: Variable<[User]?> = Variable<[User]?>(nil)
While I'm trying to bind this array of users in a UITableView, I'm getting the error Generic parameter 'Self' could not be inferred. This only occurs when I'm observing an Optional type. Why does this happen? What's the workaround for this situation?
Here is an example of how I'm doing:
private func bind() {
// Does not compile: Generic parameter 'Self' could not be inferred
users.asObservable().bind(to:
tableView.rx.items(cellIdentifier: "UserCell",
cellType: UITableViewCell.self)) { (index, user, cell) in
// Cell setup.
}.disposed(by: disposeBag)
}

Answer 1

You see this error because the compiler cannot infer the type of the Optional variable users passed into the following functions which work on generics and need to be able to infer the type.
An Optional in Swift is actually implemented as shown below. I guess in case of an Optional being potentially nil aka .none the compiler cannot infer the type for methods like e.g. items and bind(to:) which work on generics.
public enum Optional<Wrapped> : ExpressibleByNilLiteral {
/// The absence of a value.
///
/// In code, the absence of a value is typically written using the `nil`
/// literal rather than the explicit `.none` enumeration case.
case none
/// The presence of a value, stored as `Wrapped`.
case some(Wrapped)
/// Creates an instance that stores the given value.
public init(_ some: Wrapped)
//...
}
Workaround 1.): You could just use filterNil() (RxOptionals lib) to avoid the problem.
private func bind() {
users.asObservable().filterNil().bind(to:
tableView.rx.items(cellIdentifier: "UserCell",
cellType: UITableViewCell.self)) { (index, user, cell) in
// Cell setup.
}.disposed(by: disposeBag)
}
Workaround 2.): Make users non-optional. If you have no users just set an empty array as value.
let users: Variable<[User]> = Variable<[User]>([])
Workaround 3.): Use Nil-Coalescing Operator ?? in map function like
private func bind() {
users.asObservable().map { optionalUsers -> [User] in
return optionalUsers ?? []
}
.bind(to:
tableView.rx.items(cellIdentifier: "UserCell",
cellType: UITableViewCell.self)) { (index, user, cell) in
// Cell setup.
}.disposed(by: disposeBag)
}
Sidenote: Variable is deprecated in the latest version of RxSwift
Just for reference:
Implementation of items
public func items<S: Sequence, Cell: UITableViewCell, O : ObservableType>
(cellIdentifier: String, cellType: Cell.Type = Cell.self)
-> (_ source: O)
-> (_ configureCell: #escaping (Int, S.Iterator.Element, Cell) -> Void)
-> Disposable
where O.E == S {
return { source in
return { configureCell in
let dataSource = RxTableViewReactiveArrayDataSourceSequenceWrapper<S> { (tv, i, item) in
let indexPath = IndexPath(item: i, section: 0)
let cell = tv.dequeueReusableCell(withIdentifier: cellIdentifier, for: indexPath) as! Cell
configureCell(i, item, cell)
return cell
}
return self.items(dataSource: dataSource)(source)
}
}
}
Implementation of bind(to:)
public func bind<O: ObserverType>(to observer: O) -> Disposable where O.E == E {
return self.subscribe(observer)
}

Answer 2

I had the same error without an optional type. It happened because I was using a UICollectionViewCell subclass in a UITableView.