Erasing Combine Subjects Property Wrapper

ErasedSubjects.swift

#if canImport(Combine)
import Combine

@propertyWrapper
/// Provides a property wrapper whose projected value is a `PassthroughSubject`, but
/// the wrapped value is just an `AnyPublisher`. This allows you to make a property with
/// availability declared for only iOS 13+ and hide the actual publisher.
public struct ErasedPassthroughSubject<Output, Failure: Error> {
    private var value: Any?

    @available(iOS 13.0, tvOS 13.0, OSX 10.15, *)
    public var wrappedValue: AnyPublisher<Output, Failure> {
        return passthroughSubject().eraseToAnyPublisher()
    }

    @available(iOS 13.0, tvOS 13.0, OSX 10.15, *)
    public var projectedValue: PassthroughSubject<Output, Failure> {
        return passthroughSubject()
    }

    /// Creates the underlying `PassthroughSubject` automatically.
    public init() {
        if #available(iOS 13.0, tvOS 13.0, OSX 10.15, *) {
            self.value = PassthroughSubject<Output, Failure>()
        } else {
            self.value = nil
        }
    }

    @available(iOS 13.0, tvOS 13.0, OSX 10.15, *)
    private func passthroughSubject() -> PassthroughSubject<Output, Failure> {
        return (value as? PassthroughSubject) ?? PassthroughSubject()
    }
}

@propertyWrapper
/// Provides a property wrapper whose projected value is a `CurrentValueSubject`, but
/// the wrapped value is just an `AnyPublisher`. This allows you to make a property with
/// availability declared for only iOS 13+ and hide the actual publisher.
public struct ErasedCurrentValueSubject<Output, Failure: Error> {
    private var value: Any?
    private var firstValue: Output

    @available(iOS 13.0, tvOS 13.0, OSX 10.15, *)
    public var wrappedValue: AnyPublisher<Output, Failure> {
        return currentValueSubject().eraseToAnyPublisher()
    }

    @available(iOS 13.0, tvOS 13.0, OSX 10.15, *)
    public var projectedValue: CurrentValueSubject<Output, Failure> {
        return currentValueSubject()
    }

    /// Initialize with a current value.
    /// - Parameter currentValue: The current value to initialize the `CurrentValueSubject`
    /// with.
    public init(currentValue: Output) {
        firstValue = currentValue
        if #available(iOS 13.0, tvOS 13.0, OSX 10.15, *) {
            self.value = CurrentValueSubject<Output, Failure>(currentValue)
        } else {
            self.value = nil
        }
    }

    @available(iOS 13.0, tvOS 13.0, OSX 10.15, *)
    private func currentValueSubject() -> CurrentValueSubject<Output, Failure> {
        return (value as? CurrentValueSubject) ?? CurrentValueSubject(firstValue)
    }
}
#endif