dragpair.swift

public typealias MaterialRestableStream<T> = (stream: MaterialStream<T>, atRest: MaterialStream<Bool>)

public func drag(_ pan: UIPanGestureRecognizer) -> MaterialRestableStream<CGPoint> {
  return (MaterialStream<CGPoint>(named: #function) { observer in
    let dragObserver = DragObserver {
      observer.next(pan.location(in: pan.view!.superview!))
    }

    pan.addTarget(dragObserver, action: #selector(DragObserver.didPan))

    return Subscription {
      pan.removeTarget(dragObserver, action: #selector(DragObserver.didPan))
    }

  }, MaterialStream<Bool>(named: "\(#function).activity") { observer in
    observer.next(!(pan.state == .began || pan.state == .changed))

    let dragObserver = DragObserver {
      observer.next(!(pan.state == .began || pan.state == .changed))
    }

    pan.addTarget(dragObserver, action: #selector(DragObserver.didPan))

    return Subscription {
      pan.removeTarget(dragObserver, action: #selector(DragObserver.didPan))
    }
  })
}