spring-directly-manipulable.swift

    let pan = UIPanGestureRecognizer()
    let rotate = UIRotationGestureRecognizer()
    let scale = UIPinchGestureRecognizer()
    [pan, rotate, scale].forEach { $0.delegate = self }

    let directlyManipulable = DirectlyManipulable(view: square,
                                                  containerView: view,
                                                  panGestureRecognizer: pan,
                                                  rotationGestureRecognizer: rotate,
                                                  scaleGestureRecognizer: scale)
    let positionSpring = Spring(to: createProperty(withInitialValue: view.center),
                                initialValue: directlyManipulable.draggable.property,
                                threshold: 1)
    runtime.write(directlyManipulable.draggable.velocityStream, to: positionSpring.initialVelocity)

    let rotationSpring = Spring(to: createProperty(withInitialValue: 0),
                                initialValue: directlyManipulable.rotatable.property,
                                threshold: CGFloat(M_PI / 180.0))
    runtime.write(directlyManipulable.rotatable.velocityStream, to: rotationSpring.initialVelocity)

    let scaleSpring = Spring(to: createProperty(withInitialValue: 1),
                             initialValue: directlyManipulable.scalable.property,
                             threshold: 0.01)
    runtime.write(directlyManipulable.scalable.velocityStream, to: scaleSpring.initialVelocity)

    directlyManipulable.connect(with: runtime)

    let gestures = runtime.createChild()
    gestures.write(directlyManipulable.draggable.valueStream, to: Callback { _ in })
    gestures.write(directlyManipulable.rotatable.valueStream, to: Callback { _ in })
    gestures.write(directlyManipulable.scalable.valueStream, to: Callback { _ in })
    subscription = gestures.state.subscribe { state in
      if state == .active {
        positionSpring.initialVelocity.write(.zero)
        rotationSpring.initialVelocity.write(0)
        scaleSpring.initialVelocity.write(0)
      }
    }

    let stateIsActiveStream = gestures.state._map { $0 == .atRest }
    let positionSpring$ = popSpringSource(positionSpring)
    runtime.write(positionSpring$.valve(stateIsActiveStream),
                  to: directlyManipulable.draggable.property)

    let rotationSpring$ = popSpringSource(rotationSpring)
    runtime.write(rotationSpring$.valve(stateIsActiveStream),
                  to: directlyManipulable.rotatable.property)

    let scaleSpring$ = popSpringSource(scaleSpring)
    runtime.write(scaleSpring$.valve(stateIsActiveStream),
                  to: directlyManipulable.scalable.property)