Tossable drawer transition director in Material Motion

TossableDrawerTransitionDirector.swift

func willBeginTransition(_ transition: Transition) {
  self.transition = transition
  runtime = StreamRuntime(runtime: transition.runtime)

  if transition.foreViewController.preferredContentSize != .zero {
    drawerSize = transition.foreViewController.preferredContentSize
  } else {
    drawerSize = transition.foreViewController.view.bounds.size
  }
  if transition.direction == .forward {
    transition.foreViewController.view.bounds = CGRect(origin: .zero, size: drawerSize)
  }

  let bounds = transition.containerView.bounds
  backValue = bounds.maxX + drawerSize.width / 2
  foreValue = bounds.maxX - drawerSize.width / 2

  positionMemory = Memory(initialValue: transition.direction == .forward ? backValue : foreValue)

  let layer = transition.foreViewController.view.layer
  layer.position.x = positionMemory.value

  spring = springWith(destination: transition.direction == .forward ? foreValue : backValue,
                      target: layer,
                      source: positionMemory,
                      property: positionMemory.property,
                      threshold: 1)
  springStream = spring.rubberBanded(below: foreValue, above: backValue, length: rubberBandLength)
  runtime.apply(springStream, to: layer, property: propertyOf(layer).positionX)
}

func add(_ gestureRecognizer: UIGestureRecognizer) {
  switch gestureRecognizer {
  case let pan as UIPanGestureRecognizer:
    let view = transition.foreViewController.view!

    let draggable = ADraggable(subscribedTo: pan,
                               readInitialPointFrom: view,
                               property: propertyOf(view).position)
      .x()
      .passthrough(memory: positionMemory)
      .rubberBanded(below: foreValue, above: backValue, length: rubberBandLength)
    runtime.pause(spring, whileStreamIsActive: draggable)
    runtime.apply(draggable, to: view, property: propertyOf(view).positionX)

    let velocity = AVelocity(subscribedTo: pan).x()
    runtime.apply(velocity, to: spring, property: spring.velocityProperty)
  default: ()
  }
}