fix issue #71: UIView that conforms StatefulViewController protocol cannot be destroyed

StatefulViewController/StatefulViewController.swift

@@ -12,7 +12,7 @@ public enum StatefulViewControllerState: String {
 public protocol BackingViewProvider {
     /// The backing view, usually a UIViewController's view.
     /// All placeholder views will be added to this view instance.
-    var backingView: UIView { get }
+    var backingView: UIView! { get }
 }
 
 /// StatefulViewController protocol may be adopted by a view controller or a view in order to transition to

StatefulViewController/StatefulViewControllerImplementation.swift

@@ -4,13 +4,13 @@ import UIKit
 // MARK: Default Implementation BackingViewProvider
 
 extension BackingViewProvider where Self: UIViewController {
-    public var backingView: UIView {
+    public weak var backingView: UIView! {
         return view
     }
 }
 
 extension BackingViewProvider where Self: UIView {
-    public var backingView: UIView {
+    public weak var backingView: UIView! {
         return self
     }
 }

StatefulViewController/ViewStateMachine.swift

@@ -52,8 +52,8 @@ public class ViewStateMachine {
     }()
 
     /// The view that should act as the superview for any added views
-    public let view: UIView
-    
+    public weak var view: UIView?
+
     /// The current display state of views
     public fileprivate(set) var currentState: ViewStateMachineState = .none
 
@@ -162,9 +162,14 @@ public class ViewStateMachine {
     // MARK: Private view updates
 
 	fileprivate func showView(forKey state: String, animated: Bool, completion: (() -> ())? = nil) {
+        guard let superview = self.view else {
+            completion?()
+            return
+        }
+
         // Add the container view
-        containerView.frame = view.bounds
-        view.addSubview(containerView)
+        containerView.frame = superview.bounds
+        superview.addSubview(containerView)
 
         let store = viewStore