Responder: improve the responder hierarchy configuration

This improves the responder hierarchy handling to be more correct.  One
of the largest changes occurs in the `ViewController` type which gains
more of its interfaces.  These are needed for presenting a view
controller and to have the parent-child relationship for the view
controllers.  These impact the view hierarchy and thus must be present.

Author: compnerd

Sources/SwiftWin32/App and Environment/Application.swift

@@ -44,6 +44,9 @@ open class Application: Responder {
 
   // Responder Chain
   override public var next: Responder? {
+    // The next responder is the application delegate, but only if the
+    // application delegate is an instance of `Responder` and is not a `View`,
+    // `ViewController`, or the `Application` object itself.
     if let responder = self.delegate as? Responder,
        !(self.delegate is View), !(self.delegate is ViewController),
        !(self.delegate === self) {

Sources/SwiftWin32/View Controllers/ViewController.swift

@@ -280,6 +280,39 @@ public class ViewController: Responder {
   public func didMove(toParent viewController: ViewController?) {
   }
 
+  // MARK - Managing Child View Controllers in a Custom Controller
+
+  /// An array of view controllers that are children of the current view
+  /// controller.
+  public private(set) var children: [ViewController] = []
+
+  /// Adds the specified view controller as a child of the current view
+  /// controller.
+  public func addChild(_ controller: ViewController) {
+    self.children.append(controller)
+    controller.parent = self
+  }
+
+  /// Removes the view controller from its parent.
+  public func removeFromParent() {
+    self.parent?.children.remove(object: self)
+    self.parent = nil
+  }
+
+  // MARK - Getting Other Related View Controllers
+
+  /// The view controller that presented this view controller.
+  public private(set) var presentingViewController: ViewController?
+
+  /// The view controller that presented this view controller.
+  public private(set) var presentedViewController: ViewController?
+
+  /// The view controller that presented this view controller.
+  public private(set) var parent: ViewController? {
+    willSet { self.willMove(toParent: newValue) }
+    didSet { self.didMove(toParent: self.parent) }
+  }
+
   // MARK -
 
   override public init() {
@@ -288,7 +321,18 @@ public class ViewController: Responder {
   // MARK - Responder Chain
 
   override public var next: Responder? {
-    return view?.superview
+    // If the view controller's view is the root view of a window, the next
+    // responder is the Window object.
+    if self.view is Window { return view }
+    // If the view controller is presented by another view controller, the next
+    // responder is the presenting view controller.
+    return self.presentingViewController
+  }
+}
+
+extension ViewController: Equatable {
+  public static func ==(_ lhs: ViewController, _ rhs: ViewController) -> Bool {
+    return lhs === rhs
   }
 }
 

Sources/SwiftWin32/Views and Controls/View.swift

@@ -921,8 +921,13 @@ public class View: Responder {
   // MARK - Responder Chain
 
   public override var next: Responder? {
-    if let parent = self.superview { return parent }
-    return nil
+    // If the view is the root view of a `ViewController`, the next responder is
+    // the view controller; otherwise, the next responder is the view's
+    // superview.
+
+    // FIXME(compnerd) how do we determine if we are the root view of a view
+    // controller?
+    return self.superview
   }
 
   // MARK - Trait Environment

Sources/SwiftWin32/Windows and Screens/Window.swift

@@ -254,6 +254,13 @@ public class Window: View {
   public class var didResignKeyNotification: NSNotification.Name {
     NSNotification.Name(rawValue: "UIWindowDidResignKeyNotification")
   }
+
+  // MARK - Responder Chain
+
+  override public var next: Responder? {
+    // The window's next responder is the `Application` object.
+    Application.shared
+  }
 }
 
 extension Window {