refactor(iOS): migrate RNSStackController to Swift (#81)

- **Migrate RNSStackScreenController to Swift**
  Stage 1 - currently there is parallel type StackController.
  In next stage I'll remove old implementation & rename the new one.
  

- **Add missing @objc annotations**
  This is required for these methods to be visible from ObjC code
  

- **Remove old implementation & rename the new one to use naming conv.**
  

- **Add missing setNeedsUpdateOfChildViewControllers method**

Author: kkafar

ios/gamma/RNSReactMountingTransactionObserving.h

@@ -0,0 +1,7 @@
+import Foundation
+
+@objc
+protocol ReactMountingTransactionObserving {
+  func reactMountingTransactionWillMount()
+  func reactMountingTransactionDidMount()
+}

ios/gamma/ReactMountingTransactionObserving.swift

@@ -8,7 +8,6 @@
 #import <react/renderer/components/rnscreens/Props.h>
 #import <react/renderer/components/rnscreens/RCTComponentViewHelpers.h>
 
-#import "RNSStackController.h"
 #import "RNSStackScreenComponentView.h"
 #import "RNScreens-Swift.h"
 
@@ -111,7 +110,7 @@ - (void)mountingTransactionWillMount:(const facebook::react::MountingTransaction
                 withSurfaceTelemetry:(const facebook::react::SurfaceTelemetry &)surfaceTelemetry
 {
   _hasModifiedReactSubviewsInCurrentTransaction = false;
-  [_controller reactMountingTransactionWillMount:transaction];
+  [_controller reactMountingTransactionWillMount];
 }
 
 - (void)mountingTransactionDidMount:(const facebook::react::MountingTransaction &)transaction
@@ -127,7 +126,7 @@ - (void)mountingTransactionDidMount:(const facebook::react::MountingTransaction
 
     [_controller setNeedsUpdateOfChildViewControllers:childViewControllers];
   }
-  [_controller reactMountingTransactionDidMount:transaction];
+  [_controller reactMountingTransactionDidMount];
 }
 
 @end

ios/gamma/stack/RNSScreenStackHostComponentView.mm

@@ -0,0 +1,41 @@
+import Foundation
+import UIKit
+
+@objc
+class RNSStackController : UINavigationController, ReactMountingTransactionObserving {
+  private var pendingChildViewControllers: [RNSStackScreenController]?
+  
+  @objc
+  func setNeedsUpdateOfChildViewControllers(_ viewControllers: [RNSStackScreenController]) {
+    pendingChildViewControllers = viewControllers
+  }
+  
+  @objc
+  func updateChildViewControllersIfNeeded() {
+    if pendingChildViewControllers != nil {
+      updateChildViewControllers()
+    }
+  }
+  
+  @objc
+  func updateChildViewControllers() {
+    guard let pendingChildViewControllers = pendingChildViewControllers else {
+      fatalError("[RNScreens] Pending update must not be nil while it is forced!")
+    }
+    
+    setViewControllers(pendingChildViewControllers, animated: true)
+    self.pendingChildViewControllers = nil
+  }
+  
+  // MARK: ReactMountingTransactionObserving
+  
+  @objc
+  func reactMountingTransactionWillMount() {
+    // noop
+  }
+  
+  @objc
+  func reactMountingTransactionDidMount() {
+    updateChildViewControllersIfNeeded()
+  }
+}