⭐️ Impl: Add RNIModalPresentationState

Related:
* `TODO:2023-03-31-18-33-34` - Unify/streamline/consolidate logic for invoking modal focus/blur.
* `TODO:2023-03-31-18-34-23` - Impl. "modal presentation state" (i.e. `RNIModalPresentationState`).

ios/src_library/React Native/RNIModal/RNIModalPresentationState.swift

@@ -0,0 +1,122 @@
+//
+//  RNIModalPresentationState.swift
+//  react-native-ios-modal
+//
+//  Created by Dominic Go on 4/7/23.
+//
+
+import Foundation
+
+
+public enum RNIModalPresentationState: String {
+  case INITIAL;
+
+  case PRESENTING_PROGRAMMATIC;
+  case PRESENTING_UNKNOWN;
+
+  case PRESENTED;
+  case PRESENTED_UNKNOWN;
+
+  case DISMISSING_GESTURE;
+  case DISMISSING_PROGRAMMATIC;
+  case DISMISSING_UNKNOWN;
+
+  case DISMISS_GESTURE_CANCELLING;
+
+  case DISMISSED;
+
+  // MARK: - Computed Properties
+  // ---------------------------
+
+  var isDismissing: Bool {
+    switch self {
+      case .DISMISSING_UNKNOWN,
+           .DISMISSING_GESTURE,
+           .DISMISSING_PROGRAMMATIC:
+        return true;
+
+      default:
+        return false;
+    };
+  };
+
+  var isDismissingKnown: Bool {
+    self.isDismissing && self != .DISMISSING_UNKNOWN;
+  };
+
+  var isPresenting: Bool {
+    switch self {
+      case .PRESENTING_PROGRAMMATIC,
+           .PRESENTING_UNKNOWN,
+           .DISMISS_GESTURE_CANCELLING:
+        return true;
+
+      default:
+        return false;
+    };
+  };
+
+  var isNotSpecific: Bool {
+    switch self {
+      case .PRESENTING_UNKNOWN,
+           .DISMISSING_UNKNOWN:
+        return true;
+
+      default:
+        return false;
+    };
+  };
+
+  // MARK: - Computed Properties - Alias
+  // -----------------------------------
+
+  var isDismissViaGestureCancelling: Bool {
+    self == .DISMISS_GESTURE_CANCELLING;
+  };
+
+  var isDismissingViaGesture: Bool {
+    self == .DISMISSING_GESTURE
+  };
+
+  var isPresented: Bool {
+    self == .PRESENTED;
+  };
+
+  var isDismissed: Bool {
+    self == .DISMISSED;
+  };
+};
+
+public struct RNIModalPresentationStateMachine {
+
+  var state: RNIModalPresentationState = .INITIAL;
+  var statePrev: RNIModalPresentationState = .INITIAL;
+
+  var onDismissWillCancel: (() -> Void)?;
+  var onDismissDidCancel: (() -> Void)?;
+
+  mutating func set(state nextState: RNIModalPresentationState){
+    let prevState = self.state;
+    self.statePrev = prevState;
+
+    if prevState.isDismissingViaGesture && nextState.isPresenting {
+      self.state = .DISMISS_GESTURE_CANCELLING;
+      self.onDismissWillCancel?();
+
+    } else if prevState.isDismissViaGestureCancelling && nextState.isPresented {
+      self.state = nextState;
+      self.onDismissDidCancel?();
+
+    } else if !prevState.isNotSpecific && nextState.isNotSpecific {
+      /// Do not over-write specific/"known state", with non-specific/"unknown
+      /// state", e.g.
+      ///
+      /// * ✅: `PRESENTING_PROGRAMMATIC` -> `PRESENTING_UNKNOWN`
+      /// * ❌: `DISMISSING_GESTURE` -> `DISMISSING_UNKNOWN`
+      return;
+
+    } else {
+      self.state = nextState;
+    };
+  };
+};