add helpers to observe theme changes

Author: mikhailChelbaev

Sources/ComponentsKit/Helpers/SwiftUI/ThemeChangeObserver.swift

@@ -0,0 +1,48 @@
+import SwiftUI
+
+/// A SwiftUI wrapper that listens for theme changes and automatically refreshes its content.
+///
+/// `ThemeChangeObserver` ensures that its child views **rebuild** whenever the theme changes,
+/// helping to apply updated theme styles dynamically.
+///
+/// ## Usage
+///
+/// Wrap your view inside `ThemeChangeObserver` to make it responsive to theme updates:
+///
+/// ```swift
+/// @main
+/// struct Root: App {
+///   var body: some Scene {
+///     WindowGroup {
+///       ThemeChangeObserver {
+///         Content()
+///       }
+///     }
+///   }
+/// }
+/// ```
+///
+/// ## Performance Considerations
+///
+/// - This approach forces a **full re-evaluation** of the wrapped content, which ensures all theme-dependent
+///   properties are updated.
+/// - Use it **at a high level** in your SwiftUI hierarchy (e.g., wrapping entire screens) rather than for small components.
+public struct ThemeChangeObserver<Content: View>: View {
+  @State private var themeId = UUID()
+  @ViewBuilder var content: () -> Content
+  
+  public init(content: @escaping () -> Content) {
+    self.content = content
+  }
+  
+  public var body: some View {
+    self.content()
+      .onReceive(NotificationCenter.default.publisher(
+        for: Theme.didChangeThemeNotification,
+        object: nil
+      )) { _ in
+        self.themeId = UUID()
+      }
+      .id(self.themeId)
+  }
+}

Sources/ComponentsKit/Helpers/UIKit/NSObject+ObserveThemeChange.swift

@@ -0,0 +1,80 @@
+import Foundation
+import Combine
+
+extension NSObject {
+  /// Observes changes to the `.current` theme and updates dependent views.
+  ///
+  /// This method allows you to respond to theme changes by updating view properties that depend on the theme.
+  ///
+  /// You can invoke the ``observeThemeChange(_:)`` method a single time in the `viewDidLoad`
+  /// and update all the view elements:
+  ///
+  /// ```swift
+  /// override func viewDidLoad() {
+  ///   super.viewDidLoad()
+  ///
+  ///   style()
+  ///
+  ///   observeThemeChanges { [weak self] in
+  ///     guard let self else { return }
+  ///
+  ///     self.style()
+  ///   }
+  /// }
+  ///
+  /// func style() {
+  ///   view.backgroundColor = UniversalColor.background.uiColor
+  ///   // ...
+  /// }
+  /// ```
+  ///
+  /// > Note: There is no need to update components from the library as they observe the changes internally.
+  ///
+  /// ## Cancellation
+  ///
+  /// The method returns an ``AnyCancellable`` that can be used to cancel observation. For
+  /// example, if you only want to observe while a view controller is visible, you can start
+  /// observation in the `viewWillAppear` and then cancel observation in the `viewWillDisappear`:
+  ///
+  /// ```swift
+  /// var cancellable: AnyCancellable?
+  ///
+  /// func viewWillAppear() {
+  ///   super.viewWillAppear()
+  ///   cancellable = observeThemeChange { [weak self] in
+  ///     // ...
+  ///   }
+  /// }
+  /// func viewWillDisappear() {
+  ///   super.viewWillDisappear()
+  ///   cancellable?.cancel()
+  /// }
+  /// ```
+  ///
+  /// - Parameter apply: A closure that will be called whenever the `.current` theme changes.
+  ///   This should contain logic to update theme-dependent views.
+  /// - Returns: An `AnyCancellable` instance that can be used to stop observing the theme changes when needed.
+  @discardableResult
+  public func observeThemeChange(_ apply: @escaping () -> Void) -> AnyCancellable {
+    let cancellable = NotificationCenter.default.publisher(
+      for: Theme.didChangeThemeNotification
+    )
+      .receive(on: DispatchQueue.main)
+      .sink { _ in
+        apply()
+      }
+    self.cancellables.append(cancellable)
+    return cancellable
+  }
+  
+  fileprivate var cancellables: [Any] {
+    get {
+      objc_getAssociatedObject(self, Self.cancellablesKey) as? [Any] ?? []
+    }
+    set {
+      objc_setAssociatedObject(self, Self.cancellablesKey, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
+    }
+  }
+  
+  private static let cancellablesKey = "themeChangeObserverCancellables"
+}