The Composable Architecture

Co-authored-by: Stephen Celis <stephen.celis@gmail.com>

Sources/ComposableArchitecture/UIKit/IfLetUIKit.swift

@@ -0,0 +1,70 @@
+import Combine
+
+extension Store {
+  /// Subscribes to updates when a store containing optional state goes from `nil` to non-`nil` or
+  /// non-`nil` to `nil`.
+  ///
+  /// This is useful for handling navigation in UIKit. The state for a screen that you want to
+  /// navigate to can be held as an optional value in the parent, and when that value switches
+  /// from `nil` to non-`nil` you want to trigger a navigation and hand the detail view a `Store`
+  /// whose domain has been scoped to just that feature:
+  ///
+  ///     class MasterViewController: UIViewController {
+  ///       let store: Store<MasterState, MasterAction>
+  ///       var cancellables: Set<AnyCancellable> = []
+  ///       ...
+  ///       func viewDidLoad() {
+  ///         ...
+  ///         self.store
+  ///           .scope(state: \.optionalDetail, action: MasterAction.detail)
+  ///           .ifLet(
+  ///             then: { [weak self] detailStore in
+  ///               self?.navigationController?.pushViewController(
+  ///                 DetailViewController(store: detailStore),
+  ///                 animated: true
+  ///               )
+  ///             },
+  ///             else: { [weak self] in
+  ///               guard let self = self else { return }
+  ///               self.navigationController?.popToViewController(self, animated: true)
+  ///             }
+  ///           )
+  ///           .store(in: &self.cancellables)
+  ///       }
+  ///     }
+  ///
+  /// - Parameters:
+  ///   - unwrap: A function that is called with a store of non-optional state whenever the store's
+  ///     optional state goes from `nil` to non-`nil`.
+  ///   - else: A function that is called whenever the store's optional state goes from non-`nil` to
+  ///     `nil`.
+  /// - Returns: A cancellable associated with the underlying subscription.
+  public func ifLet<Wrapped>(
+    then unwrap: @escaping (Store<Wrapped, Action>) -> Void,
+    else: @escaping () -> Void
+  ) -> Cancellable where State == Wrapped? {
+    self
+      .scope(
+        state: { state in
+          state
+            .removeDuplicates(by: { ($0 != nil) == ($1 != nil) })
+            .handleEvents(receiveOutput: { if $0 == nil { `else`() } })
+            .compactMap { $0 }
+        },
+        action: { $0 }
+      )
+      .sink(receiveValue: unwrap)
+  }
+
+  /// An overload of `ifLet(then:else:)` for the times that you do not want to handle the `else`
+  /// case.
+  ///
+  /// - Parameter unwrap: A function that is called with a store of non-optional state whenever the
+  ///   store's optional state goes from `nil` to non-`nil`.
+  /// - Returns: A cancellable associated with the underlying subscription.
+  public func ifLet<Wrapped>(
+    then unwrap: @escaping (Store<Wrapped, Action>) -> Void
+  ) -> Cancellable where State == Wrapped? {
+    self.ifLet(then: unwrap, else: {})
+  }
+}